Overland T490 cartridge tape drive and autoloader

[Peter Coghlan via cctalk]

Many years ago, a friend gave me an Overland T490 tape drive which has
some kind of autoloader attached which takes ten tapes.  I was told it
came out of a Tandem system.  The tapes are square cartridges similar
but different to a DEC TK50.  I can't find very much information about
the drive on the web but there are some hints that it might be compatible
with an IBM 3480.  It powers up nicely and the fan works and after a
short pause, a green LED illuminates.  There are only two buttons on
the front, "unload" and "format".

There are two DD50 connectors on the back.  One had a terminator plugged
into it labelled "SCSI differential".  The other had a ridiculously long
cable with DD50 plugs on it connected, lending further credence that
this is a differential (pre-LVD I expect) SCSI device.

I would like to get this drive working with my Alpha or VAX VMS systems
but I have never had any luck getting them to talk to it.  Recently,
I tried a using a DD50-HD68 cable I found somewhere to connect it to
a differential SCSI card in my Alphaserver 800 but I could not get VMS
to see the drive.  Not knowing what SCSI id the drive is likely to be
using makes it hard to know where to start looking for it.

There are no switches on the outside of the drive which could be used
to set the SCSI id so I opened it up to see if I could find any hints
inside.  I didn't see anything that looked like it could be used to
set the SCSI id inside either.  What I did find is that the interface
board had a connector labelled "SCSI differential" which had two short
lengths of ribbon cables plugged into it leading to the two DD50
connectors on the rear panel and another connector labelled
"SCSI single ended" with nothing attached.  There were also two ten way
jumper packs which were labelled "DI" and "SE" on each side.

So, not having any luck with differential so far, I tried moving the
two jumper packs from "DI" to "SE" and moving the ribbon cable to the
"SCSI single ended" socket.  I used a short, known good DD50-DD50 SCSI
cable to connect the drive to my VAX 4000-100A and replaced the
differential terminator with a known good single-ended terminator.
VMS didn't see the drive.  VMS has a utility called scsi_info which can
be used to send a SCSI inquiry command and read mode pages etc.  Trying it
against each unused SCSI id results in "device timeout" every time.  The
system disk is on the same SCSI bus before the tape drive and a SCSI
scanner can be connected after it on the bus.  Both devices work fine so
the SCSI bus cabling and termination is in good shape on both sides of
the tape drive.  I've tried moving the system disk SCSI id from 0 to 1,
changing the initiator SCSI id from 6 to 7 and replacing the scanner with
a terminator in case there is any sort of SCSI id conflict but scsi_info
still doesn't show up anything that could be the tape drive.

Does anyone have any information about this drive, particularly
whether it should behave like a standard SCSI tape drive and what
SCSI id and/or lun it is expected to use or if there is some trick
required to get it to start talking? Maybe it doesn't like SCSI
inquiry commands?

Extra bonus points awarded for details on how to control the autoloader.

Maybe I did some damage to it when I was trying to get it to work when
I first got it?

Regards,
Peter Coghlan.

RE: Source for replacement caps in H744 regulators

[Peter Coghlan via cctalk]

>
>> -Original Message-
>> From: cctalk  On Behalf Of Christian Corti
>> via cctalk
>> Sent: 06 January 2022 10:02
>> To: General Discussion: On-Topic and Off-Topic Posts
>> 
>> Subject: Re: Source for replacement caps in H744 regulators
>> 
>> On Thu, 6 Jan 2022, r...@jarratt.me.uk wrote:
>> > If I can't find 10V rated ones, then, generally up to what sort of
>> > voltage rating can I go? Of course, physical size will be a factor,
>> > but electrically can it affect operation of the regulator if the rated
> voltage is
>> too high?
>> 
>> If you need to ask this, are you sure you want to do electronics repair?
>
> I am gradually learning.
>

I'm slowly picking this sort of stuff up as I go along too.

I think that asking questions like this is the right thing to do when we
are not sure about something and is not at all an indication that someone
is somehow not suited to the task.  Quite the reverse in fact.

If we don't do this sort of work ourselves, it's not going to be easy
to get someone else to do it and make sure it is done well for a
reasonable price.   Any contemporary electronics repair outfit, assuming
one can be found, would probably recommend dumping the whole thing and
replacing it with a "modern" power supply.

>
>> And what makes you think that you need to replace these caps at all?
>

I appreciate that there is lots of bogus advice out there which suggests
that all capacitors need to be replaced before even starting to investigate
the cause of problems but I think Rob has demonstrated that he is trying
to figure out what is going on rather than just blindly replacing stuff.

>
> One of the H744 regulators whines and I have been told it could be the ESR
> on these caps. I have measured the ESR on these particular ones (out of
> circuit) and it seems higher than the expected values printed on the meter
> and also the ESR is not stable, it fluctuates randomly. This suggests the
> cap is not in great condition.
>

I guess from that the large capacitance of these units, they are probably
filtering 50Hz or 100Hz ripple and high ESR is probably going to lead to
higher than normal levels of ripple on the output which could possibly
cause some inductive component somewhere to buzz or whine.

I'm not sure if it is easy to measure the ripple on the output or if
it is specified anywhere how much ripple can be tolerated before logic
errors start to become a problem.

Maybe it would be good to replace the capacitors in just the unit which
is whining to begin with and see if it makes a difference?

To address the original question, I think I read somewhere that electrolytic
capacitors that are used for voltages a lot lower than their rating may
fail to maintain their dielectric formation which may in turn result in
increased leakage.

Here's a thought.  Could the whining be coming from an inductive filter
before the capactors because of excessive leakage through the capacitors
rather than because of high ESR? Would they benefit from re-forming?

Regards,
Peter Coghlan.

>
>> 
>> Christian
>

H7821 power supply in MicroVAX 3100, SCSI disk enclosures and others

[Peter Coghlan via cctalk]

Speaking of problems with DEC power supplies...

Some time ago I posted about problems with five 1800uF capacitors in multiple
H7821 power supplies leaking electrolyte, particularly when the machines are
stored in the wrong orientation.

One of my MicroVAX 3100 machines has a slightly different looking H7821 power
supply with a shiney case instead of the matt cases on all the other ones.
The five capacitors in this one seemed to be in good shape although on closer
inspection there may be slight traces of leakage on some of them.

However, after running happily for five to ten minutes, it went pop with the
4A glass mains input fuse blackened.  Access is difficult enough but not
impossible.  This PSU consists of a double sided main PCB where the high power
components are and four daughter boards permanently attached at right angles
to the main board and blocking easy access to many of the heatsink mounted
components.  There are no component references and it is difficult to trace
the circuit because the tracks on the component side of the main PCB are
hidden by the larger components, heatsinks etc.  There was dirty black dust
accumulated by the fans everywhere and it was difficult to clean off because
of all the obstructions.  Starting at the mains input, I tried looking for
shorted capacitors, bridge rectifier, chopper transistor, diodes etc that
might cause the fuse to blow but failed to find anything obviously wrong.
I also looked at the output rectifiers and smoothing capacitors and found
nothing obviously wrong there either.

I decided to replace the fuse and try the old series light bulb trick.  My
150W bulb glowed at about a fifth of it's full brilliance and the power
supply seemed to function reasonably well, the green LED came on, the fans
turned and the unloaded +5V and +12V outputs looked good.  However the
brightness of the bulb suggested too much current was being drawn.  An
el-cheapo digital multimeter suggested there was only 28V across the power
supply mains input.  A slightly better model suggested it was closer to 90V
(out of 220V), however the likely wacky waveform could be confusing the meter.

I left it running in this state for a little while but pulled out the mains
lead when I thought I got a slight whiff of cooking electronics.  It was quite
difficult to quickly check that the input smoothing capacitors had discharged
enough and probe various likely components with fingers to see which might be
getting hot before they had time to cool down again.  After a couple of
attempts, I identified a MOV/varistor/surge arrestor thing in some thick
heatshrink rubber stuff as a likely suspect.  After unsoldering it, I found
it was cracked and blackened on the underside where I couldn't see because
of the heatshrink covering it.  I replaced it with the equivalent one out
of another H7821 which had been damaged by capacitor leakage.

When I turned the power on, the bulb glowed much less, however, there were
unexplained slight variations in it's brightness.  While I was studying this,
there was a slight cracking sound accompanied by a wisp of smoke and the
bulb suddenly got much brighter.  The replacement MOV thing had failed.  It
was connected across one of the main input smoothing capacitors, the other
capacitor also having one which was unaffected.

This fault was starting to look a bit familiar to me.  In a different DEC
power supply, I had a triac used to switch the power supply automatically
between 220V and 110V mains input go short circuit causing a similarly placed
MOV and fuse to fail when the unit was powered from 220V.  I tried looking for
a similar functioning triac in the H7821 which was not easy to do as explained.
The best way to identify this component seems to be to look for a three legged
high power component (which may or may not be on a heatsink - the previous one
wasn't) which is more or less directly connected between the (neutral?) AC
input to the bridge rectifier and the point where one end of each of the input
main smoothing capacitors are connected together.  Trying to follow the PCB
traces was nearly impossible but buzzing out lightly suspects with a
continuity tester identified a MAC220-8 mounted on the same heatsink as the
main chopper device plus a temperature sensing thermistor right at the input
end of the power supply as very likely to be it.  It wasn't shorted which was
disappointing but I decided to take it out of circuit anyway.  It was necessary
to unsolder the MAC220-8 and the thermistor using a solder-sucker to free the
leads, unscrew the heatsink from the PCB and lean it over to one side in order
to be able to unscrew the MAC220-8 from it.

After soldering the thermistor back in, replacing the MOV again and powering
up with the triac out, the light bulb glowed slightly for a brief period at
switch on but after that there was no visible illumination while the power
supply worked nicely.  However, the power supply should only work on 220V
with the triac missing

DEC 3000/600 Alphaserver problems

[Peter Coghlan via cctalk]

I have a rackmount DEC 3000/600 Alphaserver containing a H7816-BA power supply
which doesn't work.  As I mentioned before, this power supply is a real pig
to work on.

I also have two tabletop DEC 3000/600 Alphaservers which contain H7816-AA
power supplies which are similar but not identical.  The main difference
seems to be that the H7816-AA power supplies contain four integrated fans
(and lots of dust) while the H7816-BA relies on one large fan external to
it.  I am hoping that comparisons between a working H7816-AA and the failed
H7816-BA will help diagnose the cause of the failure.

So I dug out one of the tabletop machines, took the cover off and powered
it up to ensure the power supply actually works before I go any further.
The green LED came on, the fans turned and the diagnostic LEDs lit up.
After running for less than a minute, there was a cracking/popping noise
and a small spark visible from somewhere around three unmarked orange
surface mount components on the I/O board (not in the power supply) plus
a whiff of cooked electronics.  The power supply kept running and the
diagnostic LEDs remained lit but I powered off quickly and poked around
with my finger to try to find a hot component without success.  I examined
the area with a magnifying glass under a strong light but I was unable to
find anything damaged.

I wonder would these orange components be tantalum supply decoupling
capacitors?  They seem to have a small pip in the solder at the positive
ends.  After powering back on, two of them had 5.0V across them and there
was 4.87V across the third slightly smaller one, there were no further
fireworks (so far anyway) and the SROM mini-console works.

I have had similar experiences with one or both of my two tabletop units
in the past and I was unable to track down the culprits then either.
Neither unit works properly due to cache failure issues.  I wonder could
it be plausable that the caches are not functioning correctly due to
prior failure of several supply decoupling capacitors on the system board?

Unless these devices have markings on the underside, I have no idea what
their capacitance values are.  There is probably no point in trying to
measure them in-circuit if there are several all over the board in parallel
with each other.  Unsoldering them and trying to measure them then isn't
going to work either if they have failed.   Any suggestions for something
to replace them with that is less likely to go bang in the future?  Could
I get away with using components with leads to replace them instead of
surface mounted ones or would that introduce too much inductance?

Regards,
Peter Coghlan.

Re: DEC 3000/600 Alphaserver problems

[Peter Coghlan via cctalk]

On Tue, 11 Jan 2022 13:57:07 -0600, Jon Elson wrote:

On 1/11/22 10:40 AM, Peter Coghlan via cctalk wrote:

I have a rackmount DEC 3000/600 Alphaserver containing a H7816-BA power supply
which doesn't work.  As I mentioned before, this power supply is a real pig
to work on.

I also have two tabletop DEC 3000/600 Alphaservers which contain H7816-AA
power supplies which are similar but not identical.  The main difference
seems to be that the H7816-AA power supplies contain four integrated fans
(and lots of dust) while the H7816-BA relies on one large fan external to
it.  I am hoping that comparisons between a working H7816-AA and the failed
H7816-BA will help diagnose the cause of the failure.

So I dug out one of the tabletop machines, took the cover off and powered
it up to ensure the power supply actually works before I go any further.
The green LED came on, the fans turned and the diagnostic LEDs lit up.
After running for less than a minute, there was a cracking/popping noise
and a small spark visible from somewhere around three unmarked orange
surface mount components on the I/O board (not in the power supply) plus
a whiff of cooked electronics.  The power supply kept running and the
diagnostic LEDs remained lit but I powered off quickly and poked around
with my finger to try to find a hot component without success.  I examined
the area with a magnifying glass under a strong light but I was unable to
find anything damaged.

I wonder would these orange components be tantalum supply decoupling
capacitors?  They seem to have a small pip in the solder at the positive
ends.  After powering back on, two of them had 5.0V across them and there
was 4.87V across the third slightly smaller one, there were no further
fireworks (so far anyway) and the SROM mini-console works.


The orange ones are usually Sprague Oxi-Caps, designed to replace Tantalum
with Niobium Oxide, and they generally do NOT have the classic Tantalum
issue with catastrophic failure after periods of disuse.  When the do fail,
they generally turn black.  Some Tantalum caps were made black, some yellow.
Note, the bar polarity marker is usually on the PLUS side on Tantalums and
Oxi-Caps.
Yes, failed decoupling caps could lead to intermittent failures.



Thanks Jon.

Here is a badly focused picture of the suspect components:

http://www.beyondthepale.ie/cctech/p1010198.jpg

They are just beside where the flash reflected :-(

My first problem is I don't know exactly where I saw the spark other than it
seemed to be associated with one of the orange components in the middle of
the picture.

My second problem is that there were no lasting effects visible.  I can't
find anything that's turned black or looks damaged in any way.

Regards,
Peter.



Jon

RE: DEC 3000/600 Alphaserver problems

[Peter Coghlan via cctalk]

Thanks Eugene.

Perhaps I should have more clearly stated that whatever short was there cleared
itself pretty much as soon as it happened, leaving no evidence of where it
it had been and nothing to go on to locate it.  There is no sign of anything
getting hot afterwards.

The machine still seems to work as well as it did before the failure so
it seems unlikely it was an active component or a component in series with
a power rail that failed.

I have seen a non-surface mounted tantalum bead capacitor short in an ADM-5
terminal. It produced not much noise, lots of smoke pouring out of it and a
little black hole in the top of it as far as I recall, a bit like a Rifa
capacitor failing in fact.  On another occasion I had one explode violently
with a very loud bang leaving nothing but the leads sticking out of the board
in some other machine that I don't recall the name of.

This particular incident was somewhere between the two above.  It was not
very loud or violent but it was over very quickly indeed and I can't find
what happened.

Regards,
Peter.

W2HX wrote:
>
> A bad tant will show 0V across it and it will show 0V (or very close to it)
> across all the other tants used as filter caps on that same rail. I would
> turn it back on and measure all of your rails. See which is affected. Then
> pull out every card in the system that uses that rail (probably all cards
> use all rails). See if the supply comes back up. If not, problem is in the
> supply. If yes, put one card back in at a time and check the rails. It will
> help you ID the bad card. 
>
> For the bad board, look at the schematic for all the caps (tants) on the
> affected rail. If there are only a few, pull them all out and see if that
> solves the problem. The system will likely function just fine missing some
> filter caps. Take each cap and put it on an ohm meter. If the result is
> 0 ohms, that's your bad one. Pull the rest back and replace the bad one.
>
> If there are many tants on a board then I like the smoke-'em-out method.
> Pull the board, put it on your bench. If the bad rail is, say +15V, take a
> very stout bench supply, set it to 5V and current limited to very low, 50mA.
> Connect your bench supply to the card and slowly increase current. The
> supply will be voltage limited to something near zero but not quite because
> of the short in the tant. You may also need to increase the voltage too
> (even though it may still be zero). Likely would have to exceed 1A-2A or
> more amps to get the effect you want. The bad tant will heat up and
> hopefully let the magic smoke out. Or just get hot to the touch. If you had
> an IR camera, that would be a much more elegant approach but I just get
> them to smoke. Do wear eye protection as sometimes they explode. 
> 
> Here is a video where I used this technique just a few days ago. You can see
> me increasing the current and you can see how easy it is to identify the bad
> cap. There were 15 tants on this board, making it very difficult to locate
> the bad one otherwise.
> https://youtu.be/Fh8fGgh4yKY
>
>
> Sometimes there is more than one bad one. In this case, you may need to
> increase the current to many more amps. If you notice one smoking before
> any others, remove it (don't bother replacing it yet) and continue with the
> current to locate the next bad one, etc.
> 
> By your description, it appears that the rail on the stock power supply is
> plenty stout enough so it isn't being shorted but it is working to help you
> reveal the bad component. In case the power supply has any inductors that
> might suffer from a shorted tant, I would remove and use my own bench supply
> as described above.
> 
> Good luck!
> 
> 73 Eugene W2HX
> Subscribe to my Youtube Channel: https://www.youtube.com/c/w2hx-channel/videos
>
> 
> 
> -Original Message-
> From: cctalk  On Behalf Of Peter Coghlan via 
> cctalk
> Sent: Tuesday, January 11, 2022 11:41 AM
> To: General Discussion: On-Topic and Off-Topic Posts 
> Subject: DEC 3000/600 Alphaserver problems
> 
> I have a rackmount DEC 3000/600 Alphaserver containing a H7816-BA power
> supply which doesn't work.  As I mentioned before, this power supply is a
> real pig to work on.
> 
> I also have two tabletop DEC 3000/600 Alphaservers which contain H7816-AA
> power supplies which are similar but not identical.  The main difference
> seems to be that the H7816-AA power supplies contain four integrated fans
> (and lots of dust) while the H7816-BA relies on one large fan external to
> it.  I am hoping that comparisons between a working H7816-AA and the failed
> H7816-BA will help diagnose the cause of the failure.
>
> So I dug out one of the tabletop machines, took the cover off and powered
> it up to ensure the

Re: DEC 3000/600 Alphaserver problems

[Peter Coghlan via cctalk]

On Wed, 12 Jan 2022 at 11:02:47 -0600 Jon Elson via cctalk wrote:

On 1/11/22 5:17 PM, Peter Coghlan via cctalk wrote:


Thanks Jon.

Here is a badly focused picture of the suspect components:

http://www.beyondthepale.ie/cctech/p1010198.jpg


WOW, I don't know what those are!  They look like big 
ceramic caps to me, but if they don't have any polarity 
marking, then they can't be Tantalum.  Big multilayer 
ceramics are prone to cracking due to board flexing, and 
that can cause catastrophic failure, too.




Thanks again Jon.

They do have spikes of solder sticking out from the positive ends
which can be seen in the picture.  There are no circuit references on
the I/O board where the picture was taken.  However there are markings
on the system board including little + signs at the same ends as the
solder spikes on similar capacitors on that board.

I managed to desolder one of them from the middle of the picture using
two soldering irons.  I am not very good at this surface mount stuff :-(
It came out ok though.  I must have got lucky and picked a good one
because it measured 47uF on the capacitance range on my multimeter.

Regards,
Peter.



Jon

Re: DEC 3000/600 Alphaserver problems

[Peter Coghlan via cctalk]

On Wed, 12 Jan 2022 at 11:02:47 -0600 Jon Elson wrote:

On 1/11/22 5:17 PM, Peter Coghlan via cctalk wrote:


Thanks Jon.

Here is a badly focused picture of the suspect components:

http://www.beyondthepale.ie/cctech/p1010198.jpg


WOW, I don't know what those are!  They look like big 
ceramic caps to me, but if they don't have any polarity 
marking, then they can't be Tantalum.  Big multilayer 
ceramics are prone to cracking due to board flexing, and 
that can cause catastrophic failure, too.




Thanks again Jon.

They do have spikes of solder sticking out from the positive ends
which can be seen in the picture.  There are no circuit references on
the I/O board where the picture was taken.  However there are markings
on the system board including little + signs at the same ends as the
solder spikes on similar capacitors on that board.

I tried desoldering one from the middle of the picture using two
soldering irons - I'm not very good at this surface mounted stuff :-(
It came out ok though. I must have got lucky and picked a good one
because it measured 47uF in the capacitance section on my multimeter.

Regards,
Peter.

Re: DEC 3000/600 Alphaserver problems

[Peter Coghlan via cctalk]

On Wed, 12 Jan 2022 at 19:30:58 +, Peter Coghlan via cctalk wrote:

On Wed, 12 Jan 2022 at 11:02:47 -0600 Jon Elson via cctalk wrote:

On 1/11/22 5:17 PM, Peter Coghlan via cctalk wrote:


Thanks Jon.

Here is a badly focused picture of the suspect components:

http://www.beyondthepale.ie/cctech/p1010198.jpg


WOW, I don't know what those are!  They look like big 
ceramic caps to me, but if they don't have any polarity 
marking, then they can't be Tantalum.  Big multilayer 
ceramics are prone to cracking due to board flexing, and 
that can cause catastrophic failure, too.




Thanks again Jon.

They do have spikes of solder sticking out from the positive ends
which can be seen in the picture.  There are no circuit references on
the I/O board where the picture was taken.  However there are markings
on the system board including little + signs at the same ends as the
solder spikes on similar capacitors on that board.

I managed to desolder one of them from the middle of the picture using
two soldering irons.  I am not very good at this surface mount stuff :-(
It came out ok though.  I must have got lucky and picked a good one
because it measured 47uF on the capacitance range on my multimeter.



I looked around some more with the magnifying glass and found the tiniest
crack in the body of one of the smaller orange components, the vertical one
in the middle of the picture.  I desoldered it and measured it.  It was
9.73uF and had 35.7 kOhms leakage (the 47uF one had no measurable leakage).
It is not one of the three I suspected initially and it has 12V across it
in operation, possibly associated with the nearby AUI connector.  I presume
it is a 10uF unit and despite having released some magic smoke, it seems to
be continuing to function reasonably well.

I guess failure of these sort of capacitors is probably not anything to do
with the cache failures I am getting then.

Time to get back to the power supply I think.

Regards,
Peter.





Jon

Re: DEC 3000/600 Alphaserver problems

[Peter Coghlan via cctalk]

On Wed, 12 Jan 2022 at 17:28:33 -0600 Richard Schauer via cctalk wrote:

On Wed, 12 Jan 2022, Peter Coghlan via cctalk wrote:

They do have spikes of solder sticking out from the positive ends


Yes, those are tantalum caps.  The "hat" as I've always called it marks 
the positive end, and it should be part of the case, so it won't melt away 
in solder.  These things are a bit obsolete and aren't particularly 
common, but not impossible either.  I use a 100uF 0805-size (2mm x 1.25mm) 
one of these on a weird board we build for a customer at work; getting 
that much capacitance in a part that small always amazes me!




This stuff dates from around 1994 I think. My 47uF component is about
6 or 7mm x 3 or 4mm.  Maybe a problem with squeezing a lot of capacitance
in could be that it wants to pop out again!



I hit Ebay to see if I could find something vaguely like what you have. 
The first listings that looked plausible were 194453105893 and 
203613355685.  I'm not saying they're the right size, or voltage, or 
anything like it, but it'll get you a manufacturer's part number that you 
can search for and find like ones.  His price seems high too.  You could, 
of course, sub an ordinary tantalum.




I probably need to peer closely at all of mine and figure out how many I
need.  There are about 50 of these things in each of three machines :-(




I managed to desolder one of them from the middle of the picture using
two soldering irons.  I am not very good at this surface mount stuff :-(
It came out ok though.  I must have got lucky and picked a good one
because it measured 47uF on the capacitance range on my multimeter.


Two irons is a perfectly valid method of removing SMT parts, if you're 
quick!  Ever heard of a hot tweezer?  It's two little irons, hinged at the 
top.  I have a variety of tips for mine and prefer it to hot air for most 
small things.  (Well, for routine work I usually have a chisel tip on my 
iron, and can get across both ends of something up to 0805-size and sweep 
it off the board.  1206 and bigger, you have to get creative, or use the 
hot tweezer.)




The hot tweezer sounds very nice but I think I'll stick with what I have
for now.  These machines have other problems and won't ever do anything
useful unless I get the cache issues sorted out so I don't necessarily
want to invest in new tools for them to end up not getting them repaired.

Many thanks.

Regards,
Peter.



Richard Schauer
KF9VP

Re: AOL diskettes

[Peter Coghlan via cctalk]

On Tue, 18 Jan 2022 at 13:38:32 -0700, Adam Thornton via cctalk wrote:
>
> I wish I'd kept some.  I had some AOL CDs from slightly later that made
> decent coasters for decades.  Although I guess with the shutter, the floppy
> wouldn't really have made a very good coaster.
>

I can only conclude you needed something to save the surface on one of these...

https://www.thisiswhyimbroke.com/floppy-disk-table/

Regards,
Peter Coghlan.

>
> Adam
>

Re: IBM 6731 Diskette Module

[Peter Coghlan via cctalk]

On Wed, 19 Jan 2022 at 04:06:59 -0800, D. Resor via cctalk wrote:
>
> There was also a 5.25" diskette which was nicknamed "IPL" for "initial
> program load", and an interface board which was installed into the
> typewriter and was referred to as the "IBM Typewriter Modularity Option".
>

Hi Don,

Apologies if you know this already but IPL / initial program load
is IBM-speak for "boot".

Regards,
Peter Coghlan.

Re: AOL diskettes

[Peter Coghlan via cctalk]

On Wed, 19 Jan 2022 at 12:00:33 +0100, Peter Corlett via cctalk wrote:
>On Tue, Jan 18, 2022 at 04:47:36PM -0700, Grant Taylor via cctalk wrote:
>> On 1/18/22 2:21 PM, Peter Coghlan via cctalk wrote:
>>> https://www.thisiswhyimbroke.com/floppy-disk-table/
>> I like it!
>
> The ratios are wrong: it's about twice as thick as it ought to be. It's
> apparently been designed by somebody who has seen a picture of a floppy but
> never used one.
>

Maybe the idea is that when the top gets marked, you can sand it down and
paint it again multiple times so you don't need to use those matching
coasters that are so hard to get nowadays?

>
>> But I hate the price.
>
> Get a LACK table from IKEA (€6.99) and adorn it with 3D prints, or even old
> floppies? I'd advise against trying to machine patterns into it though as
> that low-end stuff is basically made of laminated cheese.
>

Peter,

I'm shocked to see you recommend IKEA!

Have you not suffered from (physical) IKEA catalogue spam the same way we have
over here?

(BTW, trying to contact IKEA to get it sorted was very much like trying to
contact Google except that Google doesn't have sacrificial call centre workers
in a different country to intercept the complaints and lose them, Google just
doesn't take calls from anyone.)

Regards,
Peter Coghlan.

More switchmode power supply grief - Cisco IGS router

[Peter Coghlan via cctalk]

I have a Cisco IGS router which hasn't worked for a long time.  When I was
last using it several years ago, it occasionally crashed and restarted.
This turned out to be due to a poor contact on a connection in the
cable going from the output of the power supply to the main board.  I
cleaned the contact more than once but it was difficult to make it good
enough to ensure reliability.

When I switched it on more recently, it was completely dead, no LEDs, no fan
noise, no anything.  I put it in the naughty pile and it sat there for a few
years before I got around to looking at it.

Today I finally managed to check it out.  The ceramic F4A mains input fuse
beside the power switch on the back panel had blown.  When I opened it up,
I found a POWER-ONE MAP80-4000 power supply.  The main chopper transistor
labelled Q1 on the PCB is almost a dead short.  It is a large plastic
packaged FET mounted on a piece of aluminium which is in turn screwed to
the case for heatsinking.  Unfortunately, there are no markings on it so
I have no idea what to replace it with :-(

As Q1 is shorted across all three terminals, whatever drives it may be
damaged too :-(

After finding screw heads hidden under the label, I managed to extract
the PCB from the case and found some corrosion underneath, possibly from
leaking electrolytic capacitors :-(

There are lots of data sheets available for this power supply on the web
but they concentrate on the specifications for the unit and don't say
anything about the components :-(

There are also lots of people offering to sell power supplies like this
for way more than I am interested in spending on this project :-(

I could replace the power supply with a different one, however, I don't
have anything to hand that will fit in the approx 5-6cm headroom :-(

Anyone have any suggestions?

Regards,
Peter Coghlan.

Re: More switchmode power supply grief - Cisco IGS router

[Peter Coghlan via cctalk]

"Maciej W. Rozycki" wrote:
> On Thu, 3 Feb 2022, Peter Coghlan via cctalk wrote:
>
>> Today I finally managed to check it out.  The ceramic F4A mains input fuse
>> beside the power switch on the back panel had blown.  When I opened it up,
>> I found a POWER-ONE MAP80-4000 power supply.  The main chopper transistor
>> labelled Q1 on the PCB is almost a dead short.  It is a large plastic
>> packaged FET mounted on a piece of aluminium which is in turn screwed to
>> the case for heatsinking.  Unfortunately, there are no markings on it so
>> I have no idea what to replace it with :-(
>> 
>> As Q1 is shorted across all three terminals, whatever drives it may be
>> damaged too :-(
>> 
>> After finding screw heads hidden under the label, I managed to extract
>> the PCB from the case and found some corrosion underneath, possibly from
>> leaking electrolytic capacitors :-(
>
>  This PSU seems to have been used across various Cisco devices and I had 
> one fail several years ago in a WS-C1202 Ethernet/FDDI switch, also having 
> suffered from leaking caps.  Back then RS still had it in stock at a hefty 
> price of £489.60: <https://uk.rs-online.com/web/p/products/8338173/>, so I 
> chose to recap the failed one and thanfully it has continued working ever 
> since.  You may try to enquire with the manufacturer.
> 
>  Good luck with bringing yours back to life!
>

Thanks Maciej.  I'll be surprised if the manufacturer has records of
what they used for the chopper in power supplies 30 years ago - who
knows if their design has changed between then and now.  I'll be even
more suprised if they will talk to me about it but I guess it's worth
a try.

Regards,
Peter.

> 
>  FWIW,
> 
>   Maciej

Re: More switchmode power supply grief - Cisco IGS router

[Peter Coghlan via cctalk]

Phil Blundell wrote:
>
> Does that PSU have a PWM controller IC, or is it built entirely from 
> discretes?
> If there is an IC driving the chopper transistor then you may be able to get
> some clues about the likely characteristics of the transistor from the IC
> datasheet.
>

It's all discretes.  There are only about five or six TO-92 thingies in there
which are likely to be small signal transistors plus a few TO-220 style items
mounted on the heatsink that could be drivers and/or output rectifiers.  Come
to think of it, I suppose a FET is unlikely to need a driver on a heatsink?

>
> Is it definitely a FET?  Some, particularly older, designs used
> bipolar transistors there.
>

Even though there are no markings visible on the transistor, it's marked
S,D,G on the PCB!

If it was a bipolar, I'd probably chance a BU208A from the TV spares box.
What could go wrong :-)

>
> As you say there is a fairly high likelihood that other components on the
> primary side will have blown up as well so you might be looking at a fairly
> extensive repair.  Are there any other obscure, unmarked devices or is this
> the only one?

I've been a bit reluctant to handle the thing much to examine it closely,
between dust from the fan, stray electrolyte and heatsink compound where the
internal heatsinking used to be screwed to the case, it's very messy :-(

Except for the two main reservoir capacitors which seem to be sound, the
other electrolytics all seem to be Nichicon units and they all seem to have
leaked :-( I think lots of components are going to need to be desoldered
from the board to clean it.  That's going to get smelly :-( At least access
is very straightforward compared to a H7821 and way better than a H7816 :-)
No live heatsinks in this thing!

The bridge rectifier doesn't seem to be shorted.  The other heatsinked
components and small signal components seem to have markings on them so
it might not be too difficult to find replacements for those where needed,
provided I don't manage to swipe the numbers off them when cleaning the
up the leaked electrolyte...

Oh, there's a couple of RIFA capacitors with cracked cases in there too
but they're not input filters so no magic smoke escaped yet...

Regards,
Peter Coghlan.

>
> p.
>

Re: More switchmode power supply grief - Cisco IGS router

[Peter Coghlan via cctalk]

What's a "Mean-Well module"?  I somehow find myself imagining:

"I put a new module in my router.  It blew up.  At least it meant well"  :-)

(My IGS has the rather heavy and over-engineered divider shelf with the
main board underneath and the fan and power supply above.)

Regards,
Peter Coghlan

Jonathan Chapman via cctalk wrote:
>
> With that much carnage, I'd probably drop a Mean-Well module in there. I
> believe there would be enough room for one or two in the IGS I have
> (taller white box, divider "shelf" over the mainboard, I don't know if
> there was a lower profile model).
>
> Thanks,
> Jonathan
>
> --- Original Message ---
>
> On Saturday, February 5th, 2022 at 09:17, Phil Blundell via cctalk 
>  wrote:
> 
> > On Thu, Feb 03, 2022 at 06:06:10PM +, Peter Coghlan via cctalk wrote:
> >
> > > Today I finally managed to check it out. The ceramic F4A mains input fuse
> > >
> > > beside the power switch on the back panel had blown. When I opened it up,
> > >
> > > I found a POWER-ONE MAP80-4000 power supply. The main chopper transistor
> > >
> > > labelled Q1 on the PCB is almost a dead short. It is a large plastic
> > >
> > > packaged FET mounted on a piece of aluminium which is in turn screwed to
> > >
> > > the case for heatsinking. Unfortunately, there are no markings on it so
> > >
> > > I have no idea what to replace it with :-(
> > >
> > > As Q1 is shorted across all three terminals, whatever drives it may be
> > >
> > > damaged too :-(
> >
> > Does that PSU have a PWM controller IC, or is it built entirely from 
> > discretes?
> >
> > If there is an IC driving the chopper transistor then you may be able to get
> >
> > some clues about the likely characteristics of the transistor from the IC
> >
> > datasheet. Is it definitely a FET? Some, particularly older, designs used
> >
> > bipolar transistors there.
> >
> > As you say there is a fairly high likelihood that other components on the
> >
> > primary side will have blown up as well so you might be looking at a fairly
> >
> > extensive repair. Are there any other obscure, unmarked devices or is this
> >
> > the only one?
> >
> > p.
>

Re: More switchmode power supply grief - Cisco IGS router

[Peter Coghlan via cctalk]

"Maciej W. Rozycki" wrote:
>On Sat, 5 Feb 2022, Peter Coghlan via cctalk wrote:
>
> > >  This PSU seems to have been used across various Cisco devices and I had 
> > > one fail several years ago in a WS-C1202 Ethernet/FDDI switch, also 
> > > having 
> > > suffered from leaking caps.  Back then RS still had it in stock at a 
> > > hefty 
> > > price of £489.60: <https://uk.rs-online.com/web/p/products/8338173/>, so 
> > > I 
> > > chose to recap the failed one and thanfully it has continued working ever 
> > > since.  You may try to enquire with the manufacturer.
> > > 
> > >  Good luck with bringing yours back to life!
> > >
> > 
> > Thanks Maciej.  I'll be surprised if the manufacturer has records of
> > what they used for the chopper in power supplies 30 years ago - who
> > knows if their design has changed between then and now.  I'll be even
> > more suprised if they will talk to me about it but I guess it's worth
> > a try.
> 
>  I've checked their web site and they continue to offer a MAP80-4000G, 
> which I suppose is just a minor update from the original, and is visually 
> the same, as it seems from the datasheet dated 2021.  So I'm fairly sure 
> they have all the data.  Those discrete semiconductor components do not 
> change much, I was able to order exact replacements for another faulty 
> device from 1990s a few years ago.
> 
>  They may offer you a paid repair or replacement service or if you're kind 
> enough and they are reasonable, they may identify the single part for you.
> 
>  NB Mouser has the MAP80-4000G in stock actually, so if all else fails and 
> you are determined to get the IGS running, you can pick a brand new one: 
> <https://www.mouser.co.uk/ProductDetail/784-MAP80-4000G>.
> 

Thanks again Maciej.

I've bit the bullet and unsoldered most of the electrolytic capacitors and
other components that seem to be in trouble.  When I got the chopper
transistor out, I found that if I hold it at exactly the right angle to
the light, I can see that there are faint markings on it after all and it
turns out to be an IRFPE50 HEXFET.  I was wrong about the cracked RIFAs
not being mains filters too, they are actually across L-E and N-E and
they seem to have between a few hundred kOhms and a few MOhms leakage too.
The driver transistor looks like it might be ok.

Regards,
Peter Coghlan.


>  Good luck!
> 
>   Maciej

Re: Rack Discussion Continued - Slide lubricant

[Peter Coghlan via cctalk]

I have several difficult slides in my H960 rack.

What is the best lubricant for the slides?

I was wondering if graphite would work better than oil due to the fact 
that it won't pick up dirt and dust.




Powdered graphite for lubricating locks?

I wouldn't like to have conductive stuff like that anywhere it might
get sucked in by fans and deposited on PCBs.

Regards,
Peter Coghlan.

Re: AlphaServer 2100s available

[Peter Coghlan via cctalk]

It's worth what someone is willing to pay for it and this will vary widely.

I got one for EUR 20 in 2006.  I didn't think this was a bargain because
I was under the impression I was getting it for free before I travelled
to pick it up.  The previous owner had no further use for it and needed
the space it was taking up.

I think it's a nice machine although an Alphaserver 2100A would be nicer.
It looks and sounds like a real computer except the front panel is a bit
pathetic.  It is great for heating the room in winter.  However, it doesn't
seem to have any commercial value.

Regards,
Peter Coghlan.



What is an AlphaServer 2100 worth? There was one at RePC in Seattle a 
couple weeks ago and I was thinking about purchasing it.


On 4/14/22 2:36 PM, Antonio Carlini via cctalk wrote:

On 12/04/2022 16:34, Dave Wade G4UGM via cctalk wrote:

Folks,
Does no one fancy a go at this. Had zero interest...
Dave


(OFFLIST, I think)

I'm assuming the machine is safe, at least for the moment?

I've actually got less room now than when I had to let it go, so 
hopefully it can just sit in a corner and be a useful table end or 
something for a while?


I'm currently struggling with a uVAX 3600 PSu and a VAX 4000 PSU, so 
if I ever fix those, maybe I can help with the AS2100 ...



Antonio

Re: AlphaServer 2100s available

[Peter Coghlan via cctalk]

We occasionally hear of aged computers being employed in the nuclear
power industry, certain military applications or long lifed medical
equipment for example.  I imagine that these machines can have a
significant commercial value long after their contemporaries which are
not involved in these roles.  As far as I know, the Alphaserver 2100
was not commonly used for this sort of work and this is why I mentioned
that I thought it was unlikely to have any commercial value.  If the
opposite was true, it would probably have a bearing on the current day
value of the machine and the price may have been outside of the reach
of any of us.

So we are probably left with what a hobbyist / enthusiast / collector
or scrap dealer will pay for one, which as I said I believe will vary
widely (certainly for the former anyway).  If you want to know what
a scrap dealer will pay for one, you are probably asking the wrong
people although certain people on this list did occasionally comment
on the price of scrap.  I haven't noticed any discussion along these
lines for a long time now though.  They may have become tired of
repeating more or less what I am saying here.

The fact that the thing is big, heavy and relatively difficult to ship
is likely to further reduce the achievable price as more interested
parties are less likely to be located nearby.

If it's sitting in a recyclers with a $100 price tag and not moving,
the answer to the original question is probably "less that $100",
however, this information was not provided with the original question.
Somebody some distance away might be willing to spring for more than
$100 for it but may find the cost and effort of getting it shipped to
be prohibitive.

My Alphaserver 2100 is probably worth nothing.  I am pretty certain it
is not located close to anyone else who would be interested in it.  If
I wanted to get rid of it, I would probably have to pay someone to take
it away or bring it to the recycling depot myself and cross my fingers
that it would be accepted as domestic electronic equipment.  On the
other hand, if someone approached me looking to buy it from me, they
would probably have to put at least four or maybe five figures in front
of the decimal point before I would be motivated enough to think about
letting it go.  That's pretty widely varying.

Sorry if it seems unhelpful but I find questions about the value of
items that there is only a tiny market for to be pretty much impossible
to answer.

Regards,
Peter Coghlan.

Alan Perry via cctalk wrote:
I wouldn't expect commercial value to come into the discussion on this 
list. I am wondering what other hobbyists pay in order to gauge whether 
the price that a local recycler is asking for one (which was around 
$100) is fair.


On 4/15/22 3:30 AM, Peter Coghlan via cctalk wrote:
It's worth what someone is willing to pay for it and this will vary 
widely.


I got one for EUR 20 in 2006.  I didn't think this was a bargain because
I was under the impression I was getting it for free before I travelled
to pick it up.  The previous owner had no further use for it and needed
the space it was taking up.

I think it's a nice machine although an Alphaserver 2100A would be nicer.
It looks and sounds like a real computer except the front panel is a bit
pathetic.  It is great for heating the room in winter.  However, it 
doesn't

seem to have any commercial value.

Regards,
Peter Coghlan.



What is an AlphaServer 2100 worth? There was one at RePC in Seattle a 
couple weeks ago and I was thinking about purchasing it.


On 4/14/22 2:36 PM, Antonio Carlini via cctalk wrote:

On 12/04/2022 16:34, Dave Wade G4UGM via cctalk wrote:

Folks,
Does no one fancy a go at this. Had zero interest...
Dave


(OFFLIST, I think)

I'm assuming the machine is safe, at least for the moment?

I've actually got less room now than when I had to let it go, so 
hopefully it can just sit in a corner and be a useful table end or 
something for a while?


I'm currently struggling with a uVAX 3600 PSu and a VAX 4000 PSU, so 
if I ever fix those, maybe I can help with the AS2100 ...



Antonio

Re: Advice on Desoldering an IC

[Peter Coghlan via cctalk]

I had to desolder a fairly large number of 14/16 pin ICs from a two layer
PCB recently.  I had an basic soldering iron and a spring loaded solder
sucker thing.  I also had some wick but never having had good results with
this, I decided not to bother trying it this time.

I was aware of the idea of cutting the pins to make removal easier but I
preferred to keep the ICs intact in case they turned out not to be faulty.

My first attempt was pretty bad and did quite a bit of damage.  By the
tenth time around, I was making a much nicer job of it so what worked
for me was practice.

I ended up propping the PCB up vertically, placing the sucker squarely over
the joint on the solder side, heating the IC pin on the component side with
the soldering iron and releasing the plunger in the sucker after allowing the
solder a little time to melt.  Sometimes this cleared all the solder out of
the hole but usually it did not.  I found it useful to get the tip of the
solder sucker to seal well around the joint with little or no gaps for air to
get through.  I kept going around the pins until I could see through most of
the holes when holding the board up to the light.  At this point many of the
pins were still stuck to the sides of the holes so I then applied the
soldering iron to each pin in turn on the solder side, pushing the pin gently
away from the side of the hole.  When the solder melted, the pin would then
move to the centre of the hole and sometimes come free entirely.  Then I would
go back to the first procedure with the solder sucker to clear more solder out
of the holes.  Using no more than finger pressure, I would try to pull the IC
out while dabbing the soldering iron on any joints that seemed to be still
holding it in place.  After a several interations, the IC would pop out, maybe
at one end only first.  A bit tedious and not at all quick but it worked well
enough in the end.

Unfortunatly, the next time I have to do some desoldering, I will have
probably forgotten what worked well this time and have to learn it all over
again.  I will probably also forget that it would be useful to get some
practice in first...

Regards,
Peter Coghlan.

Rob Jarratt via cctalk wrote:
> I am trying to remove an IC from my PDP 11/24 CPU, a DS8641. I am really
> struggling to desolder it. I am using the technique of applying fresh solder
> and then removing it. But after multiple cycles of this I think I am
> starting to damage the PCB.
> 
>  
> 
> I am using a fairly cheap desoldering station (this one
> https://cpc.farnell.com/duratool/d00672/desoldering-station-uk-eu-plug/dp/SD
> 01384?st=duratool%20desoldering). Its spec in terms of vacuum pressure is
> equivalent to that of the professional Hakko ones though. I am also trying a
> hand desoldering pump. None of these are able to clear many of the holes of
> solder, although some are doing better than others. Nevertheless, the IC
> remains stubbornly unmoving.
> 
>  
> 
> Are there any tips for removing ICs?
> 
>  
> 
> Thanks
> 
>  
> 
> Rob
> 

H7816-BA power supply (Was Re: H7821 power supply in MicroVAX 3100, SCSI disk enclosures and others)

[Peter Coghlan via cctalk]

A few months ago, I mentioned a faulty H7816-BA power supply in a rackmount
DEC 3000/600 Alphaserver.  I finally got around to looking at it again
recently.  I hoped I had an identical working power supply to compare it to
but I didn't.  It turns out what I had is two working H7816-AA power supplies
in tabletop models of DEC 3000/600 Alphaservers.  These are very similar but
not identical to the H7816-BA.  The case layouts are different and the -AA
has four integral fans while the -BA has a extra board mounted under the lid
to power a large external fan instead.  There may be differences on the main
circuit boards, or there may not be.

The faulty H7816-BA was emitting an audiable click just after power on as
if it was trying to start and tripping out.  If I put a meter on the +3.3V
(orange), +5V (red) or +12V (brown) outputs, there was a very brief voltage
kick visible coincident with the click.  However, there didn't seem to be any
activity visible on the -12V (blue) output.  This was the first good clue to
where the fault lay.

With them switched off, I tried comparing resistance readings between the
-12V outputs of the -AA and -BA power supplies and 0V (in both directions,
in both diode mode and non-diode mode).  There were visible differences.
However,  when I did the same for the +3.3V, +5V and +12V outputs, the
results were pretty much identical between the -AA and the -BA.

So, I looked where the blue wire is soldered to the main PCB.  It is very
difficult to trace where it comes from but there is a 7912 voltage regulator
IC (E10) mounted on a heatsink close by and continuity tests show that this
is it's source.  Further continuity tests show that the other two terminals
of E10 seemed to be shorted together.  Aha! This is it I thought.  It wasn't.

After I unsoldered the 7912, the short was still on the board.  Suspicion
then fell on a 1000uF capacitor C45 connected across the same two points.
After unsoldering this, it was found to be innocent too.  Following the PCB
traces further back turned out to be nearly impossible put poking around
randomly with the continuity tester brought me to D21 (FEP16DT), a double
rectifer in a TO-220 case mounted on a heatsink.  This item seemed to have
all three terminals shorted.  After it was unsoldered, I found it to be
shorted on one side only, the other side giving reasonable readings for
a diode.

After refitting E10 and C45 and throwing everything back together (because
it is pretty much impossible to operate the power supply without it being
fully assembled) I found that it didn't click at power on any more and it
now stayed operating long enough to light my headlamp bulb dummy load on
the +5V output for a few seconds before cutting out, presumably when the
control circuitry noticed the lack of the -12V output due to D21 being
missing.  The green LED didn't come on.

I didn't have anything like an FEP16DT to hand.  I tried looking in a H7821
which I have already been scavanging parts from and all I could see was a
UTG5346 (I think) which might be similar but I couldn't find any information
about it anywhere so I decided not to risk trying it and breaking it.
Instead, I tried refitting the faulty FEP16DT with the lead going to the
shorted side bent up out of the way.  I thought that maybe if it was being
fed from a centre tapped winding on the chopper transformer, it might work
with only one rectifier provided the loading was not heavy.  On test, there
was some negative voltage on the -12V output but the power supply still cut
out after a few seconds and the green LED still didn't come on.  On further
reflection, the double rectifier is not being fed from a centre tapped
winding because I can't find the centre tap anywhere on the transformer
so having both rectifiers present is probably essential after all, depending
on how the circuit is supposed to work, which I can't quite figure out.

I had some used MR760 high current fast recovery rectifiers left over after
repairing a battery charger so I decided to try bodging in one of them in
place of the faulty half of the FEP16DT.  I guess the worst that would
happen is the power supply would trip out.  However, this time the power
supply continued working, my dummy load lamp glowed gently and the green
LED came on.  Yay!  I even got 5V on the white wire which I am guessing
by a process of elimination is probably the "power good" output.

Looks like I need to put an FEP16DT on my next component order.

Regards,
Peter Coghlan.


Previously I wrote:
> Flushed with success, I moved on to look at a H7816 from a DEC 3000/600
> Alphaserver.  This one shows little or no signs of life except for a barely
> audible click at switch on.  The fuse is not blown and there are no obvious
> signs of distress anywhere.
> 
> This one is a real pig to work on.  Firstly, the incoming earth connection is
> made to the lid of the power supply so when the lid is removed to work on it,
> there is no longer any earth connection to the re

DEC H7822 power supply

[Peter Coghlan via cctalk]

I'm having a lot of problems with DEC power supplies.  Newly failed ones
are getting added to the end of the queue quicker than I am managing to
pop fixed ones off the top of the queue :-(

The latest one to give trouble is a H7822 in a MicroVAX 3100.  This machine
apparantly never got deployed for the purpose it was purchased for so it
has had very little use and it is clean and pretty much dust free inside,
including in the power supply.  The two SCSI buses only worked intermittently
when I got the machine first many years ago.  Now, they seem to have failed
completely.  A single common fault seems to be responsible for both failures
but I haven't been able to find it so I use it by accessing disks across the
network instead which is a pity because it has lots of room inside for disks.

A couple of years ago, the two fans in the H7822 (which are the only fans
in the machine) stopped working leading to overheating and shutdown due
overtemperature if it was left on for too long.  This turned out to be due
to a faulty 12V zener diode in the fan control circuit.  I replaced the
zener diode and that seemed to fix this particular problem.

More recently, the machine was working away nicely when it suddenly stopped.
Powering it off and back on later didn't help.  Months later when I finally
got around to looking at it, it still failed to do anything noticable when
powered on.  I took out the power supply, opened it up, checked for blown
fuses or anything else obvious, found nothing, put it back together, applied
power (without a load) and the fans started spinning indicating it now seemed
to be working.  Thinking there might be a short on the system board, I put it
back into the MicroVAX, connected it up and powered it on.  The fans spun,
all the diagnostic LEDs lit and stayed on but the green LED on the power supply
did not light.  The machine appeared to be held in reset.  There's probably
a voltage missing from the power supply I thought.  While I was getting the
meter to check this theory, the green LED came on, the diagnostic LEDs began
counting down and the output of the POST appeared on the console terminal.

While I was making notes so that I didn't forget what happened, I noticed
the fans stopping making noise, the green LED and diagnostic LEDs had all
gone out and the console was no longer responsive.  It had failed again.
I put the meter on the front disk drive connector to check the voltages
from the power supply.  It showed +5V and +12V as if everything was normal.
I put it on the rear disk drive connector and both lines were varying around
one or two volts.  I plugged in an AUI tranceiver which happened to be to
hand and the LEDs on it varied with the meter readings.  I tried connecting a
test lamp to the +5V and +12V lines on the front disk drive connector to load
it a bit.  They held steady while the LEDs on the AUI tranceiver continued to
vary.  It seems there are at least two independent regulation systems in this
power supply.

I left things as they were and went to make more notes.  A few minutes
later, I heard the fans starting up again.  I looked at the machine and the
green LED on the power supply was on, the diagnostic LEDs were counting down,
output was starting to appear on the console and the LEDs on the AUI
tranceiver were lighting normally.

I've tried power cycling it several times since and it doesn't seem to want
to fail now.

Access to the H7822 is not great.  There are two boards (each with permanently
connected daughter boards), one mounted in the base and one mounted in the lid.
The boards are connected together at one end by about twenty flexible wires
and at the other end by two two core cables which can be unplugged to allow
the two boards to be laid out flat end to end.  I suppose I could make up
extensions to the two two core cables to enable better access for testing.
However, it doesn't look easy to devise a layout where everything can be
cooled by the fans for testing over an extended period of time.  At least
the H7822 does not seem to suffer from the leaky capacitor problem like the
H7821 does.

I have another MicroVAX 3100 which looks like it should have a H7822 in it
too but I'm not sure how this could be helpful given the symptoms.

Any suggestions on how to further diagnose this nightmare?

Regards,
Peter Coghlan.

Re: DEC H7822 power supply

[Peter Coghlan via cctalk]

I wrote:
>
> At least the H7822 does not seem to suffer from the leaky capacitor problem
> like the H7821 does.
>

Well, it's made a liar of me already.  While checking over my previous work
replacing the zener diode to make sure I didn't break something else, my
eye was drawn to a ring of sticky brown goo around the base of one of
the capacitors that looks the same as the ones that leak in the H7821.
Thankfully, it doesn't seem to have been there long and didn't get a chance
to spread around the board.  Bizarrely this capacitor has it's legs pointing
upwards and managed to leak while there are similar capacitors on the other
board with their legs pointing downwards which don't seem to have leaked (yet?).

When I compare them side by side, I can now see that the upper board in
the H7822 is identical to the single board in the H7821.  It seems that
the lower board in the H7822 is only responsible for powering the front
disk drive connector which I found to be independent of the other supplies.

Regards,
Peter Coghlan.

Re: DEC H7822 power supply

[Peter Coghlan via cctalk]

"Maciej W. Rozycki" wrote:
>On Mon, 2 May 2022, Peter Coghlan via cctalk wrote:
>
>> Thankfully, it doesn't seem to have been there long and didn't get a chance
>> to spread around the board.  Bizarrely this capacitor has it's legs pointing
>> upwards and managed to leak while there are similar capacitors on the other
>> board with their legs pointing downwards which don't seem to have leaked 
>> (yet?).
>
> Would the system have been possibly stored upside down sometime?
>

I don't think so.  It may have spent some time lying on it's side due to
deteriorating rubber feet and for ease of access and but I can't see any
reason for it ever being upside down.  Once I discovered this issue a few
years ago, I checked all my power supplies, removed any leaking capacitors
and changed to storing the machines the right way up in a vertical stack,
with newspapers between them in place of the rubber feet.  I hoped this would
prevent any capacitors which hadn't shown any signs of leaking from starting
to leak.  (Now whichever machine I want to work on always seems to be at the
bottom of the stack...)

>
>  These nasty caps do leak even while in storage (and even if never used, 
> not even soldered ever, according to one source, a repair professional).
>

They are nasty and devious.  In my case, the ones that have been stored in
any orientation but not used much seem to have fared better.  I only started
using this machine with the H7822 for extended periods for the first time a
few months ago.  One of the reasons I started using it more is because I
thought it was immune to the leaky capacitor problem!  It never saw any
serious use before that, even when it was new.

I have at least two machines with H7822 power supplies.  Even though they
have capacitors that look the same as the ones in the H7821, the ones in
the H7822 power supplies didn't seem to be showing any signs of leaking
when I examined them some time ago so I thought they might be from a batch
that was unaffected by the problem.  It seems that this was not true :-(

I unsoldered the other eight similar capacitors (four one each board) from
the H7822 yesterday evening.  I found a small amount of leakage under most
of them but it was was only evident after they were removed from the board.
In general, there was less damage visible under the ones on the lower board
with leads facing down oddly enough.  I thought one of the capacitors from
the upper board had not leaked at all.  I left the removed capacitors
standing on the bench overnight with their leads upwards and they all have
some signs of leakage visible on them today.  It's hard to draw any
conclusions.

The capacitor from the upper board that leaked enough for me to notice it
might be under greater stress when operating than the others.  I think the
same capacitor in the H7821 power supplies seems to leak more in those too.
Maybe it carries greater ripple current than the others or something like
that?  I can see that the other four similar capacitors make up PI filters
for the +5V and +12V outputs along with a pair of smoothing chokes, however
I can't quite figure out what the more leaky one is responsible for.  It
could be associated with the -12V output which might also be the supply
for the fans.  This part of the circuit is very hard to trace.

I don't have enough spare capacitors to replace the four on the lower
board.  I am going to leave those out, leave the input lead to that board
unplugged and plug the green LED into the upper board.  I hope it will then
behave just like a H7821.  There won't be any power to the front disk drive
connector but I am not using that so it doesn't matter unless the "power good"
output is affected, in which case I will have to think of something else.

I am hoping that the leaky capacitors were responsible for the odd
intermittent failures I experienced with this power supply recently.

Regards,
Peter Coghlan.

>  Maciej

Re: DEC H7822 power supply

[Peter Coghlan via cctalk]

"Maciej W. Rozycki" wrote:
>On Mon, 2 May 2022, Peter Coghlan via cctalk wrote:
>
>> Thankfully, it doesn't seem to have been there long and didn't get a chance
>> to spread around the board.  Bizarrely this capacitor has it's legs pointing
>> upwards and managed to leak while there are similar capacitors on the other
>> board with their legs pointing downwards which don't seem to have leaked 
>> (yet?).
>
> Would the system have been possibly stored upside down sometime?
>

I don't think so.  It may have spent some time lying on it's side due to
deteriorating rubber feet and for ease of access and but I can't see any
reason for it ever being upside down.  Once I discovered this issue a few
years ago, I checked all my power supplies, removed any leaking capacitors
and changed to storing the machines the right way up in a vertical stack,
with newspapers between them in place of the rubber feet.  I hoped this would
prevent any capacitors which hadn't shown any signs of leaking from starting
to leak.  (Now whichever machine I want to use always seems to be at the
bottom of the stack...)

>
>  These nasty caps do leak even while in storage (and even if never used, 
> not even soldered ever, according to one source, a repair professional).
>

In my case, the ones that have been stored in any orientation but not used
much seem to have fared better.  I only started using this machine with the
H7822 for extended periods for the first time a few months ago.  One of the
reasons I started using it more is because I thought it was immune to the
leaky capacitor problem!  It never saw any serious use before that, even
when it was new.

I have at least two machines with H7822 power supplies.  Even though they
have capacitors that look the same as the ones in the H7821, the ones in
the H7822 power supplies didn't seem to be showing any signs of leaking
when I examined them some time ago so I thought they might be from a batch
that was unaffected by the problem.  It seems that this was not the case :-(

I unsoldered the eight other similar capacitors (four one each board) from
the H7822 yesterday evening.  I found a small amount of leakage under most
of them but it was was only evident after they were removed from the board.
In general, there was less damage visible under the ones on the lower board
with leads facing down oddly enough.  I thought one of the capacitors from
the upper board had not leaked at all but I left them standing on the bench
overnight with their leads upwards and they all have some signs of leakage
visible on them today.

The capacitor from the upper board that leaked enough for me to notice it
might be under greater stress when operating than the others.  I think the
same capacitor in the H7821 power supplies seems to leak more in those too.
Maybe it carries greater ripple current than the others or something like
that?  I can see that the other four similar capacitors make up PI filters
for the +5V and +12V outputs along with a pair of smoothing chokes, however
I can't quite figure out what the more leaky one is responsible for.  It
could be associated with the -12V output which might also be the supply
for the fans.  This part of the circuit is very hard to trace.

I don't have enough spare capacitors to replace the four on the lower
board.  I am going to leave those out, leave the input lead to that board
unplugged and plug the green LED into the upper board.  I hope it will then
behave just like a H7821.  There won't be any power to the front disk drive
connector but I am not using that so it doesn't matter unless the "power good"
output is affected, in which case I will have to think of something else.

I am hoping that the leaky capacitors were responsible for the odd
intermittent failures I experienced with this power supply recently.

Regards,
Peter Coghlan.

>  Maciej

Re: DEC H7822 power supply

[Peter Coghlan via cctalk]

Hi Maciej,

Thanks for your input.

"Maciej W. Rozycki" wrote:
>
> Lying on a side would also permit leaking, I've seen an H7821 damaged in 
> storage that way.  Gravity only helps with the leads up.
>

I have lots of H7821s that were on their side for a few years and leaked a
lot but I also have H7822s that were stored the same way and did not show
any evidence of leaking until very recently.  I think I also have some
H7821s that were the right way up most of their lives and have leaked a
little.  It's getting hard to remember what happened to which :-(

>
> From experience Chemi-con SXF caps used with many DEC PSUs need to be 
> urgently replaced.  Other Chemi-con lines reported affected are LXF, SXE 
> and KME.  Products of the time from other manufacturers may be affected as 
> well.  I'd have to check what line were those that leaked in a Bel Power 
> PSU that I had to fix (I reckon you had a similar experience, right?).
>

I still have the leaky electrolytics I removed from the POWER-ONE PSU in my
Cisco IGS a while back.  I stored them with their leads up and goo seems to
be still oozing out of some of them despite their inactivity and orientation.
These ones are marked Nichicon PL(M) 4700uF/63V, 2200uF/16V and 330uF/35V and
also have markings like H8950, H9018 and H8946 - maybe these are date codes?
I also removed the smaller ones like 47uF/35V PF(M) H8952 for example but it
is less clear to me whether these were leaking too or just got leaked on by
the others.  There were only a few of them so I decided they were better out
than in.  They all have similar coloured brown sleeves like the faulty ones
in the DEC power supplies too.

There are also the leaky 10uF/35V axial electrolytics in my LK201 keyboard.
Those are in orange sleeves and marked "ESZ", whatever that is.  They have
date codes like 8612.  I thought this might be a widespread problem but
so far I have only found it in one keyboard.

>
> It was the composition of the electrolyte that was outright wrong, so I 
> doubt it's batch-related.
>

>
> Once the seal has broken I guess all odds are off.  I could imagine 
> capillary action to take effect.
>

Looks like I need to go back and recheck everything I thought wasn't leaking
last time I checked :-(

Thing is, to check them, they have to come out of the case and that involves
at least some change in orientation, except for contortionists...

>
> Heat dissipated by the cap itself under high ripple current never helps 
> and will surely speed up cap deterioration.  After all its service life 
> halvens with each 10°C temperature rise even with non-faulty parts.
>

When I was shopping for replacements, I was a bit alarmed to find that
the maximum specified "endurance" (whatever that is) I could find was
5000 hours.  This isn't much more than a long life incandescent light bulb.

>
> As a matter of interest what capacitance/voltage are those?  Are they of 
> the Chemi-con/SXF type too?
>

All the leaking ones in DEC H7821 and H7822 PSUs I have come across so far
are 1800uF/25V Chemicon with brown sleeves.  I think mine are all SXF
but I am not 100% sure of that.  There are lots of other electrolytic
capacitors in these power supplies but I've only looked closely at the
larger ones.  All of the reservoir capacitors attached to the mains bridge
rectifiers that I have seen look fine.  Maybe I need to go back and check
the smaller ones though :-(

I think I came across some LXF ones that seemed to be ok, I can't remember
where though.  I probably need to go find these and check them again :-(

Here's a thought.  Apart from the keyboard, all the ones I have seen that
are leaking are filtering the outputs of switch mode power supplies.  I
wonder does the higher frequency of the ripple they are dealing with have
a bearing on this?


I replaced the five leaking capacitors on the upper board in my H7822,
disconnected the input to the lower board and moved the LED connection to
the upper board.  The machine powered up nicely afterwards, the fans spun
and the green LED came on after a short delay.  However, the diagnostic
LEDs all come on and stay on so it appears the CPU is being made aware that
the lower board is not functioning, even though I don't need it.  I should
to do some comparisons with a H7821 and see if I can work around this.  Or
maybe I could fit some not quite to specification electrolytics from the
junk box on the lower board, just to keep it happy?

Regards,
Peter Coghlan.

>
>  Maciej
>

Re: DEC H7822 power supply

[Peter Coghlan via cctalk]

Hi Maciej,

>
> Right, my notes indicate Nichicon PL parts might be problematic too, e.g. 
> one at 4700uF/10V on 5V output of the H7826 PSU.
>

What is H7826 used in?  I don't think I have any of those.

>
> I can confirm now Nichicon PF 47uF/35V parts to be the source of an issue 
> with my Bel Power.  All four leaked.  Thankfully I was able to fully 
> revive that PSU (now in 24/7 operation).
>

I got replacement capacitors for my POWER-ONE unit but I haven't managed
to obtain a replacement IRFPE50 chopper transistor yet.  I found a sort-of
local supplier that lists them but they have a EUR 30 minimum order and
they don't stock much other stuff I need or want :-(

>
> These are likely standard type parts.  I've yet to come across a low 
> impedance axial capacitor type.  I may have missed something of course.
>

I couldn't find any suitable axial replacements so I ended up using radials
which were roughly the same height as the width of the axials.

>
> >
> > > Heat dissipated by the cap itself under high ripple current never helps 
> > > and will surely speed up cap deterioration.  After all its service life 
> > > halvens with each 10°C temperature rise even with non-faulty parts.
> > >
> > 
> > When I was shopping for replacements, I was a bit alarmed to find that
> > the maximum specified "endurance" (whatever that is) I could find was
> > 5000 hours.  This isn't much more than a long life incandescent light bulb.
> 
>  Mind that it's at 105°C.  If you keep such caps operating at 65°C (which 
> is still rather hot), then endurance raises to 8h (~9 years continuous 
> use).
> 
>  Anyway try to chase replacements specified for at least 1h at 105°C.
> Nichicon HE/UHE and Panasonic FR seem suitable replacements for Chemi-con 
> SXF, Nichicon PL, surpassing old parts in terms of ESR/impedance/ripple 
> and dimension-wise.
>

When I was trying to order them, I found it hard to get anything suitable.
I ended up ordering 15 (out of 16 in stock at Mouser) Panasonic FC series
to do just three PSUs.

>
> I don't have any H7822 PSU.  Your experience with the H7821 is the same 
> as mine though (and I still need to figure out what's wrong with one which 
> still doesn't drive its power-good line active after recapping).
>

There is a 464 or 560 Ohm resistor near the capacitor in middle of the H7821
which has been damaged by leakage in several of my PSUs.  It is in series
with a thermistor mounted on a heatsink and as far as I recall it is part
of the fan control circuit.  It might also contribute to power-good though.
Other components could be damaged too or there could be goo lying under
some components.

>
> I used 
> Nichicon HE P/N UHE1E182MHD as the replacement for those.  There does 
> appear to be COVID-related shortage of this part (600 expected at Mouser 
> 15/03/2023, ugh!), which used to be readily available in large quantities 
> several years ago.  However Panasonic FR P/N EEUFR1E182 is available in a 
> small quantity (and is better).

I should grab some EEUFR1E182 before they disappear.

>
> I came across LXF parts in one H7826 only and they were clean, but I 
> chose to replace them as a precaution anyway as I've got stuck with trying 
> to repair a couple of broken H7826 PSUs already still not working after 
> cleaning the mess and replacing broken caps (mind that I'm a software 
> engineer with enough hassle to sort out on the software side already).
>

Sounds like it was better to not take a chance on them.

>
> I've seen leaks from SXF parts on the primary side too with the H7826, so 
> it is not that they only fail on the secondary side.  Also it is lower 
> frequency ripple that's more problematic, see frequency correction factors 
> for ripple current in datasheets, because impedance is higher at lower 
> frequencies.  That's an inherent property of capacitance.
>

That makes sense.

>
> > I replaced the five leaking capacitors on the upper board in my H7822,
> > disconnected the input to the lower board and moved the LED connection to
> > the upper board.  The machine powered up nicely afterwards, the fans spun
> > and the green LED came on after a short delay.  However, the diagnostic
> > LEDs all come on and stay on so it appears the CPU is being made aware that
> > the lower board is not functioning, even though I don't need it.  I should
> > to do some comparisons with a H7821 and see if I can work around this.  Or
> > maybe I could fit some not quite to specification electrolytics from the
> > junk box on the lower board, just to keep it happy?
> 
>  The symptom is exactly like with my broken H7821.  Check the power-good 
> signal (brown wire with the H7821, possibly likewise with the H7822).  It 
> should be driven high at the TTL level.
>

The wire in the same position as the brown one on a H7821 is purple on a H7822
and they both seem to have the same function.  I ended up temporarily fitting
a two 1000uF/25V capacitors on the lower board of my H7822 and reconnecting it
to

Re: DEC H7822 power supply

[Peter Coghlan via cctalk]

Hi Maciej,

>
> > 
> > What is H7826 used in?  I don't think I have any of those.
>
> DECstation 5000/1xx systems and TURBOchannel Extender boxes.  Not sure if 
> anything else.
>

Ok.  I have VAXStation/MicroVAX and Alpha systems but no DECStations so that
explains why I haven't seen them.

>
> > Do you know what the white wire is for?  I originally thought it might
> > be power-good but only because the white wire on a H7816 seems to be
> > power-good.  On the 30-35042-01, it is slightly above 0V.  On my H7822
> > it is about 5.6V and on my H7821 it is 8.8V.
>
>  In the H7821 it's -9Vdc return pairing with -9Vdc supplied on the yellow 
> wire (an isolated circuit).  Pin numbers 14 & 13.  Try measuring voltage 
> across the suspicious connections as any reference to ground may not be 
> indicative.
>

Aha - that explains the odd and slowly varying different voltages I've
found on these leads in different machines.  It didn't dawn on me that
they could be isolated from ground.  I should have noticed that there
were two adjacent leads which behaved like this.

This should help clarify some of the puzzles I came across when I was
trying to trace the circuit of the H7821 and H7822.

Regards,
Peter.

>
>  This voltage is also present with the H7819 PSU.  It's not clear to me 
> what it is used for.
>
>  Maciej

Re: DEC H7822 power supply

[Peter Coghlan via cctalk]

Tony Duell wrote:
> On Sun, May 8, 2022 at 1:02 PM Maciej W. Rozycki via cctalk
>  wrote:
>
> >  In the H7821 it's -9Vdc return pairing with -9Vdc supplied on the yellow
> > wire (an isolated circuit).  Pin numbers 14 & 13.  Try measuring voltage
> > across the suspicious connections as any reference to ground may not be
> > indicative.
> >
> >  This voltage is also present with the H7819 PSU.  It's not clear to me
> > what it is used for.
> 
> It's only a guess (this is rather more modern than the DEC hardware I
> have), but it may be a supply for a thinwire ethernet transceiver
> circuit. This would be insolated from system ground of course, and
> I've seen something similar in an X-terminal.
>

Hi Tony,

After years of rediscovering the same failures over and over again,
I finally managed to get in the habit of making notes while I am
trying to diagnose faults.  Now I just need to get in the habit of
reading my old notes when I start to work on a new fault.  I did
that just now and I found that I have a MicroVAX 3100 model 76 whose
internal thinwire tranceiver failed last summer.  Not finding any
obvious reason for the failure, I replaced it with an external
tranceiver plugged into the AUI port and then forgot all about it.

I should check that this floating 9V output from it's H7821 PSU is
working correctly.

(I wonder why it is described as -9V on the label of the 30-35042-01 PSU
in my VAX 4000/100A?  If it had been shown as +/-9V for example or even
just plain old 9V, I would have been less inclined to assume it was
referenced to ground.)

Regards,
Peter.

>
> -tony
> 
> 
> 
> >
> >   Maciej

Re: DEC H7822 power supply

[Peter Coghlan via cctalk]

Peter Coghlan wrote:
> Tony Duell wrote:
> > On Sun, May 8, 2022 at 1:02 PM Maciej W. Rozycki via cctalk
> >  wrote:
> >
> > >  In the H7821 it's -9Vdc return pairing with -9Vdc supplied on the yellow
> > > wire (an isolated circuit).  Pin numbers 14 & 13.  Try measuring voltage
> > > across the suspicious connections as any reference to ground may not be
> > > indicative.
> > >
> > >  This voltage is also present with the H7819 PSU.  It's not clear to me
> > > what it is used for.
> > 
> > It's only a guess (this is rather more modern than the DEC hardware I
> > have), but it may be a supply for a thinwire ethernet transceiver
> > circuit. This would be insolated from system ground of course, and
> > I've seen something similar in an X-terminal.
> >
> 
> Hi Tony,
> 
> After years of rediscovering the same failures over and over again,
> I finally managed to get in the habit of making notes while I am
> trying to diagnose faults.  Now I just need to get in the habit of
> reading my old notes when I start to work on a new fault.  I did
> that just now and I found that I have a MicroVAX 3100 model 76 whose
> internal thinwire tranceiver failed last summer.  Not finding any
> obvious reason for the failure, I replaced it with an external
> tranceiver plugged into the AUI port and then forgot all about it.
> 
> I should check that this floating 9V output from it's H7821 PSU is
> working correctly.
>

I checked it.  It showed 4.4V and also seemed to be running rather
hotter than the other H7821 PSUs I've been working with.  I confirmed
that the internal thinwire tranceiver is still not working.  I swapped
out the H7821 in favour of a known working H7821 which gave me 8.9V
and the internal tranceiver then worked again.

I put the faulty H7821 into the system that the good H7821 came from
and confirmed that the internal tranceiver in that system now didn't
work and the 9V supply was around 5V.  With the different layout in
this system, I was able to see that the rear fan in the power supply
was not rotating which could account for it's higher temperature.
On the offchance that this might also account for the low 9V supply,
I tried unplugging the seized fan but this didn't make any difference.

The 9V supply comes from a 78M09 which I hoped would be faulty because
I can't make much sense of the circuit that feeds it.  I pulled the same
component from another older H7821 and it turned out to be a 7809 which
I tried fitting in place of the 78M09 but the symptoms remained the same.
Ahead of the 78M09 is another TO220 package marked D45H8 which seems to
be a transistor.  Then I am completely lost.  I can't find the rectifier
or the winding on the chopper transformer that is supposed to produce power
for this part of the circuit.  Making comparison resistance tests between
the faulty H7821 and another one showed up some discrepancies around the
78M09 / 7809 and the D45H8 however the former were probably because of
the differences between a 78M09 and a 7809 and in the latter case, the
D45H8 looked reasonable when tested out of circuit.

The faulty H7821 has seen orders of magnitude more usage than any of
my others but it still has it's original SXF electrolytics which are
not showing any signs of leakage yet that I can see without unsoldering
them.  It's been the right way up pretty much forever.

Thanks Maciej and Tony for leading me to the cause of this issue.
Another mystery (almost) solved.

Regards,
Peter.

> 
> (I wonder why it is described as -9V on the label of the 30-35042-01 PSU
> in my VAX 4000/100A?  If it had been shown as +/-9V for example or even
> just plain old 9V, I would have been less inclined to assume it was
> referenced to ground.)
> 
> Regards,
> Peter.
> 
> >
> > -tony
> > 
> > 
> > 
> > >
> > >   Maciej

Re: DEC H7822 power supply

[Peter Coghlan via cctalk]

Christian Corti via cctalk wrote:

On Tue, 10 May 2022, Peter Coghlan wrote:
> Ahead of the 78M09 is another TO220 package marked D45H8 which seems to
> be a transistor.  Then I am completely lost.  I can't find the rectifier

That TO220 part would be the rectifier. It either has two legs (simple 
diode) or three (two-wave rectifier with presumably common cathode).




I thought that too at first.  However, it is definately a transistor.

I've made a bit more progress figuring out the circuit.  There is a
large torroid with three windings on it on the output side.  I thought
it was some sort of triple wound smoothing choke for the 5V, 12V and 9V
supplies but it is not.  It turns out that this is actually the source of
the 9V supply.  It is effectively a current transformer with two primaries.
The ripple on the rectified 5V and 12V supplies gets transformed into an
isolated AC source for the 9V supply.  This AC goes to a simple rectifier
which looks like a 1N4001 but isn't, a small choke and a smoothing capacitor.
The resulting DC goes to the emitter of the D45H8 whose collector feeds the
input of the 78(M)09.  I haven't figured out how the base of the D45H8 is
controlled yet.

Regards,
Peter.



Christian
   

Re: Cctalk subscription disabled

[Peter Coghlan via cctalk]

I recall discussion of this issue on the mailing list a number of
times in the past, often involving more heat than light.  I'd suggest
searching the mailing list archives if this were possible.  I had a
quick look just now but I was only able to find this:

http://www.classiccmp.org/pipermail/cctalk/2021-May/058836.html

I'm pretty sure I know why it's happening but I'm also pretty
sure that if I try to explain, this will result in another long
fruitless argument about it.  I'll just say that I agree that
gmail is causing it in this case and leave it at that.

Regards,
Peter Coghlan.

Tom Hunter via cctalk wrote:
>
> Gmail is by far the biggest email provider and works fine with other
> Mailman based mailing lists.
> How exactly is Gmail causing it and what is the work-around?
> 
> Thanks
> Tom
> 
> On Wed, May 11, 2022 at 2:26 PM Adrian Stoness  wrote:
> 
>> gmail is causing it
>>
>> On Wed, May 11, 2022 at 1:15 AM Tom Hunter via cctalk <
>> cctalk@classiccmp.org> wrote:
>>
>>> I didn't receive any cctalk messages for a few days, so I checked my Spam
>>> folder (empty) and then the archive which had several new messages I
>>> haven't seen. I then checked my cctalk membership configuration.
>>> Surprisingly "Mail delivery" was disabled.
>>>
>>> Some time ago I received a few emails from the list server about
>>> "excessive
>>> bounces" saying that I should simply reply to the email otherwise my
>>> subscription would be disabled. I replied and didn't think too much about
>>> it. This time I got no "excessive bounces" email but my subscription got
>>> disabled.
>>>
>>> I got an otherwise reliable @gmail.com email address. Does anyone else
>>> here
>>> have problems with this? Is there some way of preventing this from
>>> happening?
>>>
>>> Thanks
>>> Tom
>>>
>>

Re: DEC H7822 power supply

[Peter Coghlan via cctalk]

It's hard to come up with suitable terms for this sort of stuff that
convey the meaning of what is going on.  I struggled with it for a while
and that was what I ended up with.

With the strict meaning of DC and AC being direct current and alternating
current and given that we are often talking about voltages rather than
currents, meanings are already getting stretched.  Besides, what's direct
about it?  Maybe it would be more accurate to use terms like steady voltage
and alternating voltage? Alternating doesn't seem like that good a term
either.  To me it suggests some sort of square wave switching very rapidly
between one extreme and another, not a nice lazy sine wave which is the
normally accepted meaning.

Given the normal usage that has evolved for the terms DC and AC rather than
their dictionary definitions, I would suggest that the current that gets
passed by a rectifier has both a DC component and an AC component.  When
this mixture is fed into a transformer primary as in this case, the DC
component does not pass through from the primary to the secondary but the
AC component does.  Transformers only work on AC, right?

Or maybe it could equally be said that a transformer can be used to convert
pulsed DC to AC?

(The transformer has to be kinda special to avoid the core getting
saturated by the large DC current (What's a "direct current current"?)
flowing through the primaries...)

Anyway, the good news is that I think I have found the source of the
problem.  One of the capacitors used to filter the (DC?, pulsed DC?,
rippled DC?, biased AC?) supply to the 9V regulator is marked 330uF/25V.
It reads 6uF on the capacitance range on my multimeter.  This can't be
helping the cause.  It's not showing any signs of leakage but it's got
a brown sleeve and the same logo as the nasty, leaky SXF capacitors but
it is marked KME.  (I said there was only one capacitor in the filter
in a previous posting.  Originally I managed to spot a little 10uF/100V
capacitor but somehow failed to notice the chubbier 330uF/25V capacitor
completing a PI filter with a small choke...)

Regards,
Peter Coghlan.


Sent from my DEC Alphaserver 800

Wayne S wrote:
>
> “ The ripple on the rectified 5V and 12V supplies gets transformed into an
> isolated AC source for the 9V supply.  ”
> Shouldn’t that be “pulsed DC” instead of “AC” as rectification
> changes AC to DC ?
> 
> 
> Sent from my iPhone
>
>> On May 11, 2022, at 01:36, Peter Coghlan via cctalk  
>> wrote:
>> 
>> The ripple on the rectified 5V and 12V supplies gets transformed into an
>> isolated AC source for the 9V supply.  
>

Re: DEC H7822 power supply

[Peter Coghlan via cctalk]

Maciej W. Rozycki wrote:
> On Thu, 12 May 2022, Peter Coghlan via cctalk wrote:
> 
>>  Anyway, the good news is that I think I have found the source of the
>> problem.  One of the capacitors used to filter the (DC?, pulsed DC?,
>> rippled DC?, biased AC?) supply to the 9V regulator is marked 330uF/25V.
>> It reads 6uF on the capacitance range on my multimeter.  This can't be
>> helping the cause.  It's not showing any signs of leakage but it's got
>> a brown sleeve and the same logo as the nasty, leaky SXF capacitors but
>> it is marked KME.  (I said there was only one capacitor in the filter
>> in a previous posting.  Originally I managed to spot a little 10uF/100V
>> capacitor but somehow failed to notice the chubbier 330uF/25V capacitor
>> completing a PI filter with a small choke...)
>
>  Likely just a general purpose capacitor.  I only have 2 330uF/25V 85°C 
> Nichicon SE parts listed for the H7821 (and I have a note about one PSU of 
> this kind having a 220uF/35V part in place of one of those too).  Also no 
> 100V parts at all, but 4 10uF/35V 105°C parts, either Teapo SE or Daewoo 
> RS.  Teapo SE and Chemi-con KME series are standard general purpose parts 
> and given the low temperature rating Nichicon SE are likely such as well.
> 
>  I guess especially for standard capacitors factories ordered whatever was 
> readily available whereas with the high-ripple low-impedance/ESR parts the 
> choice was much more limited.
> 

Hi Maciej,

I replaced the capacitor in question with a blue Nichicon SE of the same
value scavanged from another H7821 until I order some spares.  I now have
8.7V available on the 9V supply, a little less than I hoped for but it is
enough to make the internal thinwire tranceiver happy.  Now there is just
the seized fan to deal with.

Thanks again to you and Tony for leading me in the direction of the likely
source of this fault.  I would never have thought of looking in the power
supply to find it.

Regards,
Peter.

>
>   Maciej
>

RE: DEC H7822 power supply

[Peter Coghlan via cctalk]

>
> Wayne: AC DC  terminology has been well documented since the 1800's. 
> Don't try to reinvent the terms or no one will know what you are talking 
> about.
> I answered a few things below...
> 

They didn't have switch mode power supplies in the 1800s.  Terms like AC and
DC as understood back then can be used to describe what goes in to and comes
out of this power supply but they are not ideal for describing what's going
on in the middle of it.

It seems like an unusual sort of design to me.  I haven't come across the
idea of extracting ripple from one output and using it to create another
output before.  It took me a long time to figure out what was going on.
Maybe it is a common thing to do and I haven't seen it because of my
lack of experience with this sort of thing?  Could anyone who is more
familiar with switch mode power supplies comment?  Anyway, I was just
trying to describe what I've seen. 

>
> Wayne: Alternating Current  is a continuously varying sine wave. The polarity
> does reverse over time.  Perhaps Alternating Current isn't a good term and
> should be Alternating voltage instead but AC is the terminology and it
> describes the form of the wave. Is basically says that there is a zero to
> positive component and a zero to negative component of the voltage as 
> measured over time in 2 half waves
>

Ok. I doubt we will find many true sine waves inside this power supply though.

>
> Wayne:  There is no AC component.  The output from a rectifier is pulsed DC,
> either half wave or full wave. An additional circuit after the rectifier
> provides the smoothing to provide nearly pure DC. Nearly pure meaning the
> voltage remains constant and does not drop much when measured over time.
> The most pure DC source is a battery.  Transformers work on AC or Pulsed DC.
>

It depends on your definition of AC component.  How about we say the output
of a rectifier consists of a steady component plus a varying component?
I guess you could call it pulsed DC if you want.  Whatever it is, when you
put it through a transformer, what you get out the other side will have a
varying component and no steady component.  The output of the transformer
is probably vaguely sinusoidal because the performance of the core is likely
to fall off as frequency rises.  It could be described as AC.  It could not
be described as pulsed DC.  

>
> Wayne: There is no negative component of Pulsed DC so no AC.
>

As before, how about we call it a steady component plus a varying component?

>
> Wayne.  Lotta good text on this out there. Basically a variable current
> induced through a wire generates a magnetic field. Any wire placed next
> to it, has that field induced in it as well.  The catch is that the field
> has to pulse or alternate to keep generating the field and being induced
> into the other wire. It's the movement IE the up and down motion of the 
> voltage that causes the field to be induced in the other wire.
>

In a transformer, it is the variation of the current in one wire which
creates a varying magnetic field which induces a current in the other wire.

>
> Wayne:  I just got an Dec AlphaStation 200. Look like its running NT though. 
>

It's probably ok but check the power supply for leaky capacitors!
Have a look around for batteries that might leak while it was/is in
storage too!

>
> >
> > “ The ripple on the rectified 5V and 12V supplies gets transformed 
> > into an isolated AC source for the 9V supply.  ”
> > Shouldn’t that be “pulsed DC” instead of “AC” as rectification
> > changes AC to DC ?
>

No, it shouldn't.  What goes into the transformer could be described as
pulsed DC.  What comes out of the transformer cannot be described as pulsed DC.
It has a varying component and no steady component, therefore I described it
as AC.

Regards,
Peter Coghlan.

Re: DEC H7822 power supply

[Peter Coghlan via cctalk]

Toby Thain via cctalk wrote:

On 2022-05-11 7:02 p.m., Peter Coghlan via cctalk wrote:


Given the normal usage that has evolved for the terms DC and AC rather than
their dictionary definitions, I would suggest that the current that gets
passed by a rectifier has both a DC component and an AC component.  When



It does not, due to unidirectionality.



Consider the current through the rectifier as the sum of a "DC" current plus
an "AC" current.  The "DC" current has a steady positive value and the "AC"
current varies above and below zero with a magnitude less than or equal to
that of the "DC" current.

When the two are summed, the result is a varying current which does not go
below zero.  This is what I mean by it having a DC component and an AC
component.

This sort of analysis often used in electronic engineering to break down
more complex entities into simpler ones which can be analysed separately
with greater ease.  Didn't somebody have a theorem or an axiom or something
that says this is a valid way to do it? I forget who.  It was a long time
ago.  Kirchoff maybe? No, it wasn't him, maybe he was the one that said
the sum of currents into and out of nodes is zero and stuff like that...
  
It is a particularly suitable strategy for the case in hand because a

transformer will not pass "DC" from primary to secondary so the effect of
this component can be ignored (except that it could cause the transformer
core to saturate which must be allowed for.  This is only relevant to the
designer of the power supply but if I don't mention it, someone will
surely tell me that I should have.)

(I am starting to regret making the effort to accurately describe this
unusual and confusing (to me anyway) circuit for the benefit of others who
might find themselves struggling with this power supply as I was and might
find some hints on how it operates to be helpful.)

Regards,
Peter Coghlan.

Re: DEC H7822 power supply

[Peter Coghlan via cctalk]

Wayne S wrote:
>> On May 12, 2022, at 09:56, Peter Coghlan via cctalk  
>> wrote:
>> 
>> Toby Thain via cctalk wrote:
>>>> On 2022-05-11 7:02 p.m., Peter Coghlan via cctalk wrote:
>>>> Given the normal usage that has evolved for the terms DC and AC rather than
>>>> their dictionary definitions, I would suggest that the current that gets
>>>> passed by a rectifier has both a DC component and an AC component.  When
>>>> 
>>> 
>>> It does not, due to unidirectionality.
>>> 
>> 
>> Consider the current through the rectifier as the sum of a "DC" current plus
>> an "AC" current.  The "DC" current has a steady positive value and the "AC"
>> current varies above and below zero with a magnitude less than or equal to
>> that of the "DC" current.
>
> I’m really trying to understand what you’re getting at here.
> In practical terms, The assumption that there is a sum of a DC and AC
> component is incorrect.

Why? What evidence do you have that the assumption is incorrect?

If I am trying to calculate the height a sea harbour wall needs to be, can I
not consider the height of the tide and the height of the waves separately
and add them together, just to make it easier to work it out even though I
know that it would be physically impossible to distinguish the water that
makes up the tide from the water that makes up the waves?

(It's not even that bad in the case of electric current because instruments
exist which can measure the DC component of a current and the AC component
of a current separately or together.)

>
> There is no DC component in an AC wave. A wave is either AC meaning
> reversing polarity or it is DC meaning there is no reversal of polarity.
>

Can we agree that there is such a thing as an "AC current" and there
is such a thing as a "DC current" and the two of them can be added
together?

How about we take a 3V doorbell transformer and make a circuit consisting
of the secondary of the transformer, a 6V battery and a 9V light bulb all
connected in series?  How would the current in this circuit be described?
A varying current? An AC current? A DC current?

How about we use a 6V doorbell transformer and a 3V battery instead?  Would
that change how the current is described?

I would say that in both cases, the current flowing has an AC component and
it also has a DC component.

>
> And a rectifier does not sum anything.
>

I'm not saying the rectifier sums anything.  What I am saying is that the
current that goes through the rectifier can be considered as the sum of
two component currents.  It's just a way of looking at it, like considering
the tide and the waves separately in the harbour wall calculation.

>
> Are you maybe using “Rectifier” as the whole circuit?  A Rectifier
> can be a single component, like a diode used to pass one half of an
> alternating current or it can be a few components like 4 diodes connected
> as a bridge to pass both halfs.
>

Strictly speaking, a circuit involves a closed path for a current to flow in.
However, "circuit" also gets used to describe a substantial group of components
that needs to be connected to something else to form a fully closed path.  I
don't think a rectifier would be enough to be considered a whole circuit. 

>
> This may help to visualize.
> Do an experiment. If you have an Oscope take a small battery maybe 9 volt and
> wire the negative lead to the neg scope input. Touch the positive lead to the
> scope positive and watch the deflection. You should see a quick trace  rise
> and the trace should remain above the zero point.
> That is what pure DC looks like. No ripple at all. Power Supplies try to make
> that from your wall power input. 
> Next, untouch the positive and watch the trace fall. At no point does it fall
> below the zero line.
> NoW touch and untouch the positive as fast as you can and watch the trace.
> That is pulsating DC although with irregular frequency. That would be the
> output from the rectifier component.
> Reverse the leads and do it again.
> This will show the equivalent of a negative polarity.
>  
> If you have a variac, set it to 9 volts or so, plug in a light or something to
> provide a load and using your oscilloscope probes touch across the light and
> you should see a sine wave with regular frequency.  Notice the negative going
> half. That is AC.
> 
>
> It’s important to use a variac for safety ( or an isolation transformer very
> carefully) as wall voltages can hurt you.
>

A variac does not provide isolation.  If it is not wired correctly, it is
possible to end up scooping 9V from the live end of the mains supply instead
of from the neutral end. 

Re: DEC H7822 power supply

[Peter Coghlan via cctalk]

>> 
>> If I am trying to calculate the height a sea harbour wall needs to be, can I
>> not consider the height of the tide and the height of the waves separately
>> and add them together, just to make it easier to work it out even though I
>> know that it would be physically impossible to distinguish the water that
>> makes up the tide from the water that makes up the waves?
>> 
> Using this analogy if you measured 10 waves at 10 feet high you would need
> a wall 100 feet high, correct? Or 20 waves at 7 feet it would need to be
> 140 feet. 
> Not valid at all.
>

The idea is to measure the maximum height of the waves on a windy day by
examining how far up they make splash marks on a vertical pole stuck in the
seabed for the purpose.  Then consult a tide table to see how high the tide
was at the time the wave measurement was made.  Subtracting the latter figure
from the former figure gives the height of the waves above the tide.  (This
is the AC component.  The tide is the DC component.)  Adding the height of
the highest expected tide to the figure calculated for the maximum height of
the waves would give a figure for the minimum height needed for the harbour
wall.

>
>> (It's not even that bad in the case of electric current because instruments
>> exist which can measure the DC component of a current and the AC component
>> of a current separately or together.)
> I have no idea what you are talking about. What instruments are you referring
> to? I’d like to read about them.
>

A suitable moving coil ammeter will register the average value of a current
flowing through it and will indicate which direction it is flowing in.  This
is the DC component of the current.  A suitable moving iron ammeter will
(crudely) register the RMS value of the AC current flowing through it.  I'm
not certain but as far as I recall, it will also register the actual value
of DC current.  There are probably lots of fancy computerised oscilloscope
like instruments that can be used to measure all sorts of oddball current
waveforms given suitable current probes.


>> 
>> How about we take a 3V doorbell transformer and make a circuit consisting
>> of the secondary of the transformer, a 6V battery and a 9V light bulb all
>> connected in series?  How would the current in this circuit be described?
>> A varying current? An AC current? A DC current?
>>
>
> The current would be described by ohm’s law. Incidentally, if the primary
> of a xformer was actually connected it would probably blow up the battery.
>

Maybe I should have stated that I was assuming that power is applied to the
primary for the test?  Perhaps I should have also stated that I am assuming
that the light bulb is selected such that the current it draws at it's rated
voltage will not exceed the capability of either the transformer or the
battery?  I don't expect anything to blow up assuming we are understanding
each other correctly which may not be a valid assumption.

>
> You would describe it as Open. If not, its just a 6 volt DC circuit.
>

I don't understand.

>
>> How about we use a 6V doorbell transformer and a 3V battery instead?  Would
>> that change how the current is described?
> No it wouldn’t.  
>
>> I would say that in both cases, the current flowing has an AC component and
>> it also has a DC component.
> As stated and not rectified it’s not a valid circuit.
> In ps circuits there is a fuse on the secondary side of the circuit usually
> after the rectifier-filter circuit to prevent any rectifier shorts from
> damaging other components.

The transformer secondary, battery and light bulb are all connected in series
ie in a circle, end to end.  The battery is not going to be bothered by the
AC current flowing through it and the presence of the light bulb will limit
the current to a safe value.

If the battery and the transformer secondary and the light bulb were connected
in parallel ie the two terminals of the battery were connected directly to the
two terminals of the transformer secondary and the two terminals of the light
bulb, that would be a different matter.  Either the transformer or the battery
or both could be damaged in this case.  This is not what I was suggesting.

>
> Well, the previous example seemed to imply that the output from a rectifier
> was pure dc.
>

Pretty much everything I have said is to try to illustrate that what flows
through the rectifier has a DC component and an AC component.

>
> Its not, it’s pulsed DC
>

Pulsed DC sounds like something that has a DC component and an AC component.

>
> so i thought you might be referring
> to the rectifier and associated filter and possibly regulator components as
> the whole rectifier”
> Sometimes in talking about electronics the specific function of a portion
> of connected components is cited. IE filter circuit,  Detector circuit,
> rectifier circuit. It’s just shorthand speak.
>

Sorry, I didn't follow.

I'm going to bow out at this point because I don't there is anything
further I can add

[cctalk] Re: Connecting a physical terminal via LAN to Serial Port

[Peter Coghlan via cctalk]

>
> For an extra $1 now you can get integrated WiFi (and Bluetooth but
> there isn't any BT software support yet)  in the form of the $5 Pi
> Pico W, so that's one route to talking to an IP network for IoT or
> something like this.
> 
> A neat thing about the Pico is that you can do hard real-time
> interface protocols using its PIO (Programmable I/O) channels and DMA
> and still implement the whole thing in MicroPython.
>

You had me all enthusiastic there until I got to the word "MicroPython" :-(
I'm going to stay with my patch panel!

Regards,
Peter Coghlan.

[cctalk] Re: VT220 North of Boston

[Peter Coghlan via cctalk]

Boston GB, Boston USA or Boston somewhere else?

Regards,
Peter Coghlan
(No Bostons anywhere near where I am anyway).

>
> Looks like both of them are North of Boston, but one is like a 19 hour drive
> away and one is a half hour drive away.
> 
> I'd guess the half hour one. :D
> 
>  -Paul
>
>
>
>> On Aug 8, 2022, at 3:55 PM, W2HX via cctalk  wrote:
>> 
>> North Andover, MA or North Andover WI?
>> 
>> 73 Eugene W2HX
>> Subscribe to my Youtube Channel: 
>> https://www.youtube.com/c/w2hx-channel/videos
>> 
>> 
>> 
>> -Original Message-
>> From: Dave Mitton via cctalk  
>> Sent: Saturday, August 6, 2022 10:34 PM
>> To: cctalk@classiccmp.org
>> Cc: Dave Mitton 
>> Subject: [cctalk] VT220 North of Boston
>> 
>> I picked up a VT220 passing by some clean out a while ago.
>> Green phosphor, powers up and passes self test.
>> The keyboard has 3 broken function row keys, keycaps.
>> 
>> Free to pickup in North Andover.
>> 
>> Shipping would be heavy and awkward with the CRT.
>> 
>> Dave.
>> 
>> Sent from Mail for Windows
>> 
>

[cctalk] Re: COMAL language in the UK?

[Peter Coghlan via cctalk]

I'm not in the UK but I think I have a COMAL paged rom image for the BBC Micro
somewhere.  I don't think I ever tried using it, I have no documentation
for it and I don't know anything about it but if there is any interest, I will
try to find it.

Regards,
Peter Coghlan.

>
> This is a query which came into TNMOC, but I'm wondering if there are
> people here who can assist.
> 
> From: 
> Date: Thu, 1 Sept 2022 at 22:05
> Subject: Comal 80 in Great Britain
> 
> Hey there, my name is Mikkel, I am working on a book about a series of
> danish micro computers where the history of the programming language
> "Comal" will be a part of, and i just discovered that there was a UK
> Comal User groups and also a periodical called "Comal Bulletin", ive
> searched the internet i cannot find anything. But i hope that you might
> know some historians or hobbyist? or even online databases? you can
> refer me to?
> 
> I would like to know all there is to know about Comal in UK? And as a
> sidenote, if somebody knows something about danish computers in uk from
> the 70s and 80s i would of course like to hear about that to.
> 
> Hope you can help me, more info about my book is here :
> https://www.thedanishcomet.com
> 
> -- 
> -Jon
> +44 7792 149029

[cctalk] Re: TVI-921C

[Peter Coghlan via cctalk]

Mike Loewen wrote:

On Wed, 14 Sep 2022, Jonathan Katz via cctalk wrote:


I have a TVI-921C at the museum. I power it on and it beeps
continuously. One long, solid tone that does not stop until I power it
off. How do I start diagnosing this?


   Check the power supply voltages.



After checking the voltages, if it has a microprocessor, check if it is being
held reset and if the clock is running.

If it is a big lump of logic, it would probably be a lot easier to deal
with if you can find some sort of service manual / circuit diagram /
circuit description.

Regards,
Peter Coghlan.

[cctalk] Re: Is this a RIFA (uV3100-10 PSU)?

[Peter Coghlan via cctalk]

I'm working on cleaning up a bit of battery leakage in a MicroVAX 3100 
Model 10 and while it's apart I decided to look inside the PSU (an 
H7822-00).


It's nice and clean inside with no bulging caps. What it does have is an 
X2 capacitor, as shown here: https://photos.app.goo.gl/dpdqJ3tuGfsRDR3Y6.



It doesn't appear to be damaged and I can't see the word "RIFA" on it 
anywhere but I can't see two of the sides because of other components 
that get in the way.


So does anyone know for sure whether it is a RIFA brand or not, or do I 
have to desolder it to be sure?




Antonio,

It does look rather like the troublesome RIFA capacitors that come in
transparent / slightly yellow cases.  However, it seems to be lacking the
usual tell tale cracks.

If it is a RIFA, the worst it will do is create lots of smoke and stink
the place out.  This is not a big deal compared to the six/ten 1800uF 25V
electrolytics in H7821/H7822 power supplies.  If these are not already
leaking (look for a ring of brown goo around the base, sometimes only
visible after unsoldering them), they are very likely to start leaking
soon, even more likely if the power supply has not been stored in it's
natural orientation.  These can do a lot of damage to the power supply
and anything else the goo gets on.

I would prioritise sorting out the ten dodgy electrolytics and deal with
the suspected RIFA afterwards.

(I found that the top board in a H7822 is identical or nearly identical
to the single board in a H7821)

Regards,
Peter Coghlan.




Thanks

Antonio


--
Antonio Carlini
anto...@acarlini.com

[cctalk] Re: RDI BriteLite IPX keeps going to white screen

[Peter Coghlan via cctalk]

>
> I've been trying to do a little work on the RDI BriteLite IPX I have here, but
> when it runs more than a few minutes the LCD just blanks out white. The 
> machine
> seems to still respond to commands, so it seems like it's something with the
> display hardware. Even powered off and back on it won't go back to normal 
> until
> I let it sit for awhile.
>
> I suspect heat is part of the issue and it certainly feels warm; there are two
> loud cooling fans inside, but with the case off and checking airflow the fans
> do seem to be working. Having the case off doesn't make the display any 
> happier
> though. One fan is in the power supply and another fan is in the (LCD?)
> inverter board. Finger-checking large chips while in operation doesn't burn 
> the
> skin.
> 

At one time it used to be possible to buy aerosol cans of "freezer" which were
supposed to help trace faults like this in televisions etc.  Maybe it is still
available?  I have read tales of success using it however I don't ever recall
it being any help to me.  It may be that I didn't have any of the right sort
of faults.

Regards,
Peter Coghlan.

>
> Anyone familiar with this issue? I suppose I could look for a SPARCstation IPX
> to take the motherboard out of and replace this one with it, but it seems more
> like the problem is in the display, which is a custom part.
> 
> -- 
>  personal: http://www.cameronkaiser.com/ 
> --
>   Cameron Kaiser * Floodgap Systems * www.floodgap.com * ckai...@floodgap.com
> -- Fish will never understand fear of deep water. -- Tanner Greer 
> -
>

[cctalk] Re: Is this a RIFA (uV3100-10 PSU)?

[Peter Coghlan via cctalk]

On 24/10/2022 21:07, Antonio Carlini via cctalk wrote:
the bitsbox one may be a teensy bit to large but the ebay one should 
fit nicely. Neither is too expensive even with postage so I'll buy a 
few, given that I do have a fair few PSUs knocking around.


Turns out both sets of X2 caps I bought were identical and they fit 
perfectly.



As I was putting the PSU back together I thought I could hear something 
loose rattling inside. So I decided to dismantle it completely.


I managed to get the bottom board out, with some fiddling, but it would 
be much easier if I could get the fans off. However, they seem to be 
held on with some fasteners that I've not seen before. On the inside 
they are some sort of trilobe fastener that seems to yield under any 
sort of pressure (so I stopped) and on the outside they have a small 
hole with three thin slots, but my tri-wing bits only seem to turn them 
in the "tighten" direction, almost as though the whole thing is supposed 
to be "fit and forget". I suppose if I had to get the fans out of the 
way I could destroy those fittings and replace with some suitable M 
bolts. I'm just wondering now whether I'm missing a trick for removing 
the fans?




I thought those things that hold the fans on were a variation on pop rivets
but now that you mention it, I can see the "three sided slot" on the
outside.  Perhaps there is something preventing them from unscrewing, so
that vibration from the fans will not end up loostening them or something
like that?

I've taken the boards out of a whole bunch of H7821 and H7822 PSUs to
replace the electrolytics without removing the fans.  It was difficult at
first but the more I did, the easier it got.  I did come across one seized
fan along the way but I haven't a replacement yet so I haven't got around
to removing the bad one.  I should look into it.

Regards,
Peter Coghlan.



As I write this I realise that a photo or two wouldn't go amiss, so I'll 
try to take a few close ups tomorrow in daylight.


Antonio


--
Antonio Carlini
anto...@acarlini.com

[cctalk] Re: Is this a RIFA (uV3100-10 PSU)?

[Peter Coghlan via cctalk]

Antonio Carlini via cctalk wrote:

On 01/11/2022 23:10, Peter Coghlan via cctalk wrote:

I've taken the boards out of a whole bunch of H7821 and H7822 PSUs to
replace the electrolytics without removing the fans.  It was difficult at
first but the more I did, the easier it got.  I did come across one 
seized

fan along the way but I haven't a replacement yet so I haven't got around
to removing the bad one.  I should look into it.


I think I have a second  H7822 and a pair of H7821 supplies to look  at, 
so I may well end up getting some practice in!


Did you have to replace the power LED at all? I've just bench-tested the 
PSU I've swapped the X2 cap in (luckily it powers up without a load, so 
all I did was hook up a DVM and check out the voltages), and I noticed 
that the LED didn't light. I'm pretty sure I connected it back (and it 
is keyed!) so I think I may be looking for a replacement. Something in 
the back of my mind says that there is something that makes the 
replacement slightly non-trivial (funny LED, odd housing it fits in, 
some trick to getting it out ... I can't remember unfortunately).




The LED is more than just a power on indicator.  When it fails to light,
it can indicate that something is not right in the PSU.  It may tie in
with the "power good" output from the PSU.

It could be that it is failing to light because the regulation is not
working properly because there is no load.  I'd try it with a load
before anything else.  The H7822 may need a load on both boards.

To eliminate the LED from enquiries, you could try powering it from a 5V
supply via a 1KOhm resistor or something like that to see if it lights.
If it does, then further investigation in the PSU may be required.

If the LED does turn out to be bad, I forget whether it comes out by
pulling the LED out of the bezel first or taking the ring off the
rear of the bezel first, whichever works.



BTW was it just the 1800uF 25V 105degC caps (mine are brown) that you 
had to replace? Mine look fine but there are some other large 
electrolytics in there (two large brown 470uF voltage unclear, and one 
large black cap by the mains input on which I cannot see any of the 
markings).




It was only the 1800uF 25V capacitors were bad (mine are all brown too).
In an earlier posting I said there were six of them in a H7821 and ten of
them in a H7822.  I should have said five in a H7821 and nine in a H7822.
The 470uF 400V capacitors in my units were all fine.

The leaking electrolyte had damaged some other components in some cases,
especially an odd value resistor which seems to have different values in
different units, something around 464 Ohms.  It is in series with an
thermistor which seems to control the speed of the fans. I've also had
a zener diode fail which caused problems with the fans.  (If the fans
aren't operating properly, the LED might not light!)

Regards,
Peter Coghlan.



Thanks


Antonio


--
Antonio Carlini
anto...@acarlini.com

[cctalk] Re: Seeking a Sun monitor with 13W3 interface

[Peter Coghlan via cctalk]

>
> I'm seeking to buy a Sun Microsystems monitor with a 13W3 interface.
> 
> Ideally somewhere around central Europe for pickup in person. But I'm happy
> to consider shipping options as well.
>

I have a GDM1604B40 monitor (Sony Trinitron design) with a 13W3 connector.
I don't know if it works or not.  It is missing the on/off switch which I
borrowed to repair my DEC VR297.

It is located on the east coast of Ireland on the EU side of the border.
It weighs about 28kg.

Regards,
Peter Coghlan.

>
> Background: I'm looking to complete a Sun Ultra 1 build that I've been
> collecting parts for 🙂
>
>
>
> Best regards,
> 
> D.O.

[cctalk] Re: Exabyte recovery

[Peter Coghlan via cctalk]

Jon Elson via cctalk wrote:

On 11/9/22 20:52, Warner Losh via cctalk wrote:

I have a few old exabyte tapes of possible historic value. Who can I pay to
get them recovered that has the best chance of success?

Very difficult.  We were a big user of Exabyte drives for 
processing of physics experimental data.  Our experience is 
Exabyte drives had a lifetime of 1-2 years, no matter if 
they were powered on, in heavy use or just parked on a 
shelf.  Back in the day, we found outfits that would 
refurbish and test the drives for a modest cost, but I 
assume they are not doing that now.


I do have an 8200 drive here, but I have great doubts that 
it would work.




I have an Exabyte drive too, actually two drives in the one case.
One of the drives has not worked at all while it has been in my care
(the door was pulled off by someone attempting to extract the tape
from it when it failed in service before I got it).

A few years back I dug it out and found the other drive didn't work
either.  However, it was a fairly simple power supply fault and I
was able to revive it enough to read some old tapes successfully.

I'm not sure I'd trust it to try to read anything that might be of
historic value though.

Regards,
Peter Coghlan.



Jon

[cctalk] Re: Is this a RIFA (uV3100-10 PSU)?

[Peter Coghlan via cctalk]

On 15/11/2022 10:06, Antonio Carlini via cctalk wrote:

On 24/10/2022 21:07, Antonio Carlini via cctalk wrote:
the bitsbox one may be a teensy bit to large but the ebay one should 
fit nicely. Neither is too expensive even with postage so I'll buy a 
few, given that I do have a fair few PSUs knocking around.



Just a quick follow-up in case it helps someone else down the line. I've 
put the system back together and the H7822 PSU seems to work: with the 
mainboard connected the green LED lights (and the fans spin). So that's 
good.


The mainboard stays in the reset state (all diagnostic LEDs lit) so that 
might still be an issue with the PSU DC OK signal or the mainboard 
itself. However, at this stage the PSU is behaving as expected.




My experience with a H7822 is that when all the diagnostic LEDs stay lit,
DC OK is probably not getting asserted so the power supply might still
not be working 100% correctly.  In the H7822, DC OK seems to be the
purple lead and high (something around 4V-5V) means asserted.

Regards,
Peter Coghlan.




Antonio


--
Antonio Carlini
anto...@acarlini.com

[cctalk] Re: Is this a RIFA (uV3100-10 PSU)?

[Peter Coghlan via cctalk]

Maciej W. Rozycki via cctalk wrote:
> 
>  Same here, five 1800uF 25V caps to replace in the H7821 according to my 
> notes.  Overall if you spot a cap that says SXF on it, then replace it 
> right away whether already leaking or not.  These were made by Chemi-con.  
> Other Chemi-con lines reported problematic are SXE and LXF, though from 
> own experience they seem less prone.
> 
> Also Nichicon PL and PF parts may have to be replaced, I have seen the 
> former ones actually leaking in other DEC PSUs, such as the H7826, which 
> is also infested by SXF parts in various capacitances.  Both kinds leaked 
> in a generic industrial Bel Power PSU used with a Cisco device.
>

I agree that all the 1800uF capacitors should be replaced whether leaking
or not.  When I first checked all my H7821 power supplies, I found some
that looked absolutely fine.  However, after I started using them for a
while, problems began to show up and when I checked them again, they were
leaking badly.

>
> Sadly the problem with component shortage has hit capacitors as well and 
> some that used to be readily available are not anymore and prices have 
> risen too.
>

Even if replacements are not available, I would suggest immediately removing
any capacitors that are showing any signs of leakage and cleaning the affected
area in order to limit damage from the leakage.

>
> NB I wasn't aware about the problem with RIFA capacitors, it seems like I 
> might have to replace them in several PSUs I have already recapped, sigh.  
> I haven't seen any of them fail though, not at least in an obvious way.
>

I wouldn't worry too much about the RIFAs.  Their failure mode is to produce
lots of smoke a short while after switch on.  Apart from a bit of soot, they
don't do any damage to anything else while the faulty electrolytics can do a
considerable amount of damage without any visible external sign that anything
is wrong.  I haven't seen any failures of any of the RIFAs in my H7821 or
H7822 power supplies and even if they are going to fail eventually, I am
happy enough to wait for this to happen before doing anything about it because
it will be obvious to me when they do fail and the consequences of their
failure are not severe anyway.

Regards,
Peter Coghlan

>
> HTH,
>
>  Maciej

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Peter Coghlan via cctalk]

Hi Rob,

I'm only guessing here.  I think the sequence may have been that the
main switching transistor failed first as it would be under more stress
than a diode in the base circuit.  If the transistor shorted E-B-C then
the HT would become connected to the circuitry at it's base which would
be compelely unable to cope with voltages and currents involved.  This
probably resulted in the failure of the diode.  I think it may be worth
looking at the components further back the drive chain from the diode.
The inductor could be ok unless it is a very frail little thing but small
signal semiconductor components and/or resistors further back may not
have fared as well as it.

It might also be worthwhile checking for shorted rectifiers on the output
side in case this was the cause of the stress on the switching transistor.
However, the power supply might have an overcurrent trip to reduce the
possibility of this sort of damage.  If there is an overcurrent trip or
thermal trip, this may have been reset after the power supply was powered
off for a while and when it was powered on again, the already damaged
transistor could have been teed up to fail more spectacularly?  Like I
said, just guessing here.

Were there no fuses failed or cutouts cut out?  Does it look like there
should have been? I would think a shorted switching transistor should
have caused some safety device to operate.  Or is it the case of the
old adage that the faster acting transistor managed to sacrifice itself
in time to protect the quick-blow fuse from blowing?

Regards,
Peter Coghlan.

> 
> The H7842 PSU in my Rainbow failed yesterday. At first the machine just
> powered down and there was a slight burning smell, I wasn't next to the
> machine when this happened, so I didn't see or hear anything to tell me
> where the problem might be. Not being sure if there was a short in the
> machine or a problem in the PSU, I disconnected the fans, FDD and HDD and,
> probably foolishly, I applied power again to see if the machine would work.
> At this point there was a bang and a flash in the PSU.
> 
> 
> 
> On opening up the H7842 power supply I found that one of the transistors had
> completely disintegrated. It looks to be the main switching transistor, here
> is a picture of it:
> https://rjarratt.files.wordpress.com/2022/11/img_20221120_165850.jpg.  I
> have identified a source for this transistor, but if anyone can suggest a
> modern replacement that would be useful too. However, that is not my main
> problem.
> 
>  
> 
> Given that before the transistor blew up there had clearly been another
> failure somewhere else, I tried to find the original failure. There were no
> obviously damaged parts, so I just probed around near the transistor for any
> parts that were open circuit or short circuit. I found a diode connected to
> the base of the transistor that appeared to be short circuit. So, I decided
> to lift one end to check it. As I de-soldered one of the leads, the diode
> broke in two. So clearly the diode was either damaged by the failure of the
> transistor, or it was the cause of the failure. This is the diode:
> https://rjarratt.files.wordpress.com/2022/11/img_20221120_165913.jpg.
> 
>  
> 
> I can't quite make out the markings on the diode to know what to replace it
> with. I think it says "D610". Would that be the right designation? If so,
> can anyone suggest a suitable replacement please?
> 
>  
> 
> The diode seems to connect an inductor to the base of the switching
> transistor and the collector of the transistor is connected to a
> transformer. Should I be looking for other failed parts? Not sure if the
> diode failed first and then caused the transistor to fail? Or if something
> else has failed which caused these parts to fail?
> 
>  
> 
> I do know that there are no shorts in the Rainbow itself, because I have a
> spare PSU that still works fine in the same machine.
> 
>  
> 
> I blogged this here (it repeats most of that I have said above):
> https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power-supply-fai
> lure/
> 
>  
> 
> Thanks
> 
>  
>
> Rob
> 

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Peter Coghlan via cctalk]

Hi Rob,

>
> Thanks for the analysis. There doesn't seem to be anything further back from
> the diode, unless you mean further back behind the inductor?
>

I probably do.  I guess the base drive for the main switching transistor
comes from another smaller transistor via the inductor and the diode which
failed? Or maybe the diode is in parallel across the drive voltage?  Anyway,
it is this smaller transistor providing the drive that I'm suggesting
checking next as it may have got a belt of anything up to three hundred and
something volts DC when the main switching transistor shorted out an instant
before it exploded.  If that one has failed too, check further back to
whatever is driving it.

>
> I will check on the output side as you suggest.
>

The rectifiers on the output side may be tedious to figure out and test
due in part to the very low resistances of the transformer windings
feeding them unfortunately, especially if there are double rectifiers
(maybe in packages which look suspiciously like transistors) connected
to either side of centre tapped transformer windings.

>
> The circuit breaker did pop out when it failed. The onboard fuse is intact.
>

If you mean the house breaker as opposed to a breaker in the power supply,
I would guess it was probably rated at 25A or more and given the
destruction of the transistor, I would think the instantaneous current
was way higher than that.  Perhaps the fuse is a time delay one.  It
seems likely things could have been worse if the breaker wasn't able
to cut the power as soon as it did and you had to wait for the fuse
to blow :-(

>
> The house RCD triggered and cut the power to the whole house when the
> transistor exploded!
>

I guess that the transistor exploding produced a momentary cloud of
ionised gas which allowed arcing/tracking from the live transistor
terminals to something earthed (power supply case maybe?) which
triggered the RCD.  Or maybe the heatsink the switching transistor
is mounted on is earthed and the insulating washer between the
heatsink and the transistor failed causing it's destruction and
the heatsink then became the path to earth that triggered the RCD?

>
> Do you, or anyone else, have an idea what the diode could be so that I can
> find a replacement. Like I said, it seems to be marked D610, and there are
> some other ones that look to be the same elsewhere in the PSU.
>

Sorry, I have no idea what the diode is.  Hopefully Antonio will be able
to help with that.  Or if your spare power supply is the same design as
the failed one, can you look at the diode in that one?

It has dawned on me that another mechanism for the switching transistor
to have failed so spectacularly is for it to have been switched on hard
and held on for more than an instant by something driving it resulting
in it effectively shorting out the rectified mains supply through the
primary of the chopper transformer.

When you next try powering it up, it might be good to use the old
(filament) light bulb in series with the mains supply trick in case
whatever caused the initial problem is still lurking in there.  This
can be useful for locating shorted output rectifiers too.  You might
see a slight voltage rise from the outputs with working rectifiers when
power is applied through the light bulb and a lesser or no voltage rise
on an output which has a shorted rectifier or other issue that needs
closer inspection.  It also means there is no need to hide behind the
sofa when you switch it on :-)

Regards,
Peter.

> 
> Thanks
> 
> Rob 
> 
> > -Original Message-
> > From: Peter Coghlan via cctalk 
> > Sent: 20 November 2022 18:50
> > To: General Discussion: On-Topic and Off-Topic Posts
> > 
> > Cc: Peter Coghlan 
> > Subject: [cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power
> > Supply
> > 
> > Hi Rob,
> > 
> > I'm only guessing here.  I think the sequence may have been that the main
> > switching transistor failed first as it would be under more stress than a
> diode
> > in the base circuit.  If the transistor shorted E-B-C then the HT would
> become
> > connected to the circuitry at it's base which would be compelely unable to
> > cope with voltages and currents involved.  This probably resulted in the
> > failure of the diode.  I think it may be worth looking at the components
> > further back the drive chain from the diode.
> > The inductor could be ok unless it is a very frail little thing but small
> signal
> > semiconductor components and/or resistors further back may not have
> > fared as well as it.
> > 
> > It might also be worthwhile checking for shorted rectifiers on the output
> side
> > in case this was the cause of the stress on the switching transistor.
> > However, th

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Peter Coghlan via cctalk]

Don,

Does the url suggest you may have searched for a H5901 rather than a H9501?

Regards,
Peter Coghlan.

>
> NTE seems to turn up these possibilities for a H9501.
> 
> https://www.nteinc.com/search/search/search.php?ss360Query=H5901
> 
> Don Resor
> 
> Sent from someone's iPhone
> 
>> On Nov 23, 2022, at 12:04 PM, Antonio Carlini via cctalk 
>>  wrote:
>> 
>> On 21/11/2022 21:45, Antonio Carlini wrote:
>>> 
>>> I have two more I can open up and look at, but I cannot get to them tonight 
>>> and I'm probably out tomorrow too. But I think I can get to those other two 
>>> supplies on Wednesday. Hopefully at least one of them will be readable! 
>>> Otherwise I can desolder the diode from one of the other two and hopefully 
>>> find a useful marking.
>> 
>> 
>> Turns out I have three more PSUs ... and the diode markings are 
>> unfortunately invisible on all but one. That one is this one:
>> 
>> https://drive.google.com/file/d/1TeGTcBBv7KecMJ2CmNcR0O-tb3jZigy6/view?usp=share_link
>> 
>> 
>> It's not really visible there either but with the PSU out and one end 
>> desoldered I can see that it is marked H9501. I can also see that it doesn't 
>> conduct either way, which might mean that this PSU is the non-working one I 
>> know I have. Obviously the capacitor (820uF 250V electrolytic) is going to 
>> need replacing (might as well do both). But first I need to remove them and 
>> see what (if anything has happened) underneath.
>> 
>> 
>> This now goes back into my queue (behind the MicroVAXes and the H7868B PSU 
>> modules) so if you fix yours before I get to mine, please let me know what 
>> you did :-)
>> 
>> 
>> Antonio
>> 
>> 
>> -- 
>> Antonio Carlini
>> anto...@acarlini.com
>> 

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Peter Coghlan via cctalk]

It is often possible to infer the component ratings needed from the other
components around them.  A component in the base circuit of a transistor is
likely to experience lower currents and voltages than one in the collector
circuit.

In this case, we can see from Tony's diagram that there is a 2.7 Ohm resistor
in parallel with the diode.  Suppose it is a 1W resistor.  This means that
the from P = I squared R, the average current the resistor is likely to pass
is less than 1A.  From Ohm's law, V/I = R, this means the average voltage
across the resistor is likely to be no more than 2.7 Volts.

It is possible that the peak current / voltage involved could be higher than
the average for short periods of time but we have plenty of margin for error
here so we don't need to think about that too much.  A diode with a PIV of
200V should be fine here.

Regards,
Peter Coghlan

>
> Tony Duell has reverse engineered the following schematic.
> 
>  
> 
> http://www.bitsavers.org/pdf/dec/rainbow/duell_schematics/psu.pdf
> 
>  
> 
> I will go with the 1000V as you suggest anyway.
> 
> 
>
> Thanks
>
> 
>
> Rob
>
> 
>
> From: Mattis Lind  
> Sent: 25 November 2022 07:12
> To: r...@jarratt.me.uk
> Cc: General Discussion: On-Topic and Off-Topic Posts 
> Subject: Re: [cctalk] Identifying a Failed Diode in a Rainbow H7842 Power 
> Supply
> 
>  
> 
>  
>
> 
> 
>
> 
> On 24 Nov 2022, at 22:45, Rob Jarratt   > wrote:
> 
> 
> 
> Thanks for the suggestion Mattis. The UF4007 has a PIV of 1000V, I had a
> suggestion that the PIV should be 200V. Not sure what rating I should be
> going for here?
> 
>  
>
> Given that I didn’t have a schematic and this is on the primary side I went
> for the recommendation of 1000V. 200V may a bit low on the primary side
> depending on the application of the diode. On the primary there can be
> sustained voltages up to 400V and peaks that go even higher. Using a diode
> with higher PIV almost never affects the operation as long as other
> parameters stay the same. In this case the most important parameter is the
> trr. It has to be a fast recovery diode. In this case the UF4007 is slightly
> slower than the UF4004. But I doubt it has a big significance. Actually the
> A114 is much slower. 200 ns. 
> 
>  
> 
> :Mattis
> 
> 
> 
>  
> 
> Regards
> 
>  
> 
> Rob
> 
>  
> 
> From: Mattis Lind mailto:mattisl...@gmail.com> > 
> Sent: 22 November 2022 07:54
> To: r...@jarratt.me.uk  ;
> General Discussion: On-Topic and Off-Topic Posts   >
> Subject: Re: [cctalk] Identifying a Failed Diode in a Rainbow H7842 Power 
> Supply
> 
>  
> 
>  
> 
> Hello Rob!
> 
>  
> 
> 
> Given that before the transistor blew up there had clearly been another
> failure somewhere else, I tried to find the original failure. There were no
> obviously damaged parts, so I just probed around near the transistor for any
> parts that were open circuit or short circuit. I found a diode connected to
> the base of the transistor that appeared to be short circuit. So, I decided
> to lift one end to check it. As I de-soldered one of the leads, the diode
> broke in two. So clearly the diode was either damaged by the failure of the
> transistor, or it was the cause of the failure. This is the diode:
> https://rjarratt.files.wordpress.com/2022/11/img_20221120_165913.jpg.
> 
> 
> 
> 
>  
> 
>  
> 
> DEC used a lot of A114x diodes in their PSUs. They looked exactly like that
> one. Those are fast recovery diodes.
> https://pdf2.alldatasheet.com/datasheet-pdf/view/7563180/2074/A114F.html
> 
>  
> 
> I would replace it with a UF4007 or something similar.
> https://www.mouser.se/datasheet/2/849/uf4001-2578577.pdf
> 
>  
> 
>  
> 
>  
> 
> 
> I can't quite make out the markings on the diode to know what to replace it
> with. I think it says "D610". Would that be the right designation? If so,
> can anyone suggest a suitable replacement please?
> 
> 
> 
> The diode seems to connect an inductor to the base of the switching
> transistor and the collector of the transistor is connected to a
> transformer. Should I be looking for other failed parts? Not sure if the
> diode failed first and then caused the transistor to fail? Or if something
> else has failed which caused these parts to fail?
> 
>  
> 
>  
> 
> Also check all other semiconductors. Also on the outputs. If there is a 1 ohm
> fusible resistor in the base drive circuit check that one as well. In the
> VT100 PSUs it happens that it blows.
> 
>  
> 
>  
> 
> 
> 
> 
> I do know that there are no shorts in the Rainbow itself, because I have a
> spare PSU that still works fine in the same machine.
> 
> 
> 
> I blogged this here (it repeats most of that I have said above):
> https://robs-old-computers.com/2022/11/20/dec-rainbow-h7842-power-supply-failure/
> 
> 
> 
> 
> 
>  
> 
> /Mattis 
> 
> 
> Thanks
> 
> 
> 
> Rob
> 

[cctalk] Re: Identifying a Failed Diode in a Rainbow H7842 Power Supply

[Peter Coghlan via cctalk]

>
> I think that a good thing is to ramp up the input voltage slowly. Use a
> bench PSU to supply the Vstart voltage and then use a variac with
> insulation transformer to feed the rest if the supply. Use some small
> loads on the 5V and 12V outputs. Now you can safely probe the PSU and
> monitor base and collector voltages of the main switch transistor and
> see that everything looks fine.  Check output voltages. Stay below 50VAC
> input and very little harm can be done. Could be good idea to have an
> amp-meter inline with input AC from the variac to find out if there is 
> short somewhere.
>

I'm sure I recall we had a discussion about this a some time ago and I
think we came to the conclusion that a variac is not a good tool for this
sort of work, mainly because it doesn't limit current.

A series light bulb bulb on the other hand will not allow the power supply
to draw more current than the maximum current drawn by the light bulb, even
if the power supply is a complete short circuit.  This avoids the need to
monitor an ammeter and react quickly to switch off if a high current is
noticed.  It also avoids having to aquire an AC ammeter which can accurately
measure the odd current waveforms likely to be drawn by a switch mode power
supply.

An isolation transformer can be added if it turns out to be necessary to
probe the primary side of the chopper transformer.  Usually this won't
be necessary as the light bulb will give a good indication of what is
happening there.  If the bulb lights at normal brilliance and the power
supply does nothing, there is likely to be a serious short on the input
side.  If the bulb barely glows and the power supply produces reasonable
output voltages into small loads, then all is probably well.  If the bulb
glows somewhat and one or more outputs are missing, then check the
associated output rectifiers and smoothing networks for shorts.  If the
bulb pulses, the power supply could be tripping in response to an overload
somewhere.  If the bulb doesn't light and the power supply does nothing,
either something is open circuit or the chopper control circuit is not
working.

I should probably add that the bulb needs to be a mains bulb.  A 100W
bulb will limit the current to less than half an Amp (in this part of
world anyway).

Remember to check that the reservoir capacitors for the rectified mains
have discharged before handling that part of the power supply.

Regards,
Peter Coghlan.

[cctalk] Re: Guidance on repairing Dec PDP 11 System

[Peter Coghlan via cctalk]

I am looking for advice to get the 11/34 system up and running. I have 
started to put together a site to document my progress, to stay on track 
with the repair effort. The system has 2 rl02 drives, and an attached 9 
track tape drive. I had worked to repair a power supply issue at first, 
there was a problem with the main transformer, as well as one of the 
smaller voltage "bricks". Thankfully i have many systems and i was just 
able to swap in the needed working parts.




I'd suggest repairing the swapped out parts that were found to be faulty
and putting them back in the systems they came from before going any
further.  I know this seems like boring work as this doesn't seem to
contribute to any visible progress but it may be that minor differences
in ECO level, customisations, hacks etc could lead to surprises down the
road when parts from different systems are mixed and matched.  Doing this
work first also means there is less danger of running out of working parts
if more failures should occur further along.  Finally, it is going to be
easier to faulty repair parts when you have working examples of them to make
comparisons with than if you have no remaining working parts to guide you.

Regards,
Peter Coghlan.

[cctalk] Re: unpleasant odor from VT100

[Peter Coghlan via cctalk]

I had something similar with a VT220.  I didn't get around to investigating it
before the flyback transformer failed :-(

I can only suggest to run it for a short time with the cover off and the lights
out while looking for any glows / discharges around the flyback transformer,
the EHT cable, the EHT connector on the tube and the tube base connector.

Regards,
Peter Coghlan.

Ps: Anyone got a flyback transformer for a VT220?

>
> i have a VT100 that is working fine (powers on, navigates setup,
> saves/restores settings, etc) but has an unpleasant burning/ozone odor
> coming from the rear of it. i suspect something is running hot but i am not
> sure what, i do not currently have an IR camera in my hands- working on
> that. the original power supply has failed and is currently in storage, so
> the terminal is being powered by an ATX power supply and adapter board for
> the time being. this eliminates the usual R27 as being the culprit.
>
> as best i can tell, the smell is strongest just above the power
> distribution board, around the neck of the CRT. appreciate any suggestions
> you might have!

[cctalk] Re: unpleasant odor from VT100

[Peter Coghlan via cctalk]

I'm not familiar with the VT100 - I don't know where particular
components are and which ones usually cause problems in that unit.
My suggestion was aimed at CRT monitors in general.  Hopefully someone
who knows the VT100 better will reply with more specific suggestions.

If you are getting multi-kilovolt arcing, I expect you would be
hearing loud, sharp, cracking noises.  I think what you need to
look for is more of a very soft purple/blue corona discharge which
is hard to spot and probably makes little or no noise.  As I don't
know the layout of the VT100, it is hard for me to suggest places to
look for this other than in very general terms.

It could be something like a pinhole in the EHT cable, EHT connector,
flyback transformer etc allowing a corona discharge to something
nearby that happens to be at a lower potential (which is everything
where EHT is concerned).  Maybe the plastic / rubber etc insulation of
some of these components is breaking down and getting soft?  Switch off
and wait long enough for everthing to discharge before checking for this
sort of stuff with bare fingers.

If there is muck / grime / soot on the glass of the tube around the CRT
final anode connector, this could be part of the problem.  If you try
cleaning this off, don't clean off the dark grey / black aquadag coating
which is supposed to be there.  There will be a gap in the aquadag around
the final anode connector where there should just be clear glass.  Best to
clean with isopropyl alcohol or something similar that will evapourate
completely afterwards.  Whatever you use, make sure everthing EHT related
is fully dry before switching on again.  Similarly with muck around the
connections to the flyback transformer.

If the terminal has been stored in damp conditions, you could be
getting internal discharge in the flyback transformer.  If you suspect
this is the case, someone more familiar with the VT100 may be able to
advise how to get the flyback transformer dried out.

I think it's unlikely on a small monitor like the VT100 but it might
also be worth looking at the scan coils.

If there are spark gaps across some of the connections to the tube
base, either on the main PCB or on the tube base panel, if there is one,
it may be worth peering at those looking for corona discharge.  Again,
I think this is unlikely on a small monitor though.

Someone more familiar with the VT100 may be able to advise if it is
safe to disconnect the tube heaters in order to get more darkness
to look for corona discharge or if this would be likely do damage to
anything.  You could turn down the brightness so that there is less
stray light from the screen.

One more thing, give your eyes time to get accustomed to the darkness
when checking for corona discharge.  Be careful not to accidentally
get too close to the EHT components in the dark though.

The diagonal lines may be unrelated, again someone more familiar with
the VT100 would have to comment.

Regards,
Peter Coghlan.

>
>
> Peter, thank you for the reply! i had to wait a bit for it to get dark
> enough, but i just checked and i could not see any visible arcing inside
> the chassis when the terminal is powered on. there is a slight buzzing
> noise coming from the problem area but i suspect that is normal.
>
> i did notice something new and potentially enlightening though, this time
> when i turned the terminal on i could see several lines stretching
> diagonally across the screen that were not there before. i also checked the
> transformer(?) above the power distribution board while the unit was
> powered on, it does not seem to be overheating.
> 
> On Tue, Dec 20, 2022 at 1:09 PM Peter Coghlan via cctalk <
> cctalk@classiccmp.org> wrote:
> 
>> I had something similar with a VT220.  I didn't get around to
>> investigating it
>> before the flyback transformer failed :-(
>>
>> I can only suggest to run it for a short time with the cover off and the
>> lights
>> out while looking for any glows / discharges around the flyback
>> transformer,
>> the EHT cable, the EHT connector on the tube and the tube base connector.
>>
>> Regards,
>> Peter Coghlan.
>>
>> Ps: Anyone got a flyback transformer for a VT220?
>>
>> >
>> > i have a VT100 that is working fine (powers on, navigates setup,
>> > saves/restores settings, etc) but has an unpleasant burning/ozone odor
>> > coming from the rear of it. i suspect something is running hot but i am
>> not
>> > sure what, i do not currently have an IR camera in my hands- working on
>> > that. the original power supply has failed and is currently in storage,
>> so
>> > the terminal is being powered by an ATX power supply and adapter board
>> for
>> > the time being. this eliminates the usual R27 as being the culprit.
>> >
>> > as best i can tell, the smell is strongest just above the power
>> > distribution board, around the neck of the CRT. appreciate any
>> suggestions
>> > you might have!
>>

[cctalk] Re: what is on topic?

[Peter Coghlan via cctalk]

>
> Windows 2003 and XP is about as obsolete now as the IBM PC was in 1995.
> Probably more so. XP is popular on Vogons but I'm sure considerably less
> then 5% of computers actually host it.
>
> There's also the issue of beating a dead horse. There will always and
> forever (and forever...) be some esoteric issue to discuss about some
> bizzare and mostly unknown 8 bit system that 3 people on the whole of
> planet earth own (now not to brag, but chances are there are a dozen or
> less Northstar Dimension owners (never mind users) out there. And I'm
> proud to be 1). Who am I going to discuss that with?? And a larger issue
> is what? An even larger issue is why? Well I got plans ...
> 
> Lists of this sort are about tech and used by people who love it. Windows
> 11 is not germain to these conversations. But what about, and I'm just
> putting this out there, making pre UEFI shtuff -on-? I don't even know
> how I personally feel about such a delineation. But the suggestion is
> there. Nostalgic weirdos like older tech. Yes often dang old. Now a
> Thinkpad T60p doesn't exactly seem dang old. But it's pretty much dang
> obsolete (can't remember what sort of firmware it has but the processor,
> though 64 bit, is part of that hazy quasi transitional grouping that had
> more similarities to a 32 bit chip). On Tuesday, December 20, 2022,
> 10:05:06 PM EST, Fred Cisin via cctalk  wrote:  
> 
> On Wed, 21 Dec 2022, Chris via cctalk wrote:
>> Well there doesn't seem to be a great deal of activity these days, I has 
>> thought the suggestion about relaxing the rules might need discussing.
>> I know there are people still using Windows 2003 puters, or a near 
>> equivalent based on XP? But that's entirely irrelevant, as I'm quite 
>> sure you could find someone out there still utilizing an 8088/286/386. 
>> Of course that's the discretion of the sysop. As it stands there's at 
>> least 1 opinion for every ahole attached to the person who types on this 
>> board. Whateber. The way I see it dang obsolete shouldd be open for 
>> discussion.
>
> To summarize:  "On-Topic" == "Dang obsolete"
>


It seems that the previous list owner lost interest in this mailing list
and turfed us over to someone else who doesn't really say anything some
time ago.  At least I think that's what happened.  Nobody really said
anything.

So, can someone enlighten me on how to stop receiving mail from the list
until the holidays and/or this discussion are over under the new regime?

Regards,
Peter Coghlan

[cctalk] Re: AI applied to vintage interests

[Peter Coghlan via cctalk]

Fred Cisin wrote:

How well does it do on something "simple", and less esoteric, such as
translating FORTRAN to BASIC?

On Mon, 16 Jan 2023, Chuck Guzis via cctalk wrote:

How about FORTRAN to RPG?


Certainly more of a challenge!

But, a good way to quanitfy how far along they are getting on it.

The range of code traanslation goes from trivial to extreme.
The question is where is it currently on that scale?

Yes, conversion of FORTRAN-IV to Fortran-77 should be easy.
And considering that many systems ADD capoabilities, converting Fortran-77 
to FORTRAN-IV might be a bit harder.




In the 1980s, I used WATFIV.  WATFIV stood for "Waterloo FORTRAN IV" which was
actually FORTRAN-IV with some Fortran-77-like extensions such as character
variables, IF-THEN-ELSE and WHILE-ENDWHILE for example.

The WATFIV runtime library came with a subroutine (called TRANSL IIRC) which
could be used to translate code with WATFIV extensions to FORTRAN-IV.
I can't remember what it did with character variables but it translated
IF-THEN-ELSE and WHILE-ENDWHILE constructs into IF-GOTO-CONTINUE type
constructs which just worked.  I think dealing with character variables may
have required some human input but I don't really recall.



(compare that to converting 8080 to Z80, vs converting Z80 to 8080)
still relatively straight-forward, but a few additional complications.



How about translating code from Z80 which has several registers to 6502 with
rather fewer?  That would seem to need some more intelligent thinking on
how to simulate the unavailable registers without causing additional
difficulties.

Regards,
Peter Coghlan.

[cctalk] Re: AI applied to vintage interests

[Peter Coghlan via cctalk]

Peter Corlett wrote:
> On Tue, Jan 17, 2023 at 10:16:18AM +0000, Peter Coghlan via cctalk wrote:
> [...]
>> How about translating code from Z80 which has several registers to 6502
>> with rather fewer? That would seem to need some more intelligent thinking
>> on how to simulate the unavailable registers without causing additional
>> difficulties.
>
> It is often said that the 6502 has 256 registers, i.e. zero page.
>
> So e.g. LD (HL), A could be mechanically transformed into the sequence LDX
> #0, LDA (h, X), and STA a, with h and a being zero page locations. On the
> 65C02 the first two operations can be replaced with a simple LDA (h),
> although it may still be useful to index via X to simulate EXX without
> performing an expensive copy.
> 
> As it stands, that replaces a one byte instruction with five byte sequence
> which is obviously not great, but a relatively simple peephole optimiser can
> eliminate many of the redundant loads and stores so it wouldn't be quite so
> bad. After all, one important source of stores is the flags register, which
> I ignored in the code fragment. A _good_ optimiser can do a lot of clever
> analysis and transformation, and would probably be needed to handle all of
> the edge cases well, but would be too large and CPU-intensive to run on a
> Z80 or 6502 system.
> 

In the context of the original question posed, I think it would be interesting
to know of if todays technology artifical intelligence can perform these sort
of tasks by "learning how" or "figuring out how" ie without being programmed
specifically to carry out these tasks.  Even better, could it deal with more
complex difficulties such as the address space already being full and the
translated code being larger than the original code?

>
> It'd be easier to bodge a Z80 into a 6502 machine than try and translate the
> code. That's what often happened back in the day, after all.
>

If the artificial intelligence machine came up with the answer:
"It'd be too much trouble to translate the code, stick in a Z80 instead"
would that be a useful advance on having a human being come up with the same
conclusion?  Would it be able to justify it's conclusion to a beancounter?

Regards,
Peter Coghlan.

[cctalk] Re: AI applied to vintage interests

[Peter Coghlan via cctalk]

Chuck Guzis via cctalk wrote:
> On 1/17/23 19:14, Chris via cctalk wrote:
>> My worst nightmare is some fiendish AI turning me into a really beautiful
>> woman. Maybe with no arms. The Terminator future doesn't seem so bad by
>> comparison.
>>
> My worst nightmare, which is sure to become reality is for it to become
> a weapon, like most technology eventually does.
>

If these AI weapons are implemented with current computing technology, it's
hard to imagine them managing to take over the world before they
crash/panic/BSOD/bomb etc depending on their specific flavour and/or get
bricked by malware and/or fail due to leaking/dried up electrolytic capacitors
and/or batteries.  It's hard enough to keep systems going without interruption
when this is what people are actively trying to do.

In any case, how are they going to prevent the humans from cutting off their
power?

Why is it that the terminator never gets a warning light on and needs to call
maintenance?  Technology doesn't get simpler and more foolproof, it gets more
complex and therefore more likely to foul up in some unexpected way eventually.

Regards,
Peter Coghlan.

>
> --Chuck
>

[cctalk] Re: AI applied to vintage interests

[Peter Coghlan via cctalk]

I telephoned the customer service department at a railway company.  I think
the person I spoke to knew less about trains than this thing knows about
a PDP-11.  Maybe they are using AI to do telephone answering now?

Regards,
Peter Coghlan.

Jay Logue via cctalk wrote:


I don't know about guessing, but ChatGPT can deadpan bullshit with the 
best of them...


Prompt: /Describe the purpose of the DC LO signal in the DIGITAL PDP-11 
Unibus/


Response:

/The DC LO signal, also known as the "Data Bus Low" signal, is a signal 
that is used on the Unibus of the Digital PDP-11 computer. The Unibus is 
a bus architecture that was used in the PDP-11 computer to connect 
various peripheral devices, such as memory, disk drives, and other I/O 
devices, to the central processing unit (CPU)./


/The purpose of the DC LO signal is to provide the lower 8 bits of data 
that are transferred between the peripheral devices and the CPU over the 
Unibus. It is used in conjunction with the DC HI signal, which provides 
the upper 8 bits of data, to form a 16-bit wide data bus. Together, the 
DC LO and DC HI signals provide the mechanism for data transfer between 
the peripheral devices and the CPU on the PDP-11./




[snip]



--Jay
//

[cctalk] Re: scsi

[Peter Coghlan via cctalk]

Paul Berger via cctalk wrote:


Indeed not not all SCSI terminators are created equal, there where three 
types of signalling used Single Ended (SE) High Voltage Differential 
(HVD)  and Low Voltage Differential (LVD). LVD was created to squeeze a 
bit more speed out of parallel SCSI.




That's not exactly correct.  The reason for differential signalling was to be
able to use longer cables for a given speed.  As speeds increased, the
useable length of an SE SCSI cable became prohibitively short and LVD
was devised to overcome this in a way that was relatively compatible with SE.

Also, just like what is now known as World War I was originally the Great War,
what we now known as HVD was originally known as just "differential SCSI".
Older equipment which is labelled "differential" is likely to be what we
now call HVD and connecting it to older SE only equipment may not work out well.

Regards,
Peter Coghlan.

[cctalk] Re: cctalk archives not updating

[Peter Coghlan via cctalk]

It appears there was some sort of coup in July 2022. The mailing list owner
was apparantly replaced and the list was moved from where it was previously
hosted to somewhere new.  There were some vague declarations about needing to
deal with archives but I never saw any formal or informal announcement
decribing what happened to the list or the archives with any clarity back
then or since.

Regards,
Peter Coghlan

Tom Hunter wrote:
> It appears that the cctalk archives stopped updating in July 2022. See the
> link below:
> 
> https://classiccmp.org/pipermail/cctalk/
> 
> Could the new list admin please re-enable the archive feature of the
> mailing list and if possible fill in the missing months since July 2022.
> 
> Thanks and best regards
> Tom

[cctalk] Re: cctalk archives not updating

[Peter Coghlan via cctalk]

Well, I don't really know because as far as I can tell, there was nothing said
to the ordinary list members about what happened.  I assumed some sort of
emergency arose and that an announcement would be made in due course when the
pressure was off so I waited to see.  Nothing appeared.  I'm still in the dark.
You seem to know something about it?  Can you enlighten those of us that don't?

Regards,
Peter Coghlan.

Patrick Finnegan wrote:
>
> If by "coup" you mean rescued from getting turned off, then yes.
> 
> Patrick Finnegan
> 
> On Sat, Jan 28, 2023, 04:44 Peter Coghlan via cctalk 
> wrote:
> 
>> It appears there was some sort of coup in July 2022. The mailing list owner
>> was apparantly replaced and the list was moved from where it was previously
>> hosted to somewhere new.  There were some vague declarations about needing
>> to
>> deal with archives but I never saw any formal or informal announcement
>> decribing what happened to the list or the archives with any clarity back
>> then or since.
>>
>> Regards,
>> Peter Coghlan
>>
>> Tom Hunter wrote:
>> > It appears that the cctalk archives stopped updating in July 2022. See
>> the
>> > link below:
>> >
>> > https://classiccmp.org/pipermail/cctalk/
>> >
>> > Could the new list admin please re-enable the archive feature of the
>> > mailing list and if possible fill in the missing months since July 2022.
>> >
>> > Thanks and best regards
>> > Tom
>>

[cctalk] Re: Low cost logic analyzer

[Peter Coghlan via cctalk]

>
> There are some nice mixed signal oscilloscopes made by Rigol (Chinese) at a
> very affordable price.
>

Until just over a year ago, buying very afforadable Russian oil and gas also
seemed like a great idea with no downsides compared to getting these products
elsewhere.

Regards,
Peter Coghlan.

[cctalk] AT&T Customer Service

[Peter Coghlan via cctalk]

Fred Cisin wrote:

On Mon, 17 Apr 2023, AT&T Customer Service via cctalk wrote:
Sorry, I forgot to post the location.  I'm in Kent, Washington. USA.  
disregard the AT&T customer service.  Have E-mails in to them. not sure 
what going on there, but its at&tso anything is possible.


With a user name of "AT&T Customer Service", we speculated that it might 
be the Philipines, or India. :-)




Is there a country called Oxymoronia?

[cctalk] Re: Rainbow H7842 PSU Fault

[Peter Coghlan via cctalk]

>
> This seems to be because I measure a steady 0.6V on pin 6 of the transformer
> (p4, PSU Sheet 3). I just can't imagine where it might be coming from as the
> chopper won't be running. I had previously removed the transformer and there
> are no shorts between the pin 5-pin 6 winding and any of the other pins on
> the transformer. I checked all the DC outputs of the PSU when powering the
> 7812 from the bench, both on a working PSU and the non-working one. They are
> all at zero except the -12V output on the non-working PSU, which is +0.6V.
> But the voltage can't come out of nowhere.
> 
 
Is the -12V output connected to it's load while you are testing?  If so,
perhaps there is a path through the load to the the +12V or +15V supplys
available elsewhere which would forward bias the two diodes connected to
pin 6 of the transformer giving you a small positive voltage at that point.
(I would have thought it would be a bit higher though, depending on the type
of diode.  Neither of the two diodes is shorted, is it?)

[cctalk] Re: Rainbow H7842 PSU Fault

[Peter Coghlan via cctalk]

> 
> This seems to be because I measure a steady 0.6V on pin 6 of the transformer
> (p4, PSU Sheet 3). I just can't imagine where it might be coming from as the
> chopper won't be running. I had previously removed the transformer and there
> are no shorts between the pin 5-pin 6 winding and any of the other pins on
> the transformer. I checked all the DC outputs of the PSU when powering the
> 7812 from the bench, both on a working PSU and the non-working one. They are
> all at zero except the -12V output on the non-working PSU, which is +0.6V.
> But the voltage can't come out of nowhere.
> 


I've looked at it some more.  On page 5 (control module sheet 1), at the
non-inverting input of E1b, there is a 75k resistor and 16k9 I think?
resistor between -12V and V2 which is derived from Vstart.  Perhaps this
would account for the +0.6V?

(Ignore what I said earlier about the possibility of one of the two diodes
connected to pin 6 of the transformer being shorted.  The 51 Ohm (or is it a
5 Ohm?) resistor across one of them would look like a short compared to 75k.)

Regards,
Peter Coghlan.

[cctalk] Re: Rainbow H7842 PSU Fault

[Peter Coghlan via cctalk]

Hi Rob,

>
> Thanks for the various replies. Here are my responses and further
> observations.
> 
> I was asked if I had a load on the PSU while bench testing it. The answer to
> that is no.
>

I initially thought that the +0.6V on the -12V line might have been arriving
there via a load which was connected to both the -12V line and a positive
supply derived from the +15V you are applying from the bench power supply.
It seems that this cannot be the case as there is no load connected and all
the positive outputs from the PSU are at zero anyway.  An the internal to
the PSU path from Vstart to the -12V line is a much more likely explaination
for this +0.6V.

> 
> I did check the two diodes around the -12V current sensing resistor (PSU
> Sheet 3), but as was pointed out the current sensing resistor is very low
> value (51 ohms). The two diodes measure the same, in circuit, on the working
> and non-working PSUs. I suppose I may need to lift them to check them
> properly.
> 

I don't think there is any need to check these further.  I was barking up
the wrong tree whe I suggested suspecting them.

>
> I am not sure about the comment suggesting that the bench test conditions
> could result an overcurrent being sensed on the -12V output. This is because
> I use the same bench test conditions on both the working and non-working
> PSUs, and on the working one an overcurrent is not sensed.
>

It looks like the conditions in the control and overcurrent sensing parts of
the working and the non-working PSUs are different so some further
investigation may be required as to why this is the case.

> 
> I hadn't noticed the connection to -12V on the non-inverting input of E1b.
> However, I don't think this can account for the 0.6V because in both the
> working and non-working PSUs the non-inverting input to E1b is 4V.
>

See further down.

>
> However,
> when I was checking this, I noticed that I must have made a mistake, because
> the -12V output actually measures +0.4V on the working PSU (not zero as I
> first thought), but the non-working PSU measures +0.6V. Also, I noticed that
> the -12V output on the working PSU rises more slowly to +0.4V than on the
> non-working PSU where it rises more quickly to +0.6V.
>
> I still don't understand where this +ve voltage on the -12V output can come
> from though? Whether on the working PSU or the non-working PSU.
>

The 4V at the non-inverting input of E1b is linked to the -12V line via the
75k resistor and from there via a smoothing choke and the chopper transformer
secondary to the -12V line rectifier diode connected to pin 6 and on to ground
via the parallel diode/resistor combination.  This should result in a current
of about 45 microamps flowing to ground through those components mentioned.
This in turn should cause the rectifier to be forward biased and drop
approximately 0.6V across it which is characteristic of silicon diodes.
Does that make sense?

(It's rather confusing in that the -12V line rectifier is on the ground side
of the chopper transformer secondary instead of on the supply side where it
would more usually be encountered.  It's electrically all the same wherever
it is put as the components involved are in series anyway.)

It ought to be possible to measure the same 0.6V across the diode to confirm
this is where it is being dropped (and to measure the remaining 3.4V of the
4V at E1b across the 75k resistor).  As to why it is only 0.4V on the working
power supply, I haven't thought that far ahead yet :-)

I think the +0.6V on the -12V line is explainable and to be expected under the
test conditions described.  It looks like there could be something wrong in
the control circuitry which is preventing the power supply from starting up.
This might also account for the difference between the 0.4V and 0.6V.

Brent's suggestions for checking the condition around the comparators and how
they are supplied with power are good ones.  I haven't made any further
suggestions because I don't have any right now :-)

Regards,
Peter Coghlan.

[cctalk] Re: Rainbow H7842 PSU Fault

[Peter Coghlan via cctalk]

>
> Yes, that is certainly the problem. Presumably more current is flowing
> through the current sense resistor than should be.
>

Do you mean when the power supply is connected to the mains here?

If we are still talking about the test case when you are applying an external
supply to Vstart only, there should be almost zero current flowing through
the 51 Ohm current sense resistor because the chopper is not operating
because it has no HV supply.

We can calculate the current flowing through the current sense resistor
using Ohm's law given it's resistance and the voltage across it so we
don't need to presume.

>
> I thought that perhaps
> the higher positive voltage might be enough to switch on the rectifier diode
> while the lower positive voltage on the good PSU won't,
>

My feeling is that (in the test case with external Vstart only) the -12V
rectifier diode is being caused to conduct by the forward bias voltage applied
to it from Vstart/V2 (should that be Vz I wonder?) via the 16k9 resistor, the
75k resistor and the wound components.  The current available to flow through
the diode is limited to some microamps by the 75k resistor.  As Brent said,
this is very low on the diode's characteristic curve and there are likely to
be variations between the voltages dropped across different diodes.

If diode was not present in the circuit, approx 4V (which IIRC you measured
at E1b non-inverting input) should appear across the points where the diode
would have been.  It is more the characteristic of the diode which is
determining how much voltage is appearing across itself than anything else.
The other components involved have little say about what voltage appears
across the diode.  They just allow a small current to flow through the diode
to forward bias it and the diode gets to decide how much voltage then appears
across itself, depending on the magnitude of the current flowing through it
when forward biased and it's characteristic curve.

>
> but it seems to
> switch on in both cases because I measured a voltage across the current
> sense resistor in both cases. I measured 0.08V on the bad PSU and 0.01 on
> the good PSU. So this would explain why an over current is being detected.
>

0.08V across a 51 Ohm resistor suggests that approx 1.6mA is flowing through
it from Ohm's law.  0.01V across the same resistor suggests that approx 200 uA
flowing through it.  I assume this is with the power supply connected to the
mains, not using the external supply to energise Vstart only as I can't see
where 1.6mA would come from in the latter case.

1.6mA being drawn from the -12V supply is not suggestive of an overload
unless it's capacity is very limited indeed.  Perhaps this is the normal
expected loading?  200uA seems very low indeed but perhaps this is to be
expected? If the -12V line is not being produced, the load cannot draw
any current from it.  I don't think that either of these conditions should
trigger an overload response from the PSU.

(Are the 0.08V and and 0.01V voltages appearing in the same direction or
is one of them positive and the other one negative?)

(Does the resistor measure approximately 50 Ohms in both cases with the
power switched off)?

>
> What I don't now get is why the -12V output is higher on the bad PSU because
> if there is something shorted or a capacitor with high current leakage on
> the -12V output, that would surely result in a lower voltage on the -12V
> output, not a higher one?
>

Sorry, this is not very clear to me and I already get confused enough when
dealing with negative supply rails.  The -12V output being higher could
refer to it's voltage being more positive with respect to 0V or it's voltage
being more negative with respect to 0V.  In any case, I'm not sure I recall
what the voltages measured on the -12V lines of the working and non-working
power supplies were with them connected to the mains.  I think I recall they
were +0.6V and +0.4V respectively when power was applied to Vstart only.

I think it is likely that most of what we are seeing here is to be expected
and is explainable either by the test conditions created by applying power
to Vstart only or by conditions created by the presence of the fault and
not necessarily because we are close to the location of the fault.


I think the next step might be to check what is happening with the
comparators.  Are their positive power supply pins connected to Vstart or
V2 (Vz?) or somewhere else? Are their negative supply pins connected to 0V
or maybe to the -12V line or maybe somewhere else?  Are they getting suitable
volages on both supply pins that will enable them to operate under startup
conditions?  If either the positive or negative supply to one or more of
them is missing or wildly incorrect at startup, they cannot be expected to
do their jobs properly.

It might be worth doing some more testing without a mains supply connected
but with an external supply for Vstart and also an external negative supply
going to the -12V

[cctalk] Re: Rainbow H7842 PSU Fault

[Peter Coghlan via cctalk]

It seems a bit odd that a power supply from someone like DEC of that era would
be designed to depend so critically on the absolute value of a rail used
for startup purposes.

The output of the replacement 7812 varying between 12.2V and 12.4V suggest that
the 7812 itself could be faulty.  Or maybe it could be oscillating due to a
failure in the smoothing/reservoir/decoupling capacitors on either side of it,
especially the 10uF and 2.2uF capacitors connected to it's output?  It might be
useful to see what Vstart looks like with an oscilloscope.

If the value of Vstart is really critical enough to change the state of E3d,
I would suggest checking the resistors connected to the inputs and output
of E3d, especially the 360k resistor.  Assuming it is getting a reasonable
power supply on pins 3 and 12 (whose value should not be critical), the
output of E3d should depend solely on the relationship between the voltages
at it's two inputs.  If the output does not correspond with what the inputs
say it should be, then the comparator itself is probably faulty.

Regards,
Peter Coghlan.

> 
> I need to order higher tolerance ones. However, I was wondering, would a
> sensible strategy be to go through the ones that I have and find one that
> outputs a voltage closer to the spec? Or would that just be storing up
> trouble for the future?
> 
> Regards
> 
> Rob
> 
> > -Original Message-
> > From: Rob Jarratt 
> > Sent: Sunday, April 30, 2023 9:35 PM
> > To: 'Wayne S' 
> > Cc: r...@jarratt.me.uk; 'General Discussion: On-Topic and Off-Topic Posts'
> > 
> > Subject: RE: [cctalk] Re: Rainbow H7842 PSU Fault
> > 
> > Looks like the one I used is an L7812CV, which has a looser tolerance. I am
> > going to have to order some tighter tolerance parts.
> > 
> > > -Original Message-
> > > From: Wayne S 
> > > Sent: Sunday, April 30, 2023 7:42 PM
> > > To: Wayne S 
> > > Cc: r...@jarratt.me.uk; General Discussion: On-Topic and Off-Topic
> > > Posts ; Rob Jarratt
> > > 
> > > Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault
> > >
> > > The CT version is 4%, the ACT is 2%.
> > >
> > > Sent from my iPhone
> > >
> > > > On Apr 30, 2023, at 11:37, Wayne S  wrote:
> > > >
> > > > Spec sheet for the 7812 shows 2 versions - 4% and 2% tolerances. So
> > > > the
> > > voltages your measuring are within the 4% part. Maybe you do need the
> > > 2% one.
> > > >
> > > > Sent from my iPhone
> > > >
> > > >> On Apr 30, 2023, at 11:26, Rob Jarratt via cctalk
> > > >> 
> > > wrote:
> > > >>
> > > >> I think I have found a possible cause, but not too sure what to do
> > > >> about it if I have.
> > > >>
> > > >> I had to replace the 7812 regulator that supplies Vstart. The new
> > > >> one has an output that seems to vary between 12.2 and 12.4V. The
> > > >> 7812 on the good PSU outputs a steady 12.1V. I think that is enough
> > > >> to cause the E3d comparator to turn the comparator off and allow
> > > >> its output to
> > > float high.
> > > >>
> > > >> I suspect I may have used one with a wide tolerance rather than a
> > > >> narrow one, or just have a bad one. It should be a genuine part as
> > > >> it came from CPC Farnell. Not quite sure why the output varies on
> > > >> the bad
> > > one though.
> > > >>
> > > >> Regards
> > > >>
> > > >> Rob
> > > >>
> > > >>
> > > >>

[cctalk] Re: Rainbow H7842 PSU Fault

[Peter Coghlan via cctalk]

>
> The comments about the tolerance of the 7812 were right, it doesn’t
> appear to be an issue with the replacement 7812 regulator because when
> I tried using the bench PSU to feed exactly 12V to the circuit from the
> output of the 7812 the comparator still gave the wrong result. It was
> still wrong if I applied only 11V
>

What do you mean by "gave the wrong result"?  If the power supply to the
comparator is "reasonable" and the comparator's output does not reflect
what is happening at it's inputs, the comparator is faulty.

If the comparator is giving the "wrong result" because it's inputs
are telling it to, it is behaving correctly.

With no mains supply connected and a positive startup voltage applied
to Vstart and a negative startup voltage applied to the -12V line via
a 2k7 resistor, you could try shorting the inputs of the comparator
together and see whether this changes the comparator's output.  I want
to emphasise doing this without power going to the mains rectifier
feeding the chopper so that if there is real overload, the magic smoke
will not be released.

If the output if the comparator is then "correct" and this output being
"wrong" was the source of the PSU not working, then the PWM should start
up like it does in the good power supply.  This would confirm that there
is a problem in the components providing and/or mmonitoring the -12V line.
If the PWM does not start up, this suggests the problem is elsewhere.

>
> I then looked at the value of Vz on the good and bad PSUs, when applying
> 12V to the 7812 output. That was 5.4V in both the good and bad PSUs. Where
> I saw a difference was on the -12V output, it was +0.4V on the good PSU
> and 0.56V on the bad one (the voltage varied so this was an average). I
> checked the voltage drop across the current sense resistor. It is 0.01V
> on the good PSU and 0.08V on the bad PSU, which would explain the higher
> positive voltage on the -12V output and the comparator being turned on.
>
> 
>
> I am wondering if there could be a problem on the -12V output circuit
> (PSU Sheet 3). I am struggling to understand the purpose of the two
> transistors and the Zener diode there, but maybe one of them should be
> switched on and isn’t. I am also unsure now as to which diode is doing
> the rectification (to -12V). Someone said it was the one attached to
> pin 6 of the transformer, but is that right? Isn’t it the Zener diode
> half way across the page?
>

The TIP121 darlington transistor is a shunt regulator for the -12V line.
The voltage at the base of the MPSA55 transistor varies with the -12V line
because it is connected to the -12V line via the potential divider formed by
the 1k24 and 1k10 resistors.  If the voltage on the -12V line increases
in the negative direction, the voltage at the base of the MPSA55 increases
in proportion to it via the potential divider.  It's emitter voltage is
fixed by the zener diode so the MPSA55 is turned on more and it pulls more
current through the base of the TIP121 which results in the TIP121
conducting more and pulling the -12V line down closer to it's correct
voltage.  The opposite happens if the -12V goes lower than it should be,
the TIP121 is turned on less and this allows the -12V line to increase
negatively to it's correct value.

None of this circuitry should be doing very much until the chopper
transformer is producing the source for the -12V line.

I suppose if the TIP121 is sborted or the zener diode is shorted, it
could be causing problems, however, from the test results etc we have 
been given so far, I am not completely convinced there is a problem 
with the -12V line.

If a negative startup voltage is applied to the -12V line as well as
a positive voltage to Vstart to better simultate startup conditions,
it may reveal more about what is happening with the -12V line.
It might then be possible to compare the voltages across the zener
diodes in the working power supply and the non-working power supply
for example.

Regards,
Peter.
 
>
> Regards
> 
>  
> 
> Rob
> 
>  
> 
> From: Rob Jarratt  
> Sent: 02 May 2023 08:19
> To: 'Mattis Lind' ; r...@jarratt.me.uk; 'General 
> Discussion: On-Topic and Off-Topic Posts' 
> Subject: RE: [cctalk] Re: Rainbow H7842 PSU Fault
> 
>  
> 
> That’s a good idea, I will try that
> 
>  
> 
> From: Mattis Lind <  mattisl...@gmail.com> 
> Sent: Tuesday, May 2, 2023 7:55 AM
> To:   r...@jarratt.me.uk; General Discussion: 
> On-Topic and Off-Topic Posts <  
> cctalk@classiccmp.org>
> Cc: Rob Jarratt <  
> robert.jarr...@ntlworld.com>
> Subject: Re: [cctalk] Re: Rainbow H7842 PSU Fault
> 
>  
> 
>  
> 
>  
> 
> 
> 
> 
> Not quite sure what you mean here. I had advice from a friend to bench test
> the control module by providing 14V to the input of the 7812. On the good
> PSU I can see the PWM operate, on the bad one the PWM is shutdown.
> 
>  
> 
> And what happens

[cctalk] Re: Rainbow H7842 PSU Fault

[Peter Coghlan via cctalk]

> 
> 
> > -Original Message-
> > From: Peter Coghlan via cctalk 
> > Sent: 07 May 2023 10:34
> > To: General Discussion: On-Topic and Off-Topic Posts
> > 
> > Cc: Peter Coghlan 
> > Subject: [cctalk] Re: Rainbow H7842 PSU Fault
> > 
> > >
> > > The comments about the tolerance of the 7812 were right, it doesn’t
> > > appear to be an issue with the replacement 7812 regulator because when
> > > I tried using the bench PSU to feed exactly 12V to the circuit from
> > > the output of the 7812 the comparator still gave the wrong result. It
> > > was still wrong if I applied only 11V
> > >
> > 
> > What do you mean by "gave the wrong result"?  If the power supply to the
> > comparator is "reasonable" and the comparator's output does not reflect
> > what is happening at it's inputs, the comparator is faulty.
> > 
> > If the comparator is giving the "wrong result" because it's inputs are 
> > telling it
> > to, it is behaving correctly.
> 
> Sorry for the loose language. I mean that the comparator is working correctly
> but the output is not what should be expected, because its inputs are wrong.
> 
> > 
> > With no mains supply connected and a positive startup voltage applied to
> > Vstart and a negative startup voltage applied to the -12V line via a 2k7
> > resistor, you could try shorting the inputs of the comparator together and 
> > see
> > whether this changes the comparator's output.  I want to emphasise doing
> > this without power going to the mains rectifier feeding the chopper so that 
> > if
> > there is real overload, the magic smoke will not be released.
> > 
> > If the output if the comparator is then "correct" and this output being
> > "wrong" was the source of the PSU not working, then the PWM should start
> > up like it does in the good power supply.  This would confirm that there is 
> > a
> > problem in the components providing and/or mmonitoring the -12V line.
> > If the PWM does not start up, this suggests the problem is elsewhere.
> > 
> 
> I can tell you that if I remove the diode on the output of the -12V
> comparator then the PWM operates normally.
> 
> > >
> > > I then looked at the value of Vz on the good and bad PSUs, when
> > > applying 12V to the 7812 output. That was 5.4V in both the good and
> > > bad PSUs. Where I saw a difference was on the -12V output, it was
> > > +0.4V on the good PSU and 0.56V on the bad one (the voltage varied so
> > > this was an average). I checked the voltage drop across the current
> > > sense resistor. It is 0.01V on the good PSU and 0.08V on the bad PSU,
> > > which would explain the higher positive voltage on the -12V output and the
> > comparator being turned on.
> > >
> > >
> > >
> > > I am wondering if there could be a problem on the -12V output circuit
> > > (PSU Sheet 3). I am struggling to understand the purpose of the two
> > > transistors and the Zener diode there, but maybe one of them should be
> > > switched on and isn’t. I am also unsure now as to which diode is doing
> > > the rectification (to -12V). Someone said it was the one attached to
> > > pin 6 of the transformer, but is that right? Isn’t it the Zener diode
> > > half way across the page?
> > >
> > 
> > The TIP121 darlington transistor is a shunt regulator for the -12V line.
> > The voltage at the base of the MPSA55 transistor varies with the -12V line
> > because it is connected to the -12V line via the potential divider formed by
> > the 1k24 and 1k10 resistors.  If the voltage on the -12V line increases in 
> > the
> > negative direction, the voltage at the base of the MPSA55 increases in
> > proportion to it via the potential divider.  It's emitter voltage is fixed 
> > by the
> > zener diode so the MPSA55 is turned on more and it pulls more current
> > through the base of the TIP121 which results in the TIP121 conducting more
> > and pulling the -12V line down closer to it's correct voltage.  The opposite
> > happens if the -12V goes lower than it should be, the TIP121 is turned on 
> > less
> > and this allows the -12V line to increase negatively to it's correct value.
> > 
> > None of this circuitry should be doing very much until the chopper
> > transformer is producing the source for the -12V line.
> > 
> > I suppose if the TIP121 is sborted or the zener diode is shorted, it could 
> > be
> &

[cctalk] Re: Rainbow H7842 PSU Fault

[Peter Coghlan via cctalk]

>
> I am going to read your answer more carefully later. But I wanted to check
> one thing. I measured the base-emitter voltage as negative in both cases,
> and yet the TIP121 appears to be conducting on the bad PSU. Surely that
> means that the TIP121 is not working correctly?
>

I'd be very reluctant to draw any conclusions from measurements made when
the device is not biased correctly.  As Brent says, there could be current
flowing through the presumably forward biased base-collector junction.
Also, this device is a not just a straighforward transistor.  It's data sheet
says it is a package containing two transistors and some other components,
including a normally reverse biased diode between the collector and emitter
terminals.  A positive voltage of sufficient magnitude on it's emitter could
be forward biasing this diode resulting in current flow through it and
therefore also through the 20R resistors causing a voltage to be developed
across them.

I think a better way to determine if the TIP121 is causing excess current
draw on the -12V line is to make the voltage at it's emitter approximately
correct and see if it then draws enough current through itself to cause
the excess current trip to operate.  This current can then be measured
by observing the voltage across the 20R resistors (or the smoke coming
from them if they are not of sufficient power rating...).  It should also
be possible to observe the conditions around the zener diode and the MPSA55
and see if these components are behaving reasonably.

Regards,
Peter.

[cctalk] Re: Rainbow H7842 PSU Fault

[Peter Coghlan via cctalk]

>
> I will do all the suggested checks, but I won't be able to do this for a
> number of days. However, I wanted to understand something in the meantime.
> The conditions I am applying are (I think!) what would happen during
> startup, and during startup the control board has to make the PWM run,
> otherwise the main switching transistor won't operate and the transformer
> won't operate to produce the -12V in the first place. So surely in the
> startup condition I am applying (which is to supply Vstart from a bench PSU)
> is valid? Where is the flaw in my reasoning here?
>

The PSU also gives a kick start to the -12V line by applying some negative
voltage probably around -15V to it via a 2k7 resistor (on sheet 1).  If
you are not providing this, perhaps this is why the -12V line is able to
swing slightly positive and upset conditions in the -12V current sense
circuit enough to prevent the PWM from starting?

Even if you are providing this resistor limited -12V startup supply, could
it be insufficient due to a faulty component leaking current from the
-12V line to ground meaning that the -12V line does not become sufficiently
negative to fulfil the startup conditions?

Could the PWM be managing to start up and run for a cycle or two and then
stop due to a fault in the PWM circuit somehow attempting to draw too much
current from the -12V line?  I don't see any connections from the -12V line
to the PWM so this is probably not the case.

Is this the same PSU whose chopper transistor exploded a while back?  Could
there be any carbon deposits remaining on the board or conductive remnants
wedged under components etc causing leakage from the -12V line to ground?  

Regards,
Peter.

[cctalk] Re: Rainbow H7842 PSU Fault

[Peter Coghlan via cctalk]

> >
> > -Original Message-
> > From: Peter Coghlan via cctalk 
> > Sent: 10 May 2023 12:04
> > To: 'General Discussion: On-Topic and Off-Topic Posts'
> > 
> > Cc: Peter Coghlan 
> > Subject: [cctalk] Re: Rainbow H7842 PSU Fault
> > 
> > 
> > >
> > > I will do all the suggested checks, but I won't be able to do this for
> > > a number of days. However, I wanted to understand something in the
> > meantime.
> > > The conditions I am applying are (I think!) what would happen during
> > > startup, and during startup the control board has to make the PWM run,
> > > otherwise the main switching transistor won't operate and the
> > > transformer won't operate to produce the -12V in the first place. So
> > > surely in the startup condition I am applying (which is to supply
> > > Vstart from a bench PSU) is valid? Where is the flaw in my reasoning here?
> > >
> > 
> > The PSU also gives a kick start to the -12V line by applying some negative
> > voltage probably around -15V to it via a 2k7 resistor (on sheet 1).  If you 
> > are
> > not providing this, perhaps this is why the -12V line is able to swing 
> > slightly
> > positive and upset conditions in the -12V current sense circuit enough to
> > prevent the PWM from starting?
> > 
> > Even if you are providing this resistor limited -12V startup supply, could 
> > it be
> > insufficient due to a faulty component leaking current from the -12V line to
> > ground meaning that the -12V line does not become sufficiently negative to
> > fulfil the startup conditions?
> > 
> > Could the PWM be managing to start up and run for a cycle or two and then
> > stop due to a fault in the PWM circuit somehow attempting to draw too much
> > current from the -12V line?  I don't see any connections from the -12V line 
> > to
> > the PWM so this is probably not the case.
> > 
> 
> So, I did the following test. I used my bench PSU to apply +12V to the
> regulator output and -12V to the actual -12V output. Under these conditions
> the PWM operated correctly. I repeated the test on the good PSU and the
> result was the same.
> 
> The good PSU drew 13mA on the -12V line and 92mA from the regulator side.
> The bad PSU was 16mA and 86mA respectively. When I test only applying +12V
> to the regulator output, the good PSU draws about 90mA and the bad one 140mA.
> 
> It is worth repeating that the PWM operates correctly on the good PSU when
> I only apply +12V to the regulator, but not on the bad one. I guess there
> must be some kind of short somewhere, but really not sure where it might be.
> It has to be something that is pulling the -12V line slightly higher. The
> only place where this seems like it could happen is around E1b I think.
> Perhaps there is a path through the positive input to E1b to Vcc on the
> LM393 (Control Module Sheet 1). Does that seem logical?

Ok, it looks like there is not a severe leak from the -12V line to ground then.

I am puzzled by the extra current draw on Vstart by the bad PSU but I'm not
sure that tracking this down would lead us to the real problem.

On the other hand, did you mention at one point that Vstart was varying?
If this is the case, the reason for this would probably need to be found and
fixed independent of whether it leads to finding the main problem as this is
supposed to be a stable supply.

I don't think there is likely to be any serious leakage via E1b because the
link to the -12V line is via a 75K resistor which would limit any leakage
current to roughly 160uA.  Of course this applies if the resistor really
is 75K and doesn't have carbon deposits bridging the tracks and connections
around it to somewhere else.

I would suggest looking carefully at the resistors around E3d to make sure
they have the correct values, especially the 360K resistor and making sure
there is no debris etc around these components that could be bridging any
connections associated with them to somewhere else, also that no connections
have been severed.  Problems here could be leading to E3d falsely triggering
when there is no real overload.

It might be useful to check the voltages and resistor values in the -12V
regulator and compare with same in the good power supply, especially the
voltage across the zener diode.

> >
> > Is this the same PSU whose chopper transistor exploded a while back?  Could
> > there be any carbon deposits remaining on the board or conductive
> > remnants wedged under components etc causing leakage from the -12V line
> > to ground?
> 
> The component nearest to the exploded transistor is the 10uF capacitor on
> the outp

[cctalk] Re: Rainbow H7842 PSU Fault

[Peter Coghlan via cctalk]

>
> This evening I went to check Vstart for any oscillation. However, all of a
> sudden, the current draw is down to 85mA and PWM has started working. I am
> at a loss to explain it. I wondered if there might be a dry joint, but I
> have tried a few light taps and shakes and it continues to work. Perhaps
> your idea of some debris causing a short might explain it, otherwise I just
> don't know.
>

This one is a real doozie :-(

>
> I am thinking I may put it back together and test with a light bulb in series.
>

Go for it.  I can't think of anything else to suggest trying at the moment.

Regards
Peter.

Re: Scrounging - was Floating point routines for the 6809

[Peter Coghlan via cctalk]

>
> Or the time an English co-worker related the story surrounding her
> initial job interval in the US.  She described the stunned look on the
> face of the desk clerk at the local Holiday Inn when she asked to be
> knocked up at 7:30 the next morning.
>

This reminds me of the rather surprised look on my Australian colleague's
face when I said I was going to have a root in the cupboard for a missing
manual.

Regards,
Peter Coghlan

Re: Reviving VT220?

[Peter Coghlan via cctalk]

> Hi all,
>
> A colleague and I are trying to get a VT220 working again as it recently
> died on us. We are hoping to set up a few items for the mid-80s
> (including this terminal) to show the graduands what it would have been
> like if they were doing their CompSci degree 30 years ago.
>
> It looks to me like the flyback is dead. There is a lot of soot and
> there looks like there is some damage to the top of the transformer,
> better seen in the second image.
>
> http://www.cs.nott.ac.uk/~psxasj/sparse/flyback1.jpg
> http://www.cs.nott.ac.uk/~psxasj/sparse/flyback2.jpg
>

I think the soot is fairly typical of what accumulates on high voltage
components in a city environment.  I'm not sure this indicates any damage to
it but it may well be faulty without showing any signs of damage.

>
> The terminal powers on and does the usual beeping but nothing is
> displayed on the screen. Does anyone have any advice about what to do
> here? Are there any sources of compatible flyback transformers?
>

I have a VT220 which also appears to have a faulty flyback.  This results in
it drawing too much current from the power supply.  I am not certain the
flyback is faulty but I have eliminated most of the other components which
are likely to be responsible.

I have looked for replacement flybacks or equivelants but I have never found
any.  Someone mentioned that they might have VT220 parts available on the list
some time ago but I didn't get anywhere in following that up.

> 
> We have a second VT220 which exhibits the same behaviour, hopefully for
> a different reason so we can try and cobble two into one.
>

I have a second VT220 which works and I have used it to compare readings
with my faulty one.  I also tried swapping some of the suspect components
but this failed to take suspicion off the flyback.  I am reluctant to try
swapping the flybacks over in case I cause damage to the working VT220.

While the two flybacks appear likely to be electrically similar, I am not
certain of this and they are physically different - one is PCB mounted
and the other is chassis mounted with flying leads to the PCB as far as
I recall.  They have different part numbers, 16-21181-01 and 16-26299-01.

>
> Any thoughts / advice would be greatly appreciated.
>

Sorry, I haven't been able to provide much help or hope.

Regards,
Peter Coghlan

>
> Thanks,
>
> Aaron.
>
>
>
> --
> Aaron Jackson
> PhD Student, Computer Vision Laboratory
> http://cs.nott.ac.uk/~psxasj

Re: Reviving VT220?

[Peter Coghlan via cctalk]

>
> Thanks for the information, Peter. It makes me want to try and test a
> few of the components around the flyback.
>

It would also be good to check the stuff like voltages on the base of the CRT
and that the CRT heater is glowing.  You could have a problem that is nothing
to do with the flyback.  (I'm assuming the brightness control is turned up).

>
> Thanks again,
>

You're welcome.

Regards,
Peter Coghlan.

>
> Aaron.
>

Re: xv and VMS

[Peter Coghlan via cctalk]

>
> Is anyone out there using X11 on VMS and the xv image viewer?
>

Yes.  Why do you ask?

> -- 
> David Griffith
> d...@661.org
>
> A: Because it fouls the order in which people normally read text.
> Q: Why is top-posting such a bad thing?
> A: Top-posting.
> Q: What is the most annoying thing in e-mail?
>

Still a great sig!

Regards,
Peter Coghlan

Re: xv and VMS

[Peter Coghlan via cctalk]

>
> I ask because I've been scraping together patches for xv and collecting 
> them into a Github repo[1].  I've finished adding all the patches 
> collected by Greg Roelofs, patches from OpenBSD, and now I'm working on 
> eliminating unsafe calls like strcpy() and sprintf().  Could I get some of 
> you VMS people to check out my work and submit any necessary changes?
>

Great.  Thanks for asking.  I'll take a look.

>
> My overall goal here is to clean up assorted bitrot and somehow convince 
> John Bradley to loosen the license.  I believe that I have the most 
> up-to-date codebase of xv available at the moment.
>

What's not to like about the license?  I've been using using XV for free for
over 20 years within the terms of the license.  I liked it so much I decided
to register and pay up anyway a few years ago even though the license said I
could continue to use it for free.

>
> [1] https://github.com/DavidGriffith/xv
>
> -- 
> David Griffith
> d...@661.org
>
> A: Because it fouls the order in which people normally read text.
> Q: Why is top-posting such a bad thing?
> A: Top-posting.
> Q: What is the most annoying thing in e-mail?
>

Regards,
Peter Coghlan.

Re: DEC terminals and classic server computing

[Peter Coghlan via cctalk]

>
> The DEC PS/2 keyboards that you could use with the VT510, VT520, VT525
> include the LK450, LK460, and LK461.
>

My VT510 has an LK411.  I don't know if this is what it came with originally
but I suspect it is.  It works fine with it anyway.

Regards,
Peter Coghlan.

RE: Line printer art: (was Re: tape baking)

[Peter Coghlan via cctalk]

>
> >
> > If anyone's running Hercules or some other uses-EBCDIC emulator, here's
> > the link to the .tap image file.
> > 
> > https://app.box.com/s/8rxbihjjnw5zdkesym4cjwqswekufagh
> > 
> > --Chuck
>
> Sadly I don't think that's much use on Hercules as it needs an AWS or HET
> file. Is there a format converter any where?
>

I think I recall managing to do that sort of conversion with VTAPECP which is
part of something called VTAPEUTILS.  It looks like I used version 0.2.

Regards,
Peter Coghlan.

Re: Ibm rs6000 7025-f50

[Peter Coghlan via cctalk]

>
> I have a CD still in the original shrink wrap of 5765-393 AIX 4.1.4
> for Clients, LCD4-0114-00. Can't be that hard to find. I have no use
> for it myself anymore, got rid of my RS/6000 systems a few years back.
>

I have a CD which has "5765-393 AIX 4.1.4 for Server for D5 Processors" and
"LCD4-0115-00" on the label.  I'm not sure where I got it.  I think it was in
a CD drive that I rescued from a skip (dumpster).  There are some light
scratches on it but it looks like it should be ok.  Is this of any use to
anyone?

Regards,
Peter Coghlan.

RE: DECstation 220. Another Impasse

[Peter Coghlan via cctalk]

>
> I have looked at this problem a little more. I have two motherboards,
> neither of which work, but one at least produces a corrupted video pattern.
> The one that works best appears not to be writing to the video memory. When
> I look at the EMEM pin on the Paradise PVGA1A chip I can see a signal but
> the scope shows a trace that is very faint. When I look at the same pin on
> the other motherboard, I get a nice clear bright trace on the scope, using
> all the same settings on the scope. This pin is driven by a non-inverting
> buffer (74LS126). The input side of the buffer is tied to 0V, the enable
> signal comes from a custom gate array. Comparing the buffer's enable signal
> on the two boards I see the same dimming effect on the board with the
> corrupted video pattern, and no dimming on the other board.
>

A dim trace suggests that the trace is changing too rapidly to see it properly.
Try increasing the scope brightness and sweep rate and adjusting the various
triggering options to see if you can get a better trace that reveals what is
really happening at that pin.  It might be useful to try triggering the scope
from whatever clock is used locally as the signal might be synchronised to that.
Once you can see the trace properly, it should be easier to figure out where
it is coming from.

>
> I have checked the other pins on both the buffer and the gate array and I
> don't see anything suspicious.
>

It may be worth looking very closely at the power supply pins for
difficult to see spikes that might be caused by decoupling failures.
It would also be good to make sure ground is really ground at the chips
in question.

>
> I am thinking of speculatively replacing the 74LS126 because I can go and
> buy replacement parts for it, I can't replace the gate array (although I
> could conceivably swap the part on the two boards).
>

If the 74LS126 has some fault at it's input which is affecting the signal
coming from the gate array, it seems to me that it would be more likely
to load it down or up rather than cause it to change rapidly (if that is
in fact what it is doing), unless it has somehow managed to turn itself
into an oscillator.

On the other hand, if the gate array output is open collector, it could be
relying on the 74LS126 to provide a collector load and not getting it if
the 74LS126 is faulty.  This seems unlikely though.

I suppose another possibility is that the gate array output could be
tristate and not enabled leading to noise pickup from nearby traces and
components and perhaps across the PCB surface, given there was a battery
leak at some point.

Getting back to the oscillator theory, I wonder if it is possible that
the non-inverting buffer could be oscillating at a much higher frequency
than normally found on the board due to feedback from it's output to
it's input as result of the battery leakage?

I don't normally like suggesting cutting tracks but if the board already
has damaged and repaired tracks, you might feel ok about cutting the
track between the gate array and the 74LS126 to determine which of them
(or both together) is responsible for the unusual signal there.

I would be very wary of replacing the unobtainable gate array with
your only replacement until all other possiblities are eliminated in
case the gate array was damaged by a fault elsewhere.

Looks like I've provided more questions than answers :-(

Regards,
Peter Coghlan.

Re: Plotter + Tape drive

[Peter Coghlan via cctalk]

>
> It is SCSI.
>

Hi Andy,

In that case, I am interested too.  I am on the East coast of Ireland.
I don't suppose you make regular trips to North Wales or anything like that :-)

Regards,
Peter Coghlan.

Re: HP Draftmaster I, Power Supply repair

[Peter Coghlan via cctalk]

>
> I think that I'm responsible for the dead, as the plotter stood in an attic 
> for 20 years, meanwhile power was raised from 220 to 230v (+-10%) in Europe.
>

The distribution voltage had previously been specified as 220V in some European
countries and 240V in others but the specification was then changed to 230V
in all the countries involved, with a wider tolerance applied.  This allows
new equipment to be designed to a common standard which will work in any of
the countries.  As far as I understand it, there has been no deliberate change
in the actual distribution voltage in any individual European country and the
power companies continue to do what they did before the 230V standard came in.

Regards,
Peter Coghlan.

RE: BitNET (Was: RE: RIP Jerry Pournelle - Firsts)

[Peter Coghlan via cctalk]

Hi Bill and Dave,

I have fond memories of BITNET in a university environment in the early 1990s
on VM/CMS and on VAX/VMS.

Some years ago, I went searching on the web for information about it and found
almost nothing.  It was as if it had been erased from history.  I did find a
single website which contained lots of BITNET user nostalgia but it has since
disappeared.  I managed to pull some parts of it from archive.org with a view
to putting it back together eventually.  I have had no success contacting the
owner.

After a lot of digging, I managed to locate someone who once worked in EARN
(the European arm of BITNET) who had a long forgotten backup of the BITNET/EARN
network definition files which he was willing to share with me.  I haven't
quite figured out how to share these with the rest of the world yet as they
are full of email addresses, some of which may still be valid.

While the original BITNET was NJE over bisync lines, the second generation of
BITNET also allowed the use of NJE over TCP/IP while maintaining compatibility
with NJE over bisync.

The Hercules bi-sync over IP implementation is not suitable for sending over
the internet as it produces vast amounts of traffic when idle and cannot cope
with latency of more than a couple of seconds nor interruptions of any kind.
Neither is it compatible with the NJE over TCP/IP used for BITNET-II.  I have
written a patch for Hercules which allows it to present what looks like a
bisync line on the inside and to speak BITNET-II compatible NJE over TCP/IP on
the outside.  I have also patched the RSCS that comes with the publically
available VM/370 to be able to speak NJE.  Like most of my projects, these are
very close to completion but hung up on some minor snag near the end.  At the
moment, this setup is able to communicate over the internet with RSCS on
VM/ESA running on a P/390 machine and should be capable of talking to anything
else that could have connected to BITNET-II.

Software such as MAILER and MAILBOOK still exists in much updated form but will
not run on VM/370.  I have asked the current maintainers to see if they can dig
up very old versions which would be easier to press into service on VM/370 but
they have not been successful.  The chances of someone finding an old backup
are probably diminishing rapidly.

The software most commonly used for BITNET on VAX and Alpha VMS is called JNET.
A few years ago, I approached the then owner to see if they would license it
for hobbyist use in a manner similar to the VMS hobbyist license. 
Unfortunately they would not.

There is some relatively freely available software for NJE on VMS and unix
called HUJI-NJE.  It is not as capable or polished as JNET but it is workable.
I have made some tweaks to this to make it work on modern versions of VAX and
Alpha VMS but see note above on completion of projects.  HUJI-NJE also inspired
a unix-only enhanced version possibly called FUNET-NJE which was used to great
effect on EARN/BITNET in Finland.  As far as I know, this version is freely
available.  As far as I recall, there is some confusion over names and versions.

Back in the day, I remember BITNET for unix software called UREP.  I have no
direct experience of it but I recall that interaction with it from other BITNET
nodes was pretty awful.  I have not managed to find any trace of it in recent
years - I think it was probably not free software.

It is not necessary for SIMH to support synchronous connections in order to
allow NJE over TCP/IP in and out.

Regards,
Peter Coghlan.

Hi Bill,

 That's a complex topic. Basically, a BitNet connection was an IBM RSCS
link..., 
 At the low levels  that's BiSync. Real BiSync hardware is rare(ish) but the
Hercules Mainframe emulator does support bi-sync over IP so physically its
possible.

 At the networking level, whilst the original RSCS code is available its an
early version and its missing key features needed for Bitnet.
 Also the freely available versions of VM/CMS on which RSCS runs don't have
the E-Mail software used to send messages.
 
Later versions that would work are still licenced materials of IBM. I think
the same goes for the VAX software. 
As it could possibly still be used commercially to connect to current
mainframes it does not appear to be freely available.

I am also not sure how you would connect it to a mainframe, as I don't
believe SIMH supports synchronous connections.

Dave 
 
 

> -Original Message-
> From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Bill
> Gunshannon via cctalk
> Sent: 16 September 2017 01:16
> To: General Discussion: On-Topic and Off-Topic Posts
> 
> Subject: BitNET (Was: RE: RIP Jerry Pournelle - Firsts)
> 
> 
> 
> 
> While all this talk of the ARPANet is cool and brings back some fine
> memories, what about BitNET?  Anyone here remember it?
> Any chance someone has a copy of the source for a BitNET Node?
> I have seen UUCPNet and DECNet revived.  It might be f

Re: H7878 Fails Under Even Moderate Load

[Peter Coghlan via cctalk]

>
> Can anyone explain the behaviour?
>

It's hard to know what to do from a distance but here's what I think I'd look
at if I was faced with this problem.

I think failing under moderate load could be explained by one or more of the
following possibilities and probably others I haven't thought of:

- The power supply is not capable of producing sufficient current.  Check how
  the voltage across the main input smoothing capacitors which have been
  replaced varies while the load is applied.  If it dips severely, check input
  components such as filters, surge limiting devices, connectors and so on for
  breaking down under load.  If you can measure the ripple here while changing
  the load, an increase may indicate that one side of a fullwave rectifier is
  going high impedance or open circuit under load.  Also check for damage that
  might have occurred in the struggle to remove the capacitors.

- Overcurrent sensing is kicking in too soon.  Look for low value, moderate to
  high power resistors in the output current paths and check their values and
  how the voltage across them varies with applied load.  If they seem good,
  check associated small components.

- Regulation is not working correctly.  Try to figure out how the regulation
  is supposed to work and take measurements to see how it is behaving in
  reality.  Easy to say but may be difficult to do in practice.  If the PSU
  uses a chip to provide regulation and drive to a chopper device, the
  data sheet for the chip may provide some guidance on how it is supposed
  to work.  Be careful taking measurements as accidentally shorting something
  out could lead to big bangs.

- The PSU may be looking for feedback from other parts of the machine in the
  form of remote voltage sensing or remote current sensing or inputs which
  cause particular supply lines to be switched on or off or come up in a
  particular order.  If this is the case, the fault may be elsewhere in the
  machine or may be as result of operating the power supply without it being
  connected to the rest of the machine.

Hope this helps.

Regards,
Peter Coghlan.

Re: DECstation 5000/240 Thickwire Connector

[Peter Coghlan via cctalk]

>
> The thickwire connector on my DECstation 5000/240 does not have the sliding
> lock on it, instead it has standoffs with threaded screw holes (like any
> normal serial/parallel connector). This means that the thickwire adapters I
> have won't connect properly because their locking studs are blocked by the
> standoffs on the main board connector. I can remove the standoffs I suppose,
> but I was just wondering if anyone has ever come across this before?
>

This is also the case on the VAXStation/MicroVAX 2000 and on the (E?)ISA
ethernet card in my Alphaserver 2100.

Look for an AUI cable such as the BNE4E-02 which has screws on the right
angle male end and a sliding lock on the straight through female end. 
(I suspect the 02 could mean 2m long.)
 
Regards
Peter Coghlan.

RE: The origin of the phrases ATA and IDE [WAS:RE: formatting MFM drives on a IBM PC]

[Peter Coghlan via cctalk]

> > That concurs with my observation that SCSI was initially an Apple 
> > convention.
> > I can recall conversations about SASI vs. Apple SCSI.
>
> And like Fred, I don't believe that it does any such thing.
>
>Rich

Me too.  Given how Apple mangled the 50pin SCSI connector into a 25pin one,
it is hard to see how they could have been responsible for coming up with it.

Regards,
Peter Coghlan.

Re: The origin of SCSI [WAS:RE: The origin of the phrases ATA and IDE ]

[Peter Coghlan via cctalk]

The biggest problem you had was the requirement to assert ATN when 
selected properly.  Later the tag queuing caused huge headaches as 
manufacturers implemented that feature.


It eventually was made mandatory for the most part by linux, and perhaps 
Windows requiring the tag queuing drilled own to the lowest level of the 
system's use of the disk.  The capability to do that, or fake it is 
required to allow the kernel to queue commands to run, and have the OS 
continue to run till command completion.




I recall VMS having issues with SCSI disks which claimed to do tag queueing
(and bad block replacement) but didn't do it right, before I'd even heard
of linux.

Customers complained that VMS refused to work with commodity SCSI disks
and thought that it was a conspiracy to get them to buy expensive DEC branded
disks.  DEC claimed that only the disks with their firmware did tag queueing
and bad block replacement correctly.  The VMS SCSI driver supposedly had (has?)
a list of specific disks known to mess up which it would refuse to bring
online.

I wasn't well up on Sun but I expect the same issue existed there too.

Regards,
Peter Coghlan.

Re: Aaron Nabil & pdp-8.org

[Peter Coghlan via cctalk]

Jim Stephens wrote:

On 10/5/2017 11:28 PM, Vincent Slyngstad via cctalk wrote:

From: "Zane Healy via cctalk" 
Sent: Thursday, October 05, 2017 9:09 PM
I'm not sure if Aaron's website was offline prior to the last couple 
days or not, as I honestly don't remember the last time that I 
visited, but it's offline now.  Aaron once upon a time, used to work 
for what was a really great ISP, aracnet.com, and that's apparently 
where his website was still being hosted.  Aracnet has vanished under 
rather suspicious circumstances, and with it, both Aaron's website, 
and my DEC emulation pages website.


I'm not sure what's going on, but Aaron's website seems to be working 
for me this evening.


   Vince

Not even slightly working for me.  pdp8.org entered in a browser times 
out.  There are bits and pieces on archive.org.  I tried on a remote 
system in case there was a problem with the domain local to my house.


If the website is something other than the above, that would be my 
problem.  ( I also tried pdp-8.org, times out as well).  pdp8.org is 
registered to Aaron.


Time: 12:30am 10/6/2017 PDT

thanks
jim



Same here.  I get a "server failed" DNS error when trying to look up pdp8.org.

However, one of my ISP's nameservers still has the ip address cached for
pdp8.org as 216.99.193.149.  The pdp8.org webserver is still active on that ip
address, however I cannot display the site in a browser because there are
multiple websites on that ip address and the default one is some art gallery
rather than pdp8.org.

It looks to me that if the domain name hosting is moved elsewhere, the problem
should be sorted out.

If there is a danger of the webserver going away, putting 216.99.193.149 into
a hosts file for pdp8.org (and maybe www.pdp8.org) might allow a browser to
find the site.

Regards,
Peter Coghlan.

Re: DEC Alpha 3000 and OpenVMS 8.4

[Peter Coghlan via cctalk]

Douglas Taylor wrote:


I'm trying to bring up an Alpha machine, a 3000-300, the hardware is 
working fine, but I have some questions-


The system disk is an RZ26 (1.06GB) and I installed the OS and one 
layered product and the disk is full.  How much disk space is needed for 
8.4?




I have several of those disks, some in Storageworks bricks.  Back in the
VMS V7 era, I thought they would be just right for installing a system
on and not much else.  On more than one occasion, just after I had
everything set up on it just the way I wanted it, the disk started clocking
errors and shortly after that, it died in clunk-clunk-clunk mode :-(

I'd suggest looking for another disk that isn't an RZ26.

I also have at least one 4GB disk (RZ28 or RZ29? I can't remember) which
works fine once it gets going but needs a tap or a quick twist to get
it going when it is trying to spin up.  Not too practical for installing
in a machine with the cover on though.

Regards,
Peter Coghlan

Re: tandem computers cartridge tapes

[Peter Coghlan via cctalk]

Camiel Vanderhoeven wrote:
>
>On 10/14/17, 7:55 PM, "cctalk on behalf of steve shumaker via cctalk"
> wrote:
>
>
>>On 10/14/2017 10:42 AM, Camiel Vanderhoeven wrote:
>>> On 10/14/17, 7:22 PM, "cctalk on behalf of steve shumaker via cctalk"
>>> 
>>>wrote:
>>>
>>>
 found as part of the stack saved from an engineers estate: Tandem
 Computers cartridge tapes and docs

 Cartridges are appox 4x4x1 and the tape appears to be half inch.   Some
 are in boxes marked "Cartridge Tape CT-130"  All have labels indicated
 they are Tandem Computers "Site Update Tapes" with 1988, 89, and 90
 dates.  Cursory google produced nothing. Secondary label on one
 cartridge would seem to indicate they are used for software
distribution.
>>> Sounds like DLT tapes.
>>>
>>>
>>>
>>Similar shape and overall size but the shutter and leader hook are
>>different
>
>The only other format I know of that era that comes close is IBM 3480, but
>that¹s less square, more like 5² x 4². Do you have pictures online
>somewhere?
>

I have an autoloader SCSI tape drive and two sample tapes which I am told came
from a Tandem system.  The label on one of the tapes says:

"3M Royal Guard tm 1/2" tape cartridge 3480 compatible"

Regards,
Peter Coghlan.