Re: Replacement for a DEC 7474 Chip

[Chris Zach via cctalk]

I'll have to check when I get back from Norway but I believe I replaced thirty 
or so flip flops with 74ls74 chips with no problems. This was on my pdp8l and 
granted Dec used a lot of 4 and 8 input nand gates to drive loads 

On May 15, 2022 12:08:55 AM GMT+02:00, ben via cctalk  
wrote:
>On 2022-05-14 11:50 a.m., Nigel Johnson Ham via cctalk wrote:
>> AFAIR LS can only drive one unit TTL load.
>>> paul
>LS is 4 TTL, 4 ma low.
>Was there a trick of forcing the output of D flip flip
>to clear it? I was wondering if this is what kills all
>the 7474's?
>Ben.
>
>
>

-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.

RE: Replacement for a DEC 7474 Chip

[Rob Jarratt via cctalk]

Oh dear, while I was ordering an original 7474 I ordered some other parts
that were connected to the same bad chip in case other chips are damaged,
and I ordered a Fairchild 74LS08! I will ask them to change it for a
Motorola part they also have.

> -Original Message-
> From: cctalk  On Behalf Of dwight via
cctalk
> Sent: 14 May 2022 23:36
> To: Paul Koning via cctalk 
> Subject: Re: Replacement for a DEC 7474 Chip
> 
> What ever you do, don't use a Fairchild part. When I worked for Intel in
the
> 80's, we finally band using Fairchild for any latching device. They failed
on
> pullup current, even when the parts were sent back and they claimed they
> were good. We just gave up on them, we couldn't hold production while
> they figured it out.
> We had a similar problem with PowerOne, a manufacture of power supplies.
> Since it was a custom supply, we had to send someone to their plant to fix
> their final test.
> Dwight
> 
> 
> 
> From: cctalk  on behalf of Nigel Johnson
> Ham via cctalk 
> Sent: Saturday, May 14, 2022 10:50 AM
> To: Paul Koning via cctalk 
> Subject: Re: Replacement for a DEC 7474 Chip
> 
> AFAIR LS can only drive one unit TTL load.
> 
> I may have some 7474, even of that vintage, if you cannot find any
anywhere
> else.
> 
> cheers,
> 
> Nigel
> 
> 
> Nigel Johnson, MSc., MIEEE, MCSE VE3ID/G4AJQ/VA3MCU Amateur Radio,
> the origin of the open-source concept!
> Skype:  TILBURY2591
> 
> 
> On 2022-05-14 13:48, Paul Koning via cctalk wrote:
> >
> >> On May 14, 2022, at 1:41 PM, John Robertson via
> cctalk  wrote:
> >>
> >> On 2022/05/14 10:11 a.m., Rob Jarratt via cctalk wrote:
> >>> Hello,
> >>>
> >>> I have found a bad DEC 7474 chip on my M7133 board. Clearly it is a
> >>> 7474 D flip flop. The problem is I don't know which
> >>> modern series would be the best one to replace it with. I am sure I
> >>> have seen a list somewhere of modern equivalents for some DEC chip
> >>> numbers, but I can't remember where.
> >>>
> >>> If it helps at all, on the PDP 11/24 printset it is E78 on page K6
> >>> of the schematic (p157 of the PDF).
> >>>
> >>> Picture of the failed chip here:
> >>> https://rjarratt.files.wordpress.com/2022/05/damaged-dec-7474-4_li.j
> >>> pg
> >>>
> >>> Can anyone tell me what the best modern equivalent is likely to be?
> >>>
> >>> Thanks
> >>>
> >>> Rob
> >>>
> >> You are stuck with using an original 7474 family assuming this is
driving
> other early TTL. 74LS74, and others simply don't have the drive capability
to
> work.
> > I know LS has less fanout, but is it not able to drive plain 74xx at
all?  That
> doesn't sound right.  If the circuit in question runs near the fanout spec
of
> plain 74 the yes, 74LS won't work.
> >
> > Spec sheets and the actual schematic will give a definitive answer.
> >
> >paul
> >
> >

Re: Replacement for a DEC 7474 Chip

[Eric Smith via cctalk]

Those should be fine. Only the more complex parts had issues, not the
simple gates.

On Sun, May 15, 2022, 02:04 Rob Jarratt via cctalk 
wrote:

> Oh dear, while I was ordering an original 7474 I ordered some other parts
> that were connected to the same bad chip in case other chips are damaged,
> and I ordered a Fairchild 74LS08! I will ask them to change it for a
> Motorola part they also have.
>
> > -Original Message-
> > From: cctalk  On Behalf Of dwight via
> cctalk
> > Sent: 14 May 2022 23:36
> > To: Paul Koning via cctalk 
> > Subject: Re: Replacement for a DEC 7474 Chip
> >
> > What ever you do, don't use a Fairchild part. When I worked for Intel in
> the
> > 80's, we finally band using Fairchild for any latching device. They
> failed
> on
> > pullup current, even when the parts were sent back and they claimed they
> > were good. We just gave up on them, we couldn't hold production while
> > they figured it out.
> > We had a similar problem with PowerOne, a manufacture of power supplies.
> > Since it was a custom supply, we had to send someone to their plant to
> fix
> > their final test.
> > Dwight
> >
> >
> > 
> > From: cctalk  on behalf of Nigel Johnson
> > Ham via cctalk 
> > Sent: Saturday, May 14, 2022 10:50 AM
> > To: Paul Koning via cctalk 
> > Subject: Re: Replacement for a DEC 7474 Chip
> >
> > AFAIR LS can only drive one unit TTL load.
> >
> > I may have some 7474, even of that vintage, if you cannot find any
> anywhere
> > else.
> >
> > cheers,
> >
> > Nigel
> >
> >
> > Nigel Johnson, MSc., MIEEE, MCSE VE3ID/G4AJQ/VA3MCU Amateur Radio,
> > the origin of the open-source concept!
> > Skype:  TILBURY2591
> >
> >
> > On 2022-05-14 13:48, Paul Koning via cctalk wrote:
> > >
> > >> On May 14, 2022, at 1:41 PM, John Robertson via
> > cctalk  wrote:
> > >>
> > >> On 2022/05/14 10:11 a.m., Rob Jarratt via cctalk wrote:
> > >>> Hello,
> > >>>
> > >>> I have found a bad DEC 7474 chip on my M7133 board. Clearly it is a
> > >>> 7474 D flip flop. The problem is I don't know which
> > >>> modern series would be the best one to replace it with. I am sure I
> > >>> have seen a list somewhere of modern equivalents for some DEC chip
> > >>> numbers, but I can't remember where.
> > >>>
> > >>> If it helps at all, on the PDP 11/24 printset it is E78 on page K6
> > >>> of the schematic (p157 of the PDF).
> > >>>
> > >>> Picture of the failed chip here:
> > >>> https://rjarratt.files.wordpress.com/2022/05/damaged-dec-7474-4_li.j
> > >>> pg
> > >>>
> > >>> Can anyone tell me what the best modern equivalent is likely to be?
> > >>>
> > >>> Thanks
> > >>>
> > >>> Rob
> > >>>
> > >> You are stuck with using an original 7474 family assuming this is
> driving
> > other early TTL. 74LS74, and others simply don't have the drive
> capability
> to
> > work.
> > > I know LS has less fanout, but is it not able to drive plain 74xx at
> all?  That
> > doesn't sound right.  If the circuit in question runs near the fanout
> spec
> of
> > plain 74 the yes, 74LS won't work.
> > >
> > > Spec sheets and the actual schematic will give a definitive answer.
> > >
> > >paul
> > >
> > >
>
>

Re: Replacement for a DEC 7474 Chip

[Eric Smith via cctalk]

On Sat, May 14, 2022, 16:09 ben via cctalk  wrote:

> On 2022-05-14 11:50 a.m., Nigel Johnson Ham via cctalk wrote:
> > AFAIR LS can only drive one unit TTL load.
> >> paul
> LS is 4 TTL, 4 ma low.
> Was there a trick of forcing the output of D flip flip
> to clear it? I was wondering if this is what kills all
> the 7474's?
>

I don't think that worked on any TTL (or CMOS) 74x74 flip flops, except
maybe by accident if you shorted the output enough to draw Vcc down (or
ground up) enough to disrupt the FF, and then you have other problems.

Despite the logic diagram showing feedback from the outputs, all 74x74 have
buffered outputs. The recent TI data sheets show an equivalent schematic
only for the 74LS74. I can't at the moment find one for the 7474.

It seems likely to me that early pre-TTL logic families like RTL might have
had FFs with unbuffered outputs, but I haven't checked.

Re: Replacement for a DEC 7474 Chip

[Chuck Guzis via cctalk]

On 5/14/22 23:04, ben via cctalk wrote:

> Faster is not better, bigger is bett... ops wrong topic.
> 
> 74H while slower, and 74S tend to have reliable delay times looking at
> gates with min to max delays < 3 ns. This helped devices to track
> the same speed across a system.
> A 74ABT00 is 1 to 4 ns depending on the temp,phase of the moon,and what
> ever. How much margin do I need to add? Ok I guess for connecting a 386?

Sure, but if the concern was output sink current, the S or AS family can
certainly provide it.  It all depends on the position of the IC in the
circuitry.   Depending on application, you might conceivably get away
with a 74HC74; that is some parameters may not matter.   I might still
have a couple of DM7474's kicking around, but they're so old, I wouldn't
depend on them.   I even have a few Fairchild 74F74s...

--Chuck

Re: Replacement for a DEC 7474 Chip

[Chuck Guzis via cctalk]

On 5/15/22 01:16, Eric Smith via cctalk wrote:

> I don't think that worked on any TTL (or CMOS) 74x74 flip flops, except
> maybe by accident if you shorted the output enough to draw Vcc down (or
> ground up) enough to disrupt the FF, and then you have other problems.

Checking my old Moto databook, the old FFs appear to be capable of
having the outputs jammed.   At first glance, both the 7472 and 7473 FFs
had unbuffered outputs.   Most early latches, likewise.

Of course, the very early TTL was density-constrained, so FFs like the
MC512/MC563 are of course without buffered outputs.

--Chuck

RE: Replacement for a DEC 7474 Chip

[Dave Wade G4UGM via cctalk]

Folks,

I probably have some, and now I have found my ICL House Codes sheet I may even 
be able to identify them.
However I won't be in the same country as my stock for a couple of weeks.
... and they are in the UK so getting them stateside may take some time.

Dave
G4UGM

> -Original Message-
> From: cctalk  On Behalf Of Chuck Guzis via
> cctalk
> Sent: 15 May 2022 16:12
> To: cctalk@classiccmp.org
> Subject: Re: Replacement for a DEC 7474 Chip
> 
> On 5/15/22 01:16, Eric Smith via cctalk wrote:
> 
> > I don't think that worked on any TTL (or CMOS) 74x74 flip flops,
> > except maybe by accident if you shorted the output enough to draw Vcc
> > down (or ground up) enough to disrupt the FF, and then you have other
> problems.
> 
> Checking my old Moto databook, the old FFs appear to be capable of
> having the outputs jammed.   At first glance, both the 7472 and 7473 FFs
> had unbuffered outputs.   Most early latches, likewise.
> 
> Of course, the very early TTL was density-constrained, so FFs like the
> MC512/MC563 are of course without buffered outputs.
> 
> --Chuck

RE: Replacement for a DEC 7474 Chip

[Fred Cisin via cctalk]

What ever you do, don't use a Fairchild part. When I worked for Intel in 
the 80's, we finally band using Fairchild for any latching device. They 
failed on pullup current, even when the parts were sent back and they 
claimed they were good. We just gave up on them, we couldn't hold 
production while they figured it out.

We had a similar problem with PowerOne, a manufacture of power supplies.


I've heard the same thing about Intel, Shugart, Verbatim, Wabash, Tandy, 
Apple, Commodore, VW, Ford, Chevy, . . .


Just about EVERY company has released a bad product, and had their 
reputation damaged.


A better warning is: "Between xxx and yyy, we had a lot of bad parts from 
zzz."


SOME have cleaned up their act, and have made good products subsequently.

Re: Replacement for a DEC 7474 Chip

[Chris Quayle via cctalk]

On 05/15/22 18:00, cctech-requ...@classiccmp.org wrote:

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Today's Topics:

1. Replacement for a DEC 7474 Chip (Rob Jarratt)
2. Re: Replacement for a DEC 7474 Chip (John Robertson)
3. Re: Replacement for a DEC 7474 Chip (Paul Koning)
4. Re: Replacement for a DEC 7474 Chip (Nigel Johnson Ham)
5. Re: Replacement for a DEC 7474 Chip (ben)
6. Re: Replacement for a DEC 7474 Chip (Chuck Guzis)
7. Re: Replacement for a DEC 7474 Chip (ben)
8. Re: Replacement for a DEC 7474 Chip (dwight)
9. Re: Replacement for a DEC 7474 Chip (Chris Zach)
   10. RE: Replacement for a DEC 7474 Chip (Rob Jarratt)
   11. Re: Replacement for a DEC 7474 Chip (Eric Smith)
   12. Re: Replacement for a DEC 7474 Chip (Eric Smith)
   13. Re: Replacement for a DEC 7474 Chip (Chuck Guzis)


--

Message: 1
Date: Sat, 14 May 2022 18:11:32 +0100
From: "Rob Jarratt"
To: "General Discussion: On-Topic and Off-Topic Posts"

Subject: Replacement for a DEC 7474 Chip
Message-ID:<007b01d867b5$a8c90f00$fa5b2d00$@ntlworld.com>
Content-Type: text/plain;   charset="us-ascii"

Hello,



I have found a bad DEC 7474 chip on my M7133 board. Clearly it is a
7474 D flip flop. The problem is I don't know which modern series
would be the best one to replace it with. I am sure I have seen a list
somewhere of modern equivalents for some DEC chip numbers, but I can't
remember where.



If it helps at all, on the PDP 11/24 printset it is E78 on page K6 of the
schematic (p157 of the PDF).



Picture of the failed chip here:
https://rjarratt.files.wordpress.com/2022/05/damaged-dec-7474-4_li.jpg



Can anyone tell me what the best modern equivalent is likely to be?



Thanks



Rob



--

Message: 2
Date: Sat, 14 May 2022 10:41:34 -0700
From: John Robertson
To: Rob Jarratt via cctalk
Subject: Re: Replacement for a DEC 7474 Chip
Message-ID:
Content-Type: text/plain; charset=UTF-8; format=flowed

On 2022/05/14 10:11 a.m., Rob Jarratt via cctalk wrote:

Hello,

I have found a bad DEC 7474 chip on my M7133 board. Clearly it is a
7474 D flip flop. The problem is I don't know which modern series
would be the best one to replace it with. I am sure I have seen a list
somewhere of modern equivalents for some DEC chip numbers, but I can't
remember where.

If it helps at all, on the PDP 11/24 printset it is E78 on page K6 of the
schematic (p157 of the PDF).

Picture of the failed chip here:
https://rjarratt.files.wordpress.com/2022/05/damaged-dec-7474-4_li.jpg

Can anyone tell me what the best modern equivalent is likely to be?

Thanks

Rob


You are stuck with using an original 7474 family assuming this is
driving other early TTL. 74LS74, and others simply don't have the drive
capability to work. You can use 74S74 or 74F74 as they have the same
output current, the "S" is Schottky, and the other is "F"aster.

I find that https:///unicornelectronics.com is a reliable source of TTL.

John :-#)#



Original 74 series TTL can still be found, Ebay often, new old stock
and other surplus vendors.

Would need to take care replacing with later ttl series, as the gate
delay, switching times and fanout probably won't match. Always use the
original series if you can...

Chris

Re: Replacement for a DEC 7474 Chip

[Brent Hilpert via cctalk]

On 2022-May-15, at 1:16 AM, Eric Smith via cctalk wrote:
> On Sat, May 14, 2022, 16:09 ben via cctalk  wrote:
>> On 2022-05-14 11:50 a.m., Nigel Johnson Ham via cctalk wrote:
>>> AFAIR LS can only drive one unit TTL load.
paul
>> LS is 4 TTL, 4 ma low.
>> Was there a trick of forcing the output of D flip flip
>> to clear it? I was wondering if this is what kills all
>> the 7474's?
> 
> I don't think that worked on any TTL (or CMOS) 74x74 flip flops, except
> maybe by accident if you shorted the output enough to draw Vcc down (or
> ground up) enough to disrupt the FF, and then you have other problems.
> 
> Despite the logic diagram showing feedback from the outputs, all 74x74 have
> buffered outputs.

Per TI schematics from 1969: 74 standard, H and L series flip-flops are 
unbuffered. Or at least many of them are/were, in their then-original form. 
Including 7475, 7490, etc. The output transistors connect both to the pins and 
wrap back to form the FF or other purposes.

Collector-triggering was discussed a some years ago on the list in regards to a 
pdp8 front panel where DEC used collector-triggering on 74175's (IMO, bad 
design practice). From (my) empirical tests at the time, it turned out some 74S 
(Schottky) parts could be collector-triggered. However, between standard, LS, 
and S types, behaviour could vary with manufacturer and production date.


> The recent TI data sheets show an equivalent schematic
> only for the 74LS74. I can't at the moment find one for the 7474.

> It seems likely to me that early pre-TTL logic families like RTL might have
> had FFs with unbuffered outputs, but I haven't checked.

Re: Replacement for a DEC 7474 Chip

[Paul Koning via cctalk]

> On May 15, 2022, at 2:51 PM, Chris Quayle via cctech  
> wrote:
> 
> ...
> Original 74 series TTL can still be found, Ebay often, new old stock
> and other surplus vendors.
> 
> Would need to take care replacing with later ttl series, as the gate
> delay, switching times and fanout probably won't match. Always use the
> original series if you can...

That's sensible advice.  When you can't, faster devices with adequate fanout 
should work.  The exception would be in circuits with hairy timing, where the 
design relies on the actual stage delays of the components.  Most people don't 
build that way, for good reason.  Seymour Cray is the best known exception; for 
example, the CDC 6600 design is very thoroughly dependent on the circuit and 
wire delays, and either slower OR faster will break things.  And actually, it 
isn't clear to me why the thing ever works; the design files clearly 
demonstrate clashing signals that somehow apparently don't show up in reality.

paul

Re: Cctalk subscription disabled

[Eric Christopherson via cctalk]

On Wed, May 11, 2022, 4:06 PM Dave Wade G4UGM via cctalk <
cctalk@classiccmp.org> wrote:

> Jay,
> Thanks for that, I hope its fixed,
> Dave
> G4UGM
>
> > -Original Message-
> > From: cctalk  On Behalf Of jwest--- via
> > cctalk
> > Sent: 11 May 2022 13:18
> > To: 'Tom Hunter' ; 'Adrian Stoness'
> > ; cctalk-ow...@classiccmp.org
> > Cc: 'General Discussion: On-Topic and Off-Topic Posts'
> > 
> > Subject: RE: Cctalk subscription disabled
> >
> > Yes, Adrian is correct – known for some time but my time/focus has been
> > elsewhere. Addressed yesterday, should make gmail less fussy.
>

Jay,

You may also want to look at the timing the list software uses for deciding
when to notify people they've had excessive bounces; I got this on April
*24*, fully two weeks after I stopped getting messages; a similar thing
happened on March 21, but that one was only one week late:

Your membership in the mailing list cctalk has been disabled due to
excessive bounces The last bounce received from you was dated
10-Apr-2022.  You will not get any more messages from this list until
you re-enable your membership.  You will receive 1 more reminders like
this before your membership in the list is deleted.

>
> >
> >
> > J
> >
> >
> >
> > From: cctalk  On Behalf Of Tom Hunter
> > Sent: Wednesday, May 11, 2022 7:06 AM
> > To: Adrian Stoness ; cctalk-ow...@classiccmp.org
> > Cc: General Discussion: On-Topic and Off-Topic Posts
> > 
> > Subject: Re: Cctalk subscription disabled
> >
> >
> >
> > I wonder if this problem is related to DMARC anti-spam.
> >
> > Maybe the following would help the list admin:
> >
> >
> >
> > https://wiki.list.org/DEV/DMARC
> >
> > https://dmarc.org/wiki/FAQ
> >
> >
> >
> > 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
> > mailto: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


-- 
Eric Christopherson

Re: Cctalk subscription disabled

[Dennis Boone via cctalk]

 > You may also want to look at the timing the list software uses for
 > deciding when to notify people they've had excessive bounces; I got
 > this on April *24*, fully two weeks after I stopped getting messages;
 > a similar thing happened on March 21, but that one was only one week
 > late:

Modern anti-spam measures, especially google's, have gotten fairly
non-deterministic, at least to outward inspection.  It is especially
difficult to reason about delivery timing.  For example, the list server
may well have had the notice in queue trying to get google to accept it
for a number of days.  Google is known to hold things for several days,
progressively showing them to a few more people, until it decides
whether they're spam or not.  A single-address notice wouldn't seem to
be a candidate for that behavior, but _I_ wouldn't bet any money against
google comparing such a single-address message against other singles
that were statistically (or machine-learning-y) similar.

Yes, two weeks seems long, but if a number of list posts were queued up
on the list server not getting delivered to you for some days, and
google deferred some of them, and google deferred the notice, and google
held the notice after accepting it, that could actually pretty easily
add up to more than two weeks.

Mailman sends the notice at the point it makes the decision.  There are
some knobs for how many bounces over what time period result in action.

De

Re: Replacement for a DEC 7474 Chip

[Eric Smith via cctalk]

I specifically said 74x74. Early TTL flipflops were very crude by
comparison.

On Sun, May 15, 2022, 13:03 Brent Hilpert via cctalk 
wrote:

> On 2022-May-15, at 1:16 AM, Eric Smith via cctalk wrote:
> > On Sat, May 14, 2022, 16:09 ben via cctalk 
> wrote:
> >> On 2022-05-14 11:50 a.m., Nigel Johnson Ham via cctalk wrote:
> >>> AFAIR LS can only drive one unit TTL load.
> paul
> >> LS is 4 TTL, 4 ma low.
> >> Was there a trick of forcing the output of D flip flip
> >> to clear it? I was wondering if this is what kills all
> >> the 7474's?
> >
> > I don't think that worked on any TTL (or CMOS) 74x74 flip flops, except
> > maybe by accident if you shorted the output enough to draw Vcc down (or
> > ground up) enough to disrupt the FF, and then you have other problems.
> >
> > Despite the logic diagram showing feedback from the outputs, all 74x74
> have
> > buffered outputs.
>
> Per TI schematics from 1969: 74 standard, H and L series flip-flops are
> unbuffered. Or at least many of them are/were, in their then-original form.
> Including 7475, 7490, etc. The output transistors connect both to the pins
> and wrap back to form the FF or other purposes.
>
> Collector-triggering was discussed a some years ago on the list in regards
> to a pdp8 front panel where DEC used collector-triggering on 74175's (IMO,
> bad design practice). From (my) empirical tests at the time, it turned out
> some 74S (Schottky) parts could be collector-triggered. However, between
> standard, LS, and S types, behaviour could vary with manufacturer and
> production date.
>
>
> > The recent TI data sheets show an equivalent schematic
> > only for the 74LS74. I can't at the moment find one for the 7474.
>
> > It seems likely to me that early pre-TTL logic families like RTL might
> have
> > had FFs with unbuffered outputs, but I haven't checked.
>
>

PDP-8 Memory

[Mike Katz via cctalk]

I have recently acquired a PDP-8/A to go with my PDP-8/E.

Unfortunately it came with the industrial front panel and not the 
programmers panel.


Does any one have an extra PDP-8/A programmers panel they can spare?

Also, does any one have any PDP-8/A memory they can spare (or any 
Omnibus memory).


Thanks,

   Mike

Identifying an 1802 system

[John Ball via cctalk]

I just received a machine that someone found at a Vancouver second hand
store that I basically told them to buy on the single fact I don't own any
COSMAC machines and I've now had a chance to take it apart and photograph
the boards. It seems to be some sort of a kit system that the previous owner
put into a custom enclosure along with a power supply, a cassette interface
and the keyboard off of some other machine or terminal I can't identify but
the interfacing absolutely screams it was something they had available from
scrap.
It's 1802 based, has a single 2716 EPROM and a pair of Intersil D2114 1024 x
4 SRAM chips. The keyboard interfaces through a  CDP1851 PIO. The other
board on the almost empty backplane was a video board and this is what made
me think it's not just a fancy ELF clone. You have six more 2114's and an
MC6847 video display generator. The output of that seems to go to an RF
modulator-on-a-chip and then out to an F-type coax. The cassette interface
is soldered directly to the backplane but its edge connector pinout is
awfully similar to the Commodre C2N interface. The boards are all nicely
made photo etch with drilled and plated vias which leads me to believe this
was a purchased kit system and the chips all date to between 1979 and 1982
but the one thing I am missing is a brand or trademark so I don't even know
what I'm looking at. The only thing I got to go on is the name "Color
Machine" which I have not yet found anything about. I have not yet had a
chance to dump the EPROM and I'm still testing to make sure nothing will
blow up in a smoke test, plus I need to dig out a monitor with RF. I have
posted board photos over in
https://forum.vcfed.org/index.php?threads/unknown-1802-based-system.1237931/
post-1256906 in the hopes someone recognizes it but does this look familiar
to anyone here?

-John