If I remember correctly I had a problem in this area that turned out to be a loose/dry joint where the two thermistors that are across the relay joined together. In my case it was an intermittent fault though.
Regards Rob > -----Original Message----- > From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Josh Dersch > Sent: 10 October 2015 16:46 > To: General Discussion: On-Topic and Off-Topic Posts > Subject: Re: DEC H7140 (11/40) power supply revisions and general advice? > > > > On Oct 10, 2015, at 4:15 AM, Mattis Lind <mattisl...@gmail.com> wrote: > > > > 2015-10-10 5:42 GMT+02:00 Josh Dersch <dersc...@gmail.com > <mailto:dersc...@gmail.com>>: > > > >> Hey all -- > >> > >> > > First of all I haven't poked around in the PSU of my 11/44 since it > > worked fine when powered up. But the design of the H7140 is a lot > > similar to the design of the H7104, the PSU of the VAX-11/750 which I > > worked quite a lot with. > > http://www.datormuseum.se/computers/digital-equipment-corporation/vax- > > 11-750 > > <http://www.datormuseum.se/computers/digital-equipment-corporation/vax > > -11-750> > > > > > >> Once again, I find myself in over my head debugging a power supply, > >> this time an H7140 from a PDP-11/44. Here's the skinny: > >> > >> I examined the supply physically before experimenting and found a > >> capacitor on the Bias/Interface board that was leaking, bursting and > >> rather burned-out looking (not a great sign) -- this is capacitor C4 > >> in the printsets on Bitsavers ( > >> http://bitsavers.informatik.uni-stuttgart.de/pdf/dec/pdp11/1144/MP008 > >> 97_11X44sys_Dec79.pdf) Everything else looked OK physically; I > >> replaced the obviously bad capacitor at C4. > >> > >> When power is applied (plugged in, breaker switch flipped to "On") > >> the relay does not click - based on my readings of the manuals this > >> should happen after the bias voltages are up to spec. I measure > >> 308VDC on the lugs on the top of the memory board, so that's at least > something working. > >> Getting to other points to test voltages is a bit more difficult, > >> especially with those high voltages in the way, what a nice design :). > >> > > > > The previous owner said that it happens that the relays get stuck. The > > relay is there to limit the inrush current so when it clicks it > > bypasses a couple of power resistors. Regardless relay state you will > > have the full voltage over the capacitors. (at least this is the case > > in the H7104). If the relay get stuck then the power resistors get > > very hot. The relay drive circuit detects that the voltage is high enough > > then > turns on the relay. > > Maybe something in this circuitry is bad. In this case you could force > > it to on but then I think you should power it from a variac > > Thanks. I should have mentioned, but I tested out the relay and it’s not > stuck > and seems to be working fine. It definitely looks like the relay control > circuitry > is not running properly. > > > > > > > > >> Switching the front panel switch to "Local" (or any other position) > >> has no effect -- no fans, no LEDs, nothing. I've double-checked all > >> the wiring and everything looks OK. > >> > > > > Have you verified that you have the 12V BIAS voltage? If not you could > > probably generate the 12V BIAS voltage using a lab power supply just > > to see if the main switcher is working properly. > > > > I used an insulation transformer and a variac and then forced the > > relay drive to on. In the H7104 the same circuitry also enabled the > > main switcher so unless I did this there were no fun at all. I > > supplied 12V using a bench supply. It made it possible to check > > everything was working ok without any high voltages. > > Interesting idea, I’ll look into that. Thanks! > > - Josh > > > > > > > >> > >> Capacitor C4 looks to be involved with the START-UP DRIVE signal > >> circuitry (which drives the relay) so the behavior I'm seeing makes > >> sense if C4 died and took a couple of things with it (or if something > >> else died and took C4 with it). > >> > >> Here's where it gets kind of odd -- I spent some time testing diodes > >> and transistors in the related area near C4 and while doing so I > >> noticed that there are four diodes (D1-D4) listed on the schematic > >> that are missing from my board. "Missing" as in someone clipped them > >> out at some point -- there are just nubs of the leads left. I'm not > >> sure why this would have been done, but there were a number of ECOs > >> applied to this board (a few wires and resistors added) and I don't > >> want to assume that if I just put four new diodes in that it won't > >> cause other problems. Anyone know if there were other revision > >> levels of the bias/interface board that would have done away with > >> these diodes? Anyone have an H7140 they can easily crack open to > >> compare? (It's actually relatively easy to get to, if you can get to > >> the > >> supply...) > >> > >> Thanks as always, > >> Josh
