> 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/MP00897_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