DEC H7140 (11/40) power supply revisions and general advice?

[Robert Jarratt]

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-bounces at 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 <mattislind at gmail.com> wrote:
> >
> > 2015-10-10 5:42 GMT+02:00 Josh Dersch <derschjo at gmail.com
> <mailto:derschjo at 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