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

[Mattis Lind]

lördag 10 oktober 2015 skrev Josh Dersch <derschjo at gmail.com>:

>
> > On Oct 10, 2015, at 4:15 AM, Mattis Lind <mattislind at gmail.com
> <javascript:;>> wrote:
> >
> > 2015-10-10 5:42 GMT+02:00 Josh Dersch <derschjo at gmail.com <javascript:;>
> <mailto:derschjo at gmail.com <javascript:;>>>:
> >
> >> 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!



Looking yet another time at the schematic it is evident that a lot of
signal is relying on E5 collector beeing low. If the monitor circuit is
broken or not high enough voltage in (since you are running it off a
variac) you will not have this signal. To test I would ground this and run
it with a variac.

/Mattis


> - 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
>
>