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

Mattis Lind mattislind at
Sat Oct 10 06:15:22 CDT 2015

2015-10-10 5:42 GMT+02:00 Josh Dersch <derschjo at>:

> 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

> 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 (
> 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

> 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.

> 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

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