DEC H7104-D power supply tips?

Mattis Lind mattislind at
Tue Mar 15 00:46:21 CDT 2016

tisdag 15 mars 2016 skrev Josh Dersch <derschjo at>:

> Hi all --
> My call for a VAX-11/750 a month or so ago actually bore some fruit
> (locally, even!) and as of a couple of weeks ago, I now have a very nicely
> configured 11/750 system taking up most of the basement.  The previous
> owner got it after it was retired from a local(ish) university in the mid
> 1990s and it has not been powered on since then.  Apparently at the time of
> its retirement the power supplies were exhibiting "random issues."  (No
> more detail is available than that on the history...)
> At any rate, I went through the two power supplies (and the small pilot
> supply in the power controller) and found a lot of leaky capacitors (as in,
> yellow/brown goo was coming out of maybe 2/3 of them) so I went ahead and
> recapped the whole thing.
> At the moment I have things running on a dummy load in the 11/750
> chassis.  (the harnesses are still hooked to the chassis backplane, but all
> cards have been pulled, and the backplanes thoroughly checked for bent
> pins, etc.) The H7104-C (2.5V) supply seems to be working fine but the main
> 5V supply in the H7104-D is not doing so well (and as a result the other
> voltages it's supposed to be producing are also not present).  The Power
> Controller lights up the "Reg. Fail" lamp (I don't know why the 5V Fail
> lamp isn't also on) and the 5V supply emits a loud (somewhere around
> 400Hz?) whine/squeal.  I get about .3V out of it with a load.  Without a
> load there's no squeal and I get about 5.6V, but that's not particularly
> useful.
> I've double-checked everything in the H7104-D and there's nothing
> obviously wrong (no caps installed backwards, no scorched components).  At
> the moment the H7104-D is hooked up only to a dummy load, so it's not
> anything on the backplane shorting out or causing issues.
> This is another one of those cases where I've gotten myself in over my
> head with large, complicated power supplies -- anyone have any experience
> with these?  Any tips?

You can read the story of my 11/750 power supply repairs in this article:

Loud squealing noise was in my case due to short-ciricuit output rectifiers
for the 5V. Check them. Then also chech the main chopper transistors.


> Thanks as always,
> Josh

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