DEC H7104-D power supply tips?

Josh Dersch derschjo at
Mon Mar 14 23:10:15 CDT 2016

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?

Thanks as always,

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