DEC H7104-D power supply tips?

Josh Dersch derschjo at
Tue Mar 15 21:14:47 CDT 2016

On 3/14/16 10:46 PM, Mattis Lind wrote:
> 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.
> /Mattis

Thanks to you (and others) for tips.  I used the desoldering station at 
work today to remove the transistors for testing (the "motherboard" PCB 
in the 7104 is very thick and I was having no luck getting all the 
solder out of the vias with my tools at home).  I found one bad 2N6547 
in the lot (everything shorted to everything else) so I'll be replacing 
that tomorrow after a stop at Vetco on the way to work to pick up a 
replacement.  All of the diodes that I checked looked OK.  Fingers 
crossed that the transistor is the only issue here.

Thanks again,

>> Thanks as always,
>> Josh

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