VAX 4000-500 PSU Overload?

Robert Jarratt robert.jarratt at
Fri Dec 25 14:48:51 CST 2015

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at] On Behalf Of Holm
> Sent: 25 December 2015 13:15
> To: General Discussion: On-Topic and Off-Topic Posts
> Subject: Re: VAX 4000-500 PSU Overload?
> This Capacitor was bad in my PSU too and I've changed it.

In the light of what you say below, do you mean you only changed this one,
or did you change all of the ones on the logic board? Does that include the
really big ones too?

> So finally, what was wrong with my idea that you sould simply change all
> reachable electrolytic capacitors in that PSU to get it working again?

Nothing at all was wrong with your idea. I had indeed followed your advice
on the 12V board, on the basis that I could reach them and that it seemed to
be the 12V side that was having the problem, so implying that other parts of
the PSU were OK. I did not change any on the 5V side as that side seems to
be OK and I don't really like the approach of pulling and soldering them
from above all that much.

> In my Eyes it really isn't worth the effort of testing each one, change
them all
> to get that thing working halfways reliable again, it is an really bad
> anyways...

Yes, and I may yet have to change those on the 5V side, because after
posting the good news, I had one brief episode where the PSU switched itself
off again after running for about a minute. On powering it up again it
worked for a good hour without issue. So perhaps I should bite the bullet
and replace those too, and also the other small electrolytics on the logic
board too.

However, I don't think I will replace the really big ones. Generally, I get
the impression that these big ones suffer fewer problems, certainly their
ESR is fine. I am also reluctant to change them because they are generally
connected to large heat-sucking planes, making them hard for me to desolder
without making a mess of the board and possibly damaging it too.



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