VAX 4000-500 PSU Overload?

Holm Tiffe holm at
Fri Dec 25 07:15:27 CST 2015

Robert Jarratt wrote:

> Good news! It looks like I have managed to get this working again. I
> realised I had not checked the electrolytic capacitors on the main logic
> board, ones situated far from the main power output boards. I found one
> which had a higher ESR than two other identical ones, although the ESR was
> still theoretically OK according to the table on my meter it was at the
> limit, so I replaced it and the PSU now appears to work. It really goes to
> show, *all* electrolytic capacitors should be checked. Picture showing the
> one I replaced here:
> Interestingly, I had had a similar failure in another of these PSUs, which I
> handed to a friend along with a VAX 4000-300  I gave him, in the hope that
> he would be able to fix it. As that PSU was already partially dismantled
> from my previous investigations, I asked to borrow it back for the purposes
> of doing the reverse engineering. I noticed that the same capacitor on that
> PSU also has a higher ESR, but again within range of what should be OK. So,
> looks like I might try this as a fix for the second bad PSU too, if he is OK
> with me trying that.
> Regards
> Rob

This Capacitor was bad in my PSU too and I've changed it.

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?

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
desing anyways...



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