VAX 4000-500 PSU Overload?

Robert Jarratt robert.jarratt at
Wed Dec 23 14:08:54 CST 2015

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.



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