Replacing Electrolytic Capacitors on VAXmate Video Board

Rob Jarratt robert.jarratt at
Mon Dec 30 12:12:50 CST 2019

I have my VAXmate in pieces at the moment because the PSU has failed. I am
still working on finding the fault there, although it seems to be the
crowbar circuit that is shutting it down. To work out the problem I am going
to buy a DSO (Rigol DS1054Z) as my regular scope is analog and not much use
in detecting one-off events. It is an odd fault because there was a definite
bad smell after the failure, but I can't find any blown components. Whatever
blew up has presumably taken something on the PSU board with it, but I don't
know what yet. The PSU shuts down even with none of the other VAXmate boards
attached (and using a scratch HDD as a load).


Anyway, while I have the machine open, I thought I would look at the video
board capacitors too. Before the PSU failed, I noticed a bit of an
occasional wobble in the screen image and a narrowing of the image at the
top of the screen. I have measured ESR on all the electrolytics. All the
larger capacitors seem to have a low ESR compared to the table printed on my
ESR meter, some of the small ones (15uF/16V sort of size) have an ESR closer
to the values printed on my ESR meter. I don't really want to replace
everything if I don't have to, but equally I don't want any kind of failure
to ruin an irreplaceable component like the transformer. Should I just
re-cap the lot, or just the physically smaller capacitors?





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