Replacing Electrolytic Capacitors on VAXmate Video Board

[Rob Jarratt]

> -----Original Message-----
> From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Rob Jarratt via
> cctalk
> Sent: 30 December 2019 18:13
> To: General Discussion: On-Topic and Off-Topic Posts
> <cctalk at classiccmp.org>
> Subject: Replacing Electrolytic Capacitors on VAXmate Video Board
> 
> 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?
> 


Does anyone have any thoughts on this? I am not sure how extensively to
replace the electrolytics.

Thanks

Rob