1 Jan
2020
1 Jan
'20
4:41 p.m.
-----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