Another Unrelated PSU Question

Rob Jarratt robert.jarratt at
Sat Apr 18 14:23:05 CDT 2020

> -----Original Message-----
> From: cctalk <cctalk-bounces at> On Behalf Of Brent Hilpert
> cctalk
> Sent: 18 April 2020 18:58
> To: General Discussion: On-Topic and Off-Topic Posts
<cctalk at>
> Subject: Re: Another Unrelated PSU Question
> On 2020-Apr-18, at 10:03 AM, Rob Jarratt via cctalk wrote:
> > I have another PSU I have been meaning to look at for a long time.
> > This one has fairly high output ripple and some of the voltages do not
> > appear to be where they should be. I have checked all the capacitors
> > for ESR and they appear to be OK, with the exception of the two big
> > smoothing capacitors on the primary side. One of them appears to be
> > slightly bulging, but has low-ish ESR, the other has a much higher
> > ESR. Is it possible that these capacitors could be the cause of the
> outputs?
> What frequency is the ripple - mains frequency (50/60Hz) or switching
> frequency?

It looked like switching frequency to me. I think I should replace the two
primary capacitors anyway though.

> If the ripple is at mains frequency then, yes, the problem could be the
> mains/primary filtering. (Primary supply drooping far enough across mains
> cycles that the switcher is affected and unable to maintain drive.)
> Either way, you could scope the primary supply under operation (some
load), to
> see what the ripple there looks like.

I daren't put it under load in the machine itself because some of the
outputs are too far out of tolerance and it has damaged some memory modules.
I can use the load module I have from a MicroVAX 2000, but for the other
outputs I may try to lash up some resistors perhaps.

If anyone has a MicroVAX 3100 Model 95, I would really like to know which
wires on the connector are which voltage, because some seem to be too far
out to identify the nominal voltage.



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