18 Apr
2020
18 Apr
'20
10:03 a.m.
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 out-of-spec outputs?
Thanks
Rob
18 Apr
18 Apr
10:23 a.m.
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 out-of-spec outputs?
In my experience, the next step after bulging is either leaking and a sticky
mess on whatever is underneath or a big bang and sticky mess everywhere.
I'd get rid of the bulging one even if it seems otherwise good, epecially if
leaving it powered for anything other than very short periods. It will
probably heat up over time and build more pressure.
The high ESR capacitor can't really be contributing much to smoothing so
I think it needs to be replaced before any further investigating can be done.
With any luck, that might sort out all the issues but even if it doesn't, it
would have to have been eliminated anyway.
Regards,
Peter Coghlan
Thanks
Rob
10:57 a.m.
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 out-of-spec outputs?
What frequency is the ripple - mains frequency (50/60Hz) or switching frequency?
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.
12:23 p.m.
-----Original Message-----
From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Brent Hilpert
via
cctalk
Sent: 18 April 2020 18:58
To: General Discussion: On-Topic and Off-Topic Posts
<cctalk at
classiccmp.org>
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
out-of-spec
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.
Regards
Rob
2100
days inactive
2100
days old
3 comments
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participants (3)
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bhilpert＠shaw.ca -
cctalk＠beyondthepale.ie -
robert.jarratt＠ntlworld.com