5 Sep
2025
5 Sep
'25
11:35 p.m.
Thanks Scott. I removed a couple of other caps on the 12V output, tested them and then put
them back as they seemed OK. I have tested the ESR (in circuit) on all the other
electrolytics and they all seemed fine. I tested a few resistors but not particularly
rigorously though.
The resistive load I applied was calibrated to the specs of the PSU, above the minimum
load and below the maximum. I didn’t load the -12V and +15V outputs though, only +5 and
+12. The -12V output has no minimum current spec.
I can always consider the meanwell supplies but so far I have been able to repair my PSUs
and I hope to do the same here.
I am going to try installing it back in the machine today and see how it fares.
Thanks
Rob
From: Scott Baker <smbaker(a)gmail.com>
Sent: 06 September 2025 07:11
To: General Discussion: On-Topic and Off-Topic Posts <cctalk(a)classiccmp.org>
Cc: rob(a)jarratt.me.uk; Jon Elson <elson(a)pico-systems.com>
Subject: Re: [cctalk] Re: Repairing an Olivetti M24 PSU
When a power supply gives me trouble, I often pull all the electrolytic capacitors, test
them, and then install new ones if I have them (because if you're going to solder a
capacitor in, you might as well solder a new one in). A couple times, it hasn't been
the big filter capacitors that were the problem. There's some risk in
"shotgunning" component replacement like I'm suggesting in that you might
make something worse by replacing a perfectly good capacitor with a modern replacement
that isn't quite the same. But it has a good track record for me...
Also, old resistors can drift or open, so you might want to scrutinize those too.
What value is your resistive load? Some switchers specify a minimum load.
Another option is just to see if you can find a meanwell open-frame power supply or
similar at mouser or digikey that's about the same size and has the right specs and
replace it. Had to do this with an Epson QX-10 switching supply. Even though I had the
Sam's Notes for it, I gave up trying to find the obsolete semiconductors that it
needed. Smaller, more efficient, cooler than the vintage power supply.
Scott
On Fri, Sep 5, 2025 at 6:56 PM Jon Elson via cctalk <cctalk(a)classiccmp.org
<mailto:cctalk@classiccmp.org> > wrote:
On 9/5/25 18:04, Rob Jarratt wrote:
Many switching supplies have VERY tricky startup circuits
that tickle the controller to start and then power the
controller from an auxiliary winding on the transformer.
Some of these circuits might require some capacitor to fully
drain before they can start properly again. Some of these
circuits may have a really high resistance value that
charges the controller's supply cap, and then a transistor
turns off that path when the controller is running to avoid
overheating that resistor. These circuits will be at mains
potential so be careful when testing them.
Jon
-----Original
Message-----
From: Jon Elson <elson(a)pico-systems.com <mailto:elson@pico-systems.com> >
Sent: 04 September 2025 15:32
To: rob(a)jarratt.me.uk <mailto:rob@jarratt.me.uk> ; 'General Discussion:
On-Topic and Off-Topic Posts'
<cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org> >
Subject: Re: [cctalk] Re: Repairing an Olivetti M24 PSU
On 9/3/25 11:18, Rob Jarratt wrote:
can I recognise this in the future?
Switching power supplies generally radiate a ton of electrical fields at their
switching frequency. If you see insanely high frequencies in these
measurements, you can usually assume they are radiated interference. You
can also turn on the scope's bandwidth filter. I did see REAL ripple in one of
the traces, there were long straight lines with slight tilt between the noise
pulses, those are the real ripple.
Improving the ground connection at the scope probe also helps. The power
supply injects currents into the ground terminal due to capacitance between
output transformer windings, and these current flowing in the probe's ground
braid contaminates the measurement. Possibly running a HEAVY copper braid
between the scope's ground terminal and the power supply ground will
reduce the effect.
Jon
Thanks Jon and Wayne for all the advice, I will try to remember for the future.
I thought that replacing two big output capacitors on the +5V output had fixed the issue
but it hasn't. The problem seems to be intermittent. I tested it a couple of days ago
with a simple resistive load on the +5V and +12V outputs. The first two times I switched
it on it operated normally. The third time I got no output at all. It seems to start
working again after I leave it switched off for a few minutes. Over the last couple of
days I have tested it multiple times very briefly with the resistive load and it has
worked every time.
However, I am doubtful that it will always work. I don't think it can be the SCR
because I had similar symptoms with the SCR removed, so I am wondering if there is
anything else I could check for that might have this kind of all or nothing intermittent
behaviour?
-----Original Message-----
From: Jon Elson via cctalk <cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org>
>
Sent: 03 September 2025 15:39
To: cctalk(a)classiccmp.org <mailto:cctalk@classiccmp.org>
Cc: Jon Elson <elson(a)pico-systems.com <mailto:elson@pico-systems.com> >
Subject: [cctalk] Re: Repairing an Olivetti M24 PSU
Those are not real. They are conducted interference from the
switching supply getting into the scope preamp via the ground lead.
I have seen this MANY times, ignore it.
Jon
Thanks Jon, obviously I don't have enough experience to know this. How