5 Sep
2025
5 Sep
'25
6:37 p.m.
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>
Sent: 04 September 2025 15:32
To: rob(a)jarratt.me.uk; 'General Discussion: On-Topic and Off-Topic Posts'
<cctalk(a)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>
Sent: 03 September 2025 15:39
To: cctalk(a)classiccmp.org
Cc: Jon Elson <elson(a)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