> -----Original Message----- > From: Jon Elson <[email protected]> > Sent: 04 September 2025 15:32 > To: [email protected]; 'General Discussion: On-Topic and Off-Topic Posts' > <[email protected]> > Subject: Re: [cctalk] Re: Repairing an Olivetti M24 PSU > > On 9/3/25 11:18, Rob Jarratt wrote: > > > >> -----Original Message----- > >> From: Jon Elson via cctalk <[email protected]> > >> Sent: 03 September 2025 15:39 > >> To: [email protected] > >> Cc: Jon Elson <[email protected]> > >> 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 > 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? Thanks Rob
