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 <[email protected]> Sent: 06 September 2025 07:11 To: General Discussion: On-Topic and Off-Topic Posts <[email protected]> Cc: [email protected]; Jon Elson <[email protected]> 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 <[email protected] <mailto:[email protected]> > wrote: On 9/5/25 18:04, Rob Jarratt wrote: >> -----Original Message----- >> From: Jon Elson <[email protected] <mailto:[email protected]> > >> Sent: 04 September 2025 15:32 >> To: [email protected] <mailto:[email protected]> ; 'General Discussion: >> On-Topic and Off-Topic Posts' >> <[email protected] <mailto:[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] >>>> <mailto:[email protected]> > >>>> Sent: 03 September 2025 15:39 >>>> To: [email protected] <mailto:[email protected]> >>>> Cc: Jon Elson <[email protected] <mailto:[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? 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
