Ps if you didn’t get continuity for a moment in one direction then the cap is open. If you get continuity in both directions then it’s shorted. Hope this helps.
Sent from my iPhone > On Aug 31, 2025, at 13:28, Wayne S <[email protected]> wrote: > > Rob, replace c21 because of the bulge. If you want to check an electrolytic > cap with a ohmmeter ,(kinda hard with a digital one), connect the meter > across the ends. You should get either infinite resistance or a momentary > reading of zero going back down to infinite resistance as the cap charges. > Then reverse the leads and it should be infinite from the start. > On an old analog meter with a dial, you can quickly tell by see the needle > deflect full scale in one direction then settle back down to infinite. I keep > an old meter around just for this purpose. > > > Sent from my iPhone > >> On Aug 30, 2025, at 23:01, Rob Jarratt <[email protected]> wrote: >> >> >>> >>> -----Original Message----- >>> From: Wayne S <[email protected]> >>> Sent: 29 August 2025 22:57 >>> To: [email protected] >>> Cc: General Discussion: On-Topic and Off-Topic Posts >>> <[email protected]>; Robert Jarratt <[email protected]>; Scott Baker >>> <[email protected]> >>> Subject: Re: [cctalk] Re: Repairing an Olivetti M24 PSU >>> >>> Hookup your scope across the 5v and gnd on J254 and see what the trace >>> looks like. Also do +12 v. >>> And -12v on j100 >>> That will tell you if one rail is bad. >>> You should have not much ripple. >>> If there is a lot, trace that voltage rail back. >>> >> >> Thanks for the suggestions, Wayne. Something changed in the behaviour. I am >> getting some very erratic behaviour and I wonder if I am making a mistake. >> >> After getting your suggestion I checked the outputs again, because, to my >> surprise, I got some output. With no load all the outputs seemed correct, >> but as there was no load I switched on the PSU for only the briefest of >> periods. Then I added a test load and the outputs were generally just about >> in spec except for the 5V output, although +15V was also just fractionally >> below spec. With the test load, the 5V output only produced +3.9V. I used a >> 0.5R load for a 10A current, the spec says min current 6.2 and max 16.8A, so >> I wasn't overloading it. I was wondering if the fact that I had removed and >> put back some parts might have cured a dry joint, and in fact I had found >> one connector was not well attached. The +5V ripple seemed OK, the spec says >> 50mVp-p, but I was getting 400mV spikes every 16us. This is what the scope >> showed me: >> https://rjarratt.wordpress.com/wp-content/uploads/2025/08/5v-output-0.5r-load-1.png. >> One of the output caps, C21, has a slight bulge, so I will replace that. >> >> Then I noticed that I had forgotten to put back some of the components that >> I had partially removed for checking. I thought that putting them back might >> resolve the low voltage on the 5V output. But, instead, it stopped working >> altogether. The +5V output is at 0V and +12V is at 0.75V. The components I >> had lifted were R60 and D66. This made me think that maybe the SCR was being >> triggered. I lifted them again, but now the outputs are still stubbornly at >> 0V (or thereabouts). I tried removing the SCR altogether but this made no >> difference. Then at other random times the +12V will go to about 7.7V, while >> the +5V output will stay at 0. This is all quite baffling. The signal on the >> rectifying diodes seems unchanged, so something is wrong on the secondary >> side. >> >> One possible candidate is C21. I have removed it and tested it out of >> circuit, but I think my instruments may be struggling to cope with the >> relatively high capacitance value (10000uF, not 1000uF as marked). My ESR >> meter gives wildly varying ESR values, my DMM can't measure its capacitance, >> even though it is rated up to 9999uF, but if I measure resistance, it shows >> open circuit which is what I would expect. At the very least C21 is suspect >> and as it has a very slight bulge, I am going to get a new one. Otherwise, I >> am not really sure what could randomly affect the output so radically. It >> would seem though that something is shorting +5V to GND because the >> rectifying diodes are getting a signal from the output transformer. >> >> I am going to get a replacement for C21, and in the meantime I am continuing >> to investigate, but any suggestions very welcome! >> >> For reference I am referring to the parts in the marked up schematic here: >> https://rjarratt.wordpress.com/wp-content/uploads/2025/08/m24_psu_schematic-marked-up.jpg >> >> Thanks >> >> Rob >> >>
