One other thing… With it powered on and measuring voltage at the outputs but not at the plugs, wiggle the plugs a bit and see if there wierdness with them. plugs and connectors go bad often and it doesn’t seem obvious that they do but saves a lot of time just by checking them now.
Sent from my iPhone > On Aug 31, 2025, at 13:34, Wayne S <[email protected]> wrote: > > 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 >>> >>>
