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The measurements of GeoffreyReed seem to be a good starting point. Isn't there anything written on the diodes themselves? If it immediately blows a fuse, I'd start checking if any electrolytics or semiconductors in the primary part of the circuit have blown / are shorted like the bridge...

The PSU lives again and so does the Mac! :) I've thoroughly cleaned both PCBs with IPA and removed most of the dusty flux residue. Then tested it on the bench; now it reliably turned on and off. But strangely, when put back in its housing, it did its old trick again, turning off immediately...

techknight, you had hit the nail on the head concerning Q1, Q2, Q3 and the SMD electrolytics! Q1 is indeed a Zener diode "183", but the board designers have routed a track through its unconnected pin 2. So from the layout it appeared that all three pins were in use, while in reality only pins 1...

Thank you! I've tested Q101, Q301 and D101 out of circuit with a component tester; they're OK. IC101 has no short between T1 (pin 2) and T2 (pin 3). D301, all diodes and resistors measure OK in circuit. None of the caps or transformers have shorted. I'll try and desolder the two daughterboard...

Hi there, it's my first post! :) Some time ago, the PSU in my IIsi suddenly turned itself off while running. Recently I opened it up to try and fix it and found 4 leaked caps on the secondary stage. The electrolyte had corroded through the legs and affected the solder mask and some pins on...