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  1. 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 rectifiers, the main switching transistor, the clever automatic bridge rectifier/voltage doubler changeover IC (STR 80145) etc.
  2. 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 when /PFW is removed from PSTRICL. So the housing definitely affected the behavior of the circuit. At first I suspected a short circuit but eventually I discovered that the effect was caused by the earth connection. The PCB rests on three big tabs inside the housing. The one near the output wires connects th
  3. 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 and 3 are used for the diode and pin 2 is soldered to an unrelated track. Same story for Q2: it's a zener diode "133" but unconnected pin 2 is connects to an unrelated track. Q3 is a digital PNP transistor "14". I've replaced the two SMD electrolytics on the daughterboard and they were indeed co
  4. 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 caps tomorrow. Regarding Q3, yours has SMD code 133? Because mine's etched 14: https://cdn.pbrd.co/images/QzeJlakf2.jpg I'm surprised that Q1 and Q3 are diodes, as their three leads follow different tracks. The A6 diodes have pins 1 and 2 connected for example.
  5. 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 the bottom side. No unrepairable damage though. Cleaned the mess off, resoldered the oxidized wires and replaced the bad caps. Then tested the bare PCB on my desk with a fog light as load on the +5. It worked absolutely fine again: briefly touching /PFW to PSTRICL turned it on, touching /PFW to GND turned it
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