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 off. After ten minutes it still worked fine, so overjoyed I put the thing back in its enclosure. Then tested again: /PFW to PSTRICL still turned it on, but as soon as I let the connection go, it turned off.
Puzzled by this sudden change in behavior, I touched /PFW to GND to see if that would reset something. Or so I thought, because it wasn't GND -- I took the wrong hole in the connector and instead touched /PFW to -12... No smoke but then it didn't start at all anymore. My heart sank when I realized my mistake -- all my meticulous work, ruined by a single lapse of attention! About a minute later the main fuse F101 suddenly blew, completing the misery. I was quite upset by my stupid mistake and I'm still mad at myself right now...
I later reverse engineered most of the main PCB and the daughterboard PCB. All parts on the main PCB seem to test OK, so I'm not sure why the fuse blew. On the daughterboard (https://cdn.pbrd.co/...s/5dxpafFDF.jpg) I found three SMD parts with one open PN junction each. Alas Q1 and especially Q2 have such unclear markings that I can't guess what they are, thus halting my resuscitation endeavour.
Hence my questions:
- Can someone identify Q1 and Q2? Their markings may be clearer on your own unit than on mine. Q1 may be "18" or "183". Q2 is very unclear but might be "13" or "132". Q3 is definitely "14" (etched) although it's not visible in this pic.
- Does anyone have a spare PSU for sale, or the daughterboard from a broken one?
- Converting a regular new ATX power supply to IIsi may just involve some rewiring and a logic inverter between /PFW and PWR_ON. Indeed I found this post: https://68kmla.org/f...adra-630/page-3 Is this approach known to work for a IIsi?