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Small update. My UC3843 was definitely stuffed - bad readings out of circuit compared with a new UC3843. But there are other unknown problems - WIP.

I think I’ve honed in on the UC3843. I’ve removed it from the PCB and my understanding is that I should get 5V on pin 8 (VRef) if I supply 15.5V on pin 7 (Vcc) (Gnd to 5). But my little chip measures 0V on pin 8. I’ll have to send for a replacement chip all the way from Yankee Doodle land ☹️

Thanks Mitch - I’ll prioritise investigation of the LM339.

I'm a relative novice but I've managed to get mine to the point where I have +5VSB out and PFW activity when powering on but my +5V, +12V, and -12V are still all zip. To get to that stage I had to replace a lot of the zener diodes, some diodes, caps, and some resistors which had all degraded in...

Gah! ☹️

Thanks Mitch - good tips 👍 I’ll probably have to intelligently guess the values of some of the Zeners as the blown glass takes the markings with them. But will have to find other shorts first. I only have enough Power Supply knowledge to be dangerous so will be taking my sweet time and with an...

Gah - full diagnostics mode this weekend! Many thanks Mitch :) Here's what I'm facing (before cleanup)...

Helpful, not that distant into the future! I'm trying to restore a Quadra 800 with the SMP-220DB PSU. Fortunately I was able to test the logic board works well with a IIci PSU. But... the SMP-220DB PSU is a completely different story - it's rusty, mega dirty, and at one point a lot of the Zener...