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I got a bit further - I found a trace that had been eaten through, patched it, now the Mac starts off both keyboard and back power button!  Unfortunately, still no video or boot though. :( That indicates to me the possibility of more traces.. but so far I'm not seeing any that are obviously...

I was reading this very good guide: https://www.downtowndougbrown.com/2015/03/explanation-of-the-macintosh-iiiix-power-onoff-circuit/ I can see 7V at pins 1 and 14 of UB2, as expected.  When I press either of the power buttons, we still get 0V on pin 3 (output).  So I've been trying to figure...

Thanks muchly.  Yeah I am investigating these... I had a similar problem on my Lisa where battery acid had destroyed a whole raft of traces.  It's a bit harder for me to see with the Mac II given how small some traces are.  It looks like I have some corrosion problems also down by C14-C18.  ...

Hey guys, So I have this Mac II that I've had for eons, and it sat for about 8 years and the caps went bad.  I can see some light corrosion here and there, including the legs of UB1.  Anyway, it would not power on.  One of the PRAM batteries was dead, I replaced both with 3.6V (they were 3.0V...