I have a Powerbook 3400c that I thought had power board issues. There was PRAM battery corrosion on part of the board near (and a little under) the PCMCIA enclosure. I cleaned most of it off as well I could.
The power board seemed to whine when the adapter is plugged in. I replaced it but the new board also seems to whine. I think, and was told - though I have no real evidence, that this is a capacitor issue. I replaced the power board partly because the three silver capacitors on top (or bottom when it's plugged in) looked like they were starting to develop gunk. Two of the capacitors on the new one also look like they've gone. So I'm curious if anyone can offer specifics on debugging this symptom.
I also took some voltage measurements, which I did not do systematically. So if anyone could offer advice on where to take measurements, that would be helpful (and also what they should be).
And then just in general: help... The whining of the power board is really the only consistent response currently. Sometimes there's also a sort of pop/power-on noise probably coming from the screen speakers as currently it's basically just the logic board, fan, and screen inside the bottom of the case.