I've come across this forum when I found a similar question but wasn't able to solve my problem
I have a //c I haven't used in a few years, but last I did it was fine. I plugged it in a few weeks ago and for the first few minutes it was fine, booting a few disks, but then the picture started to go wavy and I had to power cycle and it was fine. After a few more it stopped putting out video and trying to boot off of the disk drive, with only the green power LED light up.
I checked the two 15v pins on the power supply, both read 19.7v, which I see is normal for this type of supply. The 7905 regulator though showed about 12v on pin 1, and 7v on pin 3, which I know is out of range 4.8 to 5.2. So I bought a new 7905, an on semi 7905 bt -5v (on left). I swapped it in, turn it on, and had picture and booted. However after that one time, it's dead again, only power LED, no drive activity or video.I checked the voltages on the new 7905, and it shows 11.9 on pine 1 and 6.94v on pin 3, so same numbers as before. I have the black common probe on pin 2, but also note it doesn't show negative?So I thought maybe power supply, I bought a new modern replacement from eBay, it shows 15.5v on output, same voltages as before though at 7905, almost 7v on pin 3. I'd opened the case on the power supply module on the MB and didn't see any obvious bad caps
I'm not sure what to try next, if anyone has any suggestions?