Hello, This seems to be a common problem but I still haven't found the answer I'm looking for. In my case something got into the power on switch and caused it to stick in the "on" (closed) position. the computer would start up and then shut down after a few seconds. I didn't know what was happening so I repeated the start up. The computer quit trying to boot after several times trying to figure it out. Then the it would power up the fans, the cdroms, the power LEDs, but I would get no signal to the monitor. I then replaced the Motherboard with a known good one and went through the same process until the new one quit the same way. That's when I found the switch was shorting and I replaced it with an identical brand new (normally open, momentary on) switch that I purchased from my local electronics supply.
Have I fried 2 good motherboards or is there a fix? I am a disabled-retired electronics technician with circuit board repair experience. I just need to know what circuit or IC to look for.