My 15" iMac G4 (700MHz) won't power up. When I first plug it in, you hear what sounds like some kind of reset sounds coming from either the optical combo drive or the hard drive and then that stops. The fan seems to work and the red light is on under the service cover on the bottom of the iMac. When you press the power on button, nothing else happens. I tried resetting the PMU and it didn't make a difference. Also tried powering up without the RAM plugged in, no change. The PRAM battery seems to work (3.65V ?).

My first thought is either the power supply or the motherboard; can anyone suggest any way determine which it could be (or if it might be something else)? BTW, another data point; pressing the caps lock key on the keyboard doesn't result in the caps lock light coming on.