Power Mac G4 doesn't start anymore

I have an old Power Mac G4 here that stopped working recently. It just doesn't start anymore when pressing the power button, not even a light or anything. It just doesn't react to the power button at all.

I have an old Power Mac G4 Digital Audio here that doesn't start anymore. The suspicion fell immediately on the power supply, as that had failed before and was already exchanged once. This time we bought an adapter that allows the use of a standard ATX power supply, but that didn't work as well. Nothing at all happens when pressing the power button, not even the fan of the new power supply starts.

I tried a PMU reset in a few variants, with or without removing the battery. This changed nothing. I also checked the battery itself, and it only provides around 3V, not 3.6V as it should.

• Can the battery alone cause this problem? I would expect battery problems to occur later, and not stop the computer completely from starting.
• Are there any ways to circumvent the power button and give the signal to start up the computer directly?
• Any other ideas on what the exact problem could be, or how I could troubleshoot this further?

Solution 1:

• Battery alone? Eh, probably not. There should be at least some signs of life if the battery were dead - I would expect the power supply to come on, at least.

• Circumvent button? Yep. Often, Apple engineers will leave little debug buttons or contacts on the motherboard that allow you to power it up. They might be buttons, or they could be two little contacts you have to bridge with a screwdriver.

Note: To do that, you would have to have the machine connected to power and open. Don't touch anything inside the machine while connected to power. That shouldn't need explaining, but anyway...

• Other ideas? Fried MB. Seriously. I had an iMac G5 (and I think and iMac G4 too) die of a fried motherboard, and it had the exact same symptoms. Before it died, there were some, uh, weird graphics problems. Like the volume control overlay coming up black and not going away. Stuff like that.