My G4 iBook is currently broken. On poweron the fan is running at full speed, and not even the openfirmware sound can be heard.
It looks like a wellknown bug of the logic board, because sometimes i can get it to boot, if i put pressure on the parts left of the touchpad.
Corey Arnold has a detailed description how to “fix” it, but it looks like i have to look at a new model soon :-(.