I have had a power up problem on my G4 iMac that has got progressively worse until the other day we had a prolonged power outage and it would not start at all. Tired PRAM reset and logic board reset - nothing. Changed PRAM battery today (nothing). Tried warming the power supply with hairdrier through top vents nothing. New power supply needed methinks and in researching came across this thread where Marco talks of re-setting the PRAM by plugging in at the same time as pressing the power on button. Hadn't heard of this technique but having nothing to lose tried it and hey presto it started up! Now it might have been a combination of factors that got it going (I'm afraid my wife needs it so not prepared to switch it off to see if I can replicate the technique but I hope by relating this history it is of help to someone who is having a similar issue