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Got an old iMac G3 that has developed a problem. On start up it reaches the grey screen, Apple logo and spinning gear wheel there is an audible click and it switches off.

The machine's not worth a big fix but if it could be something quick and easy that would be nice.

Perhaps a PMU reset, but I don't recall how that's done on iMac G3s.

I would try Big Mac's idea. On the earliest iMacs you have to disembowel the whole thing to get at the PMU. Later models, flip it over, open up the cover, the PMU is right next to the ram chips.

Hi

Luckily it is one of the later iMacs so the PMU and battery are reasonably accessable. Replaced the battery and reset PMU but it's still doing it.

"Just" before it powers off a black square appears, sort of like a KP but with no text.

any other ideas

Yuck, sounds like a power supply problem. I gotta tell you, it is not going to be worth it. There are so many components to try and test.

From posts like this one in the past, it sounds like the Flyback Transformer (or whatever it is) for the screen is failing. If this is the case, then you will be SOL short of replacing it or hooking up another monitor to the iMac. If you can bear trying to find similar posts for this on MacNN using their search, you might find more useful info on the symptoms and how to deal with it.

Yeah. ebay is chokka with G3 iMacs of much higher spec than this one. Quick and dirty solution seems to be, buy 700Mhz model for £15 of the bay and swap the HDD out.