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My iMac will switch itself off after a few hours of running

My iMac G5 20" will switch it self off after a few hours of running and I had to wait for a day or so to restart it again - I think it was running rather hot. I can not use the the normal way of starting it (pressing the start button) I had to take the back of rested on a pair of books and use the little button/switch near the four diagnostic LEDs. The fan which suppose to cool the processor is probably not working as well as it should- hence the overheating I can understand that, but why it will not start by pressing the start button?? Is it just a mechanical part or is there some circuitry attached to it? More importantly, can I fix it?

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before considering anything else with this model, you need to look at the capacitors on the logic board and within the power supply. with these models, its not a matter of if they go bad, its when will they go bad. These Capacitors are used to control power throughout the system.. you want to look for bulges or leaks on the caps themselves..

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Thanks Matthew, As far as I could see there are no leaks or bulges on the capacitors. By the way, the Mac is working perfectly after the bizarre starting ritual. Can I replace the main start button??

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The power button on this iMac really just bridges those two pins on the back, with everything unplugged, it may be worth cleaning the contacts with some isopropyl alcohol.

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try installing iStat Pro - this is a dashboard widget which will show you internal temperature sensor readings and also fan speeds - you can then see for sure if the fans aren't running or if your internal temperatures are getting abnormally high

might be your power supply shutting down due to overheating?

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Eureka! That did the trick. Thank again. Now what is left is to replace the processor cooling fan - any ideas how to get one??

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jiri will be eternally grateful.
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