iMac shuts off at random, won't reboot

M

minicoopertx

Registered
I have a G5 iMac, 1G ram, with three external HD, a MIDI keyboard, and an all-in-one Canon print/copy/scan, running Leopard.

Lately, my iMac has been turning off seemingly at random, like someone has hit the power strip (although other things plugged into the strip stay on).

Sometimes it will reboot when I turn it back on, sometimes not. When it doesn't, it will get partway through startup, then turn off again. There is no apparent pattern to the point during startup where this happens... it's different every time. Sometimes, it barely spins the HD before going dark again.

I took off the back and checked the power supply LED. It says the power supply is OK. I rebooted via the internal power button, and everything was fine for a couple of days.

I set the preferences to "reboot after power failure" so it will reboot when I'm at work. I can go through the Time Machine backup list and see how many times it shut down and rebooted during the day (TM backs up once per hour, so when there is a significant length of time between backups, I know it shut down for awhile).

The only recent addition is the installation of MoneyWell software, but this app isn't always running when the compy goes black. Could software be the culprit, or is something ominous happening in the hardware?
 
I'd put my money on the fact that it's suffering from bad capacitors. My 17" iMac G5 revision B running at 2 GHz had this problem twice in the last 6 months. Open it up again and check....if you find any capacitors that are bulging or have leaked the electrolyte, then you're affected.

You should be covered under the following repair extension program from Apple:
http://www.apple.com/au/support/imac/powersupply/repairextension/

Be sure to print this out, along with this article from Popular Science regarding the temperatures on the iMac G5:
http://www.popularmechanics.com/technology/upgrade/1707756.html

You'll note that in the article, they show a section of the inside of the iMac where it gets the hottest, as well as where the capacitors have bulged or leaked. On my iMac G5, that was the area where the capacitors were bulging.

Take all of this information with you to either the Apple Store or any authorized Apple repair center and let them know that you've done the research about it. I did this and they easily took care of my iMac even though it wasn't listed in the range of serial numbers (this problem seems to affect ALL iMac G5 models).

Good luck.
 
After it has shut down without you asking for it to do that, open console applicaiton (/Applications/Utilities) and grep for previous shutdown.
It should show a line with teh shutdown cause number. What will it show? It will be just the lines before it is restarting.
 
Gia, wouldn't the logs be a valid case if the iMac went through a shutdown process instead of just abruptly shutting off (which is what happened with my iMac)?
 
They still give some clues of what was reported as the shutdown cause. So if it would be capacitors, RAM, video RAM, a blackout, capacitors overheating or something else, it should show something interesting. Even shutdown cause 3 is informative...
 
Ah. I remember going through the logs when I had the shutdown but didn't find anything out of the ordinary. The only thing I saw was some errors regarding the USB port and my MIDIsport USB 2x2 MIDI interface. After that was the normal bootup process.

The only bad thing as time progressed was the kernel panics, and then it just wouldn't boot anymore. That was before the logic board and PSU were replaced the first time. The second time (about 4 months later) the iMac would just automatically reboot abruptly (no shutdown process) and then it stopped turning on at all. So far, it's running fine after the 2nd replacement. Let's hope it's the last at least in my case.
 
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