Spontaneous Shut-off problem (power supply?)

Question

Hi-- I have a first generation G5 iMac.
Last year, there was about a week in which it would spontaneously shut off. I read there were possibly some power supply issues with the early units, so I took it in to an Apple Store hoping they would diagnose such a problem and fix it; they ran it for a couple hours, said there were no problems, and sent me home. Over the last month, it has started happening again, and has happened at least seven times. I do regular backups and I've been lucky, but I know one of these days it's going to happen while I'm doing something important. I wouldn't mind taking it back to the Apple Store, but it's not an easy trek, and it happens so unpredictably that I think the odds of getting it to do it at an appointed time is slim, in which case I'm afraid they're just going to send me home again.
Just wanted to know if anyone else has dealt with such a problem, what the fix was, and how I might convince the tech guys to delve a little deeper even when they don't actually witness the event.
Thanks,
dave

Answer 1

Miriam--
Dave, if, in fact,as Charlie suggests, the cause of
your shutdowns is a variable power supply, it should
easily be cured by the addition of a Universal Power
Supply
While an excellent idea (I wouldn't have a computer without connecting it to a UPS any more), a UPS won't necessarily solve the problem of the unexpected shutdowns. My iMac was on a UPS and it still would shut down.
There are two kinds of UPSs. The most common is a battery backup. Most of the time, the computer is actually getting power from the mains. If the UPS detects a failure of the mains, it'll switch to providing the computer with battery power. But there will be a very small fraction of a second where there will be not power at all supplied to the computer. If the computer's power supply is healthy, it should be able to bridge that tiny interruption until it starts getting power from the UPS. But if it's not healthy enough, the computer will shut down.
The second kind of UPS is what you are describing, where the UPS provides power to the computer at all times. These aren't as common, though, and are more expensive. I'm not sure if APC even sells this kind. At least, I couldn't find any at their web site that I could be sure were this kind.
When I had the shutdown problems with my iMac, I initially thought it might be a problem with the UPS, since the iMac would shut off if the power was interrupted long enough for the UPS to notice. I could easily test it by just pulling the plug going to the UPS. Earlier, I had been able to do that without problems in order to test a script I was writing to shut down the computer in case the of a power failure (for some reason, the software built into OS X to do this wasn't working right).
But when I was having the shutdowns, the iMac would shut off right away as soon as I pulled the plug on the UPS. Luckily, I was able to test my UPS at work on another computer and try a UPS from work on my computer and rule out the UPS. Now that my power supply unit has been replaced, the iMac will once again survive the switch from the mains to the UPS's battery.
charlie