IMac DV won't power up

Question

My iMac DV 400 won't power up. When I press the power button, the light comes on for a second or so, and I can faintly hear the drive start to spin up but then before the computer is able to check any other hardware, before it makes the standard "bong" sound, it just shuts off. I can hear it make a faint clicking sound just before it shuts off. Sounds like a switch is opening. If I press the power button again, nothing happens. I have to unplug the computer and plug it back in to try to power up again, but it just does the same thing.
The computer is plugged directly into the wall (not through a ups or any other device). I tried with a different power cord, but no impact.
Any ideas for what to do to correct this problem?

Answer 1

I have heard reports suggesting that a problematic PMMU can cause the logic board battery to run down in a matter of days. At least, I recall an Apple Knowledgebase article about Power Mac G4's that said a crashed PMMU, if left in that state, drains battery in 4-6 days. I don't know if this applies to the iMac, but logic board batteries are cheap and worth a try.
If you have already updated the firmware for OS X, then there is no need to do it again. The current firmware updater is the one that's posted to Apple's site. It has not changed since 2001.
You can try the usual things that people try when machines don't power on: reseat RAM, swap out RAM, remove and reinstall the logic board, blow on it, whack it, dust it (not with a vacuum -- with canned air), check connections, check fuses on the analog board, disconnect all drives, replace the drive with a test one to see if the drive is the problem, etc.
If these are not the problem, you can repeat some steps (to see if it is an intermittant fault). Otherwise, you will need to download the appropriate service manual from this page:
http://www.whoopis.com/computer_repair/
Detailed therein is the way in which you can test (using a voltmeter, and with extreme care) whether the analog board/the power supply are good. Conducting this test will require you to have the machine powered on and will put you inches away from 26kV of voltage used to drive the CRT (at the back of the tube and on some of the components). Personally, I have never done this because I consider it too risky. If you are experienced with electronics, go ahead, otherwise I'd wait until someone else on the forum take a look at this and suggest things that I might have missed.
Based on what you said though -- clicking sounds, and power off shortly after powering up, I am guessing an internal short somewhere is causing a safety mechanism to trip out and power down your computer, esp. when it will ignore the power button. Check for a loose screw (pick it up and jiggle it, do you hear a loose part?) Check for things that might be causing a short. If you don't see anything obvious then it might be a component failure (specifically, a capacitor, a chip, or the flyback transformer we talked about earlier) that is causing the shorts. Tracing component-level faults in an iMac is beyond my technical expertise.
The iMac I picked up curbside has a screw loose. So far I haven't been able to find it, but it isn't causing any shorts, so I just left it.