[Deafy]

Has anyone had this. Press the start button and the iMac starts up and chimes. 5 seconds later it shuts down and needs to be disconnected from the power supply before you can attempt to start it again. If you try to restart with disconnecting from the power supply, the green light comes on but the iMac does nothing.
I've tried holding the C key down to boot up from a start-up disc, but it just ignores this and shuts down. I've reset the PMU chip and tried to start up and 'zapping' the PRAM. Still no good.
I've checked the power supply, the power to the logic board, battery etc and all appear to be OK.

[JohnWill]

I moved this to the Mac forum, you might get better answers over here.