boot failure

I have an intel iMac that will not boot. The light stays on for about 10 seconds and then goes off once the fans will not start. I have replaced the power supply and the logic board twice. I've also swapped parts from a good machine, including fans and cables and get the same result. Totally baffled, any suggestions?

Have you zapped PRAM and reset NVRAM since changing parts?
Have you tried running Disc Utility from the install disc?
Both worth trying before we go further.

zapping

Unfortunately I can't even get to that point. The machine shuts down within seconds of power-on. It never comes even close to post.

Ahh ok. Nightmare. It really points to a power supply problem, but if you've changed it, you should be good to go there.
Not sure what else to suggest at present, sorry.
I'm sure one of the repairers here may be able to help a bit.

thanks

It is the strangest thing. I took a good power supply and logic board from another iMac and it still died. I think it's possessed but if anyone has an answer, I'd appreciate it.

That makes me wonder if there's not a dodgy circuit elsewhere that's frying the power supply.

Check here just in case - Intel-based iMac, Intel-based Mac mini: How to reset the System Management Controller it might help