iMac G3 fails to power up

I've just been given an iMac G3 with what appears to be a power issue. It powers on for a second or two, then shuts off, the power light never going green, and it doesn't do the startup chime. I've tested the CRT with a powermac, and it is good. Not sure where to go from here, suggestions, as always are very much appreciated. :approve:

EDIT: should've added, it's a revB iMac, 266Mhz. Its been upgraded at some point with 512mb of ram and a 200 GB WD hard disk. I just pulled the hard disk out and it still doesn't boot.

'nother EDIT: Is it safe to plug in a known working ATX power supply? I did this with my powermac and it only required 1 jumper to be changed.