I've just dragged an old ATX type Dell out of the cupboard with the intent on getting it ready to use with Mach 3.

It's doing something weird, about 3 seconds after applying mains power it stops and restarts.. I've tried disconnecting all drives etc, everything but the motherboard, it still does it.. The fan on the power supply is stopping and starting at the same times. I'm thinking the power supply has died, there's no fancy motherboard sensing stuff in an ATX power supply is there? Any thoughts most welcome!