Two machines, same model PSU in each. Took "A" out and put it in "B" to confirm that it was bad. It was (fails to boot the PC). put the "B" PS back in (was good), and now it is bad (PC fails to boot).... Its a Diablotek DA400 PSU and the failure mode consists of brief power-on cycle every 4 seconds.
Anyone got any ideas? Is this what you get when you buy the cheapest PSU there is?
I had a faulty floppy drive power connector (yes, I have a floppy in the system....). It boots fine with the drive disconnected. I was in great fear that I had damaged the mobo. I suspect there is a short in the socket.