Thanks a lot!
To follow up, I would like to know the working principle of the power switch on the computer. With Windows XP, you can adjust the reponse of the computer when the power switch is press--to shut down or go to standby etc. When the operation system stop responding, you press and hold on the "power swtich" for a while then the computer would shut down... These behaviors indicate the CPU is involved in response to the simple action of "pressing the power switch". So my question is:
Is it possible that for certain motherboard, a bad CPU or memory module would prevent the power unit from starting up when you press the power switch?
I would like to know the answer because, the computer that I am trying to fix is an IBM server with separate power cable to a main board and a memory board (RAMBUS?) It is not easy to find a replacement power unit. And I don't remember the pin counts now. But do you have suggestion on how to test such a not-so-common power unit?