How should one test the power supply?

My niece's iMac EMC2133 failed suddenly whilst she was working on it. The screen went black and the computer fell silent.

It was taken to a repair shop who eventually declared it not economically reparable primarily because they could tell whether it was the power supply or logic board which had failed. Certainly they replaced the fuse at least twice on the power supply and it blew each time.

I have taken on the task of trying to repair it. When the computer is in standby mode (AC supplied but computer not turned on) LED1 does not light and any fuse placed on the board blows. This happens even when the 10 pin outlet plug from the power supply (ps) is disconnected. I concluded that certainly this ps is dead. I have obtained another second hand one (to keep her costs down) which looks very clean and of much better condition. When this board is used and its outlet plug is in place LED 1 lights but the test voltages are wrong. Pin 5 is 2.7v instead of 3.8v and pin 8 is 0v instead of 12v. I assume that these tests are made with the ps outlet plug in position and logic board connected. Is there anything else which is essential to connect for the measurements to be correct?

Until I can discover why the voltages are not correct I am reluctant to actually turn on the computer.


Tim Simmons

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