Why do my Power Supply voltages seem wrong (even w/new PSU)?

I have an iMac 20" EMC: 2210 (mid 2008 - S/N: W88282MLZE3). When started, it runs for 20 or more seconds and then just powers down. The four diagnostic LED's on the logic board all light up (#1 when connected to power, the rest in order when the device is powered up), the POST chime is heard, the display turns white, then displays the Apple logo, then the machine powers down. It will not boot any further, whether from the hard drive or from a system install or diagnostic (TTP) disc.

I assumed that the problem would either be the power supply or the logic board. Testing the PSU seemed a sensible first step, but I could get exactly zero information on the PSU's output until I found Richdave's excellent article on testing this model's PSU at:

iMac Intel 20" EMC 2210 Power Supply Output Voltage Test

Using that as my guide, I tested the PSU's output, using a digital VOM, and found that the output values vary from those specified in the article in several respects. I created a complete table of the specified and actual values of the original PSU in both stand-by and powered mode. I've highlighted the discrepant values in the table, which is included here:

Block Image


1) in standby mode, Pin 8 had no voltage where the table shows 12vdc and Pin 9 had 12vdc where the table shows a 0vdc value

2) in powered mode, Pin 2 outputs 12v where the table shows a 0vdc output and Pin 3 puts out 0vdc instead of 12vdc

3) in powered mode, Pin 4 outputs 3.4vdc where the table shows 0vdc and Pin 5 outputs 0vdc, instead of the approximate 2.2vdc shown in the table.

It's as if, in standby mode, Pins 8 and 9 are reversed and, in powered mode, Pins 2 and 3 and also Pins 4 and 5 are reversed.

Assuming that the PSU was bad because of these discrepant values, I ordered and installed a new PSU but the iMac's behaviour remained the same and the output voltages on the new PSU are identical with the original (except for stand-by mode Pin 5, which outputs 3.8vdc; an insignificant difference).

Can anyone tell me what these discrepancies mean? I'm assuming that a bad logic board is probably the correct conclusion but I'd be grateful for any relevant advice or input before I order a $250 logic board.

Regardless, please accept my sincere thanks for the time and energy you spent reading and considering this question; it is much appreciated!

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