A friend handed me an early 2006 iMac 17" for repair. She reported a loud "pop" followed by the machine powering off and refusing to power back on, with no blinking lights, etc.

"Oh," you say, "another blown cap in a power supply or on a motherboard", right? Well, I have the thing taken all the way apart and have looked at the PS and MB, and see no evidence of blown or leaky caps on either. And in case you are wondering, yes, I know just what a blown or leaky cap looks like, as I have a mound of donated Optiplexs to go through and have already found enough bad caps to float a small boat.

Any suggestions on how I might go about troubleshooting this thing, aside from dropping $80+ on a rebuilt PS only to find that it was the MB? Or should I just part the thing out for her on eBay and move on?