A long time customer dropped off her 24-inch iMac last week so we could resolve some power-related failures. The mid-2007 unit would power up and begin the boot process, but the screen would go dark after a few seconds. The service manual for this machine suggested a reset of the system management controller, or SMC, as the first troubleshooting step. This is done by unplugging the iMac for twenty seconds or so.
Unsurprisingly, this did not fix the video issue. I suspected a failure of the internal power supply, and took the machine apart to verify my hunch. With the machine open, I had clear visual access to several diagnostic light emitting diodes (LEDs). Once the machine was plugged in, the first LED would light, indicating correct trickle power from the power supply. The second and third LEDs lit up as usual. But as soon as LED four lit, indicating proper communication between the video card and logic board via a proprietary mini PCI-e slot, the machine would power down. The really weird part is that the first three LEDs remain illuminated after failure. Because they remained illuminated, I assumed that it was a logic board or video card problem, and I ordered both parts.
I installed the new video card in the original logic board, and the failure persisted. The unit chimed, the screen lit up, and the LEDs would light in proper progression; again, the machine powered off immediately after LED four lit up. Next step was to introduce the new video card to the equation, but the machine behaved even worse: it was even less responsive, having only the first LED come on when plugged in.
Analyzing my own thought process, I went back to my original hunch of power supply. I went ahead and ordered up the 250 watt power supply, installed it, and found that the unit booted right up. For my own edification, I used the process of elimination to see which one of the parts had really failed. It turned out that all three components had actually failed, and my diagnostic process was sound.