By Jon, jon@smalldog.com
I’ve been deep into every generation iMac, and a few weeks ago we discussed the replacement of the rear housing on a new iMac following damage in transit. This week, the unit had no physical damage–just an aluminum iMac that failed to power on reliably.
The iMac was received and the listed symptom simply stated: “The iMac won’t turn on.” First step with any power-related problem is to reset the System Management Controller (SMC), which is particularly simple on these iMacs: simply unplug it for about ten seconds. This didn’t resolve anything, so I dove in.
The majority of Apple desktop machines have status Light Emitting Diodes (LEDs) that can be accessed by service technicians. As I read through the service manual, I learned that when LED 1 is lit and LED 2 lights only briefly when pressing the power button, the power supply should be replaced. After following these instructions, and replacing the power supply, I was left with the same issue as before: LED 1 was lit, indicating appropriate trickle power to the logic board, and LED 2 only flashes when the power button was pressed.
In a quandary since the service manual offered no further suggestions, I contacted the extra-secret Technical Service Provider Support at Apple (tech support for technicians) for advice. They requested that I strip the unit down to the logic board, with nothing plugged in to it. It took a few minutes to strip the unit down, and again I reached a point where LED 1 lit up and LED 2 only flashed.
The next option was to to replace the main cable harness connecting the power supply to the logic board, hard drive, and inverter board. I was befuddled by this suggestion, but I reluctantly agreed to proceed. When I had replaced the rear enclosure a few weeks ago, I remembered this specific cable set–it runs behind the main frame structure in the iMac, and one has to completely “gut” the unit to replace it. This repair, which normally would take 40 minutes, turned into a two-hour learning experience!
Finally, and to my surprise, the replacement cable brought the machine back to life, just like they said.