Liam, employed by Small Dog for exactly one year today, was tasked with repairing an iMac from the South Burlington showroom floor as fast as possible. This 20” model would restart at random intervals, typically after a minute or two of activity. His first instinct was to replace a cable known to fail with some regularity—the SATA/Inverter/DC/Power Supply cable. This is probably the most complex item to replace in an aluminum iMac, and because we suspect it to be the cause of many different symptoms, we keep a few on hand.
The installation was complete a little while later, and initially it seemed to have resolved the problem. Liam put it back on the floor and hoped for the best. Unfortunately, the problem came back in an hour or so.
His next step was to replace the power supply and power harness. Neither of these parts fixed the issue. To eliminate the operating system as the cause of failure, he plugged in an external FireWire drive with a clean installation of OS X. This is perhaps the single-most important step in diagnosing a machine: by booting the computer off of a known good startup disk and seeing whether the problem remains, you can definitively isolate any symptom to hardware or software.
This time, it seemed we might have a problem with the FireWire drive, as it would not show up in the boot manager when the machine was powered up holding down the option key on the keyboard. As this external drive is bus-powered, it’s powered by the one and only cable connecting it to the iMac. It became evident pretty quickly that the iMac had no FireWire function, so immediately he knew a main logic board replacement would be in order.
When the part arrived the next day, it was installed after the paying customers’ machines were all fixed. Tipped off by the lack of FireWire function, the logic board, once again, turned out to be the culprit.