Small Dog took delivery of a giant pile of 27-inch iMacs shortly after they were announced, and the one we stationed on the South Burlington store’s showroom floor decided it wouldn’t handle Cover Flow very well. When “flowing” through album art in iTunes or a folder’s contents in a Finder window, flashing perfectly horizontal anomalies would appear on the screen. Oddly, we could not find another situation that would produce the anomalies. We played Warcraft, stressed the graphics card with the Grapher application, and ran all the diagnostics available to us, and could not come up with a definitive diagnosis.
The first step in any troubleshooting is to start the computer from a known-good startup disk, and the best tool for this is the Mac OS X install disk that comes with every Mac. Since there isn’t Cover Flow in the installer, that disk wouldn’t help us. Instead, we used our NetBoot system to boot the computer off the network. The symptom appeared that way, so we knew the issue wasn’t related to software. With any video issue, it’s always productive to plug in an external display to determine whether the issue is with the internal display. In this case, the issue could be duplicated on the external display.
At this point, we suspected the graphics card, so one was ordered and installed with no luck. The immediate reaction was that the video card was dead on arrival, but the exact same symptom was present with this card. The video card clearly was not the issue with this machine.
The service manual suggests swapping RAM at this point, but we’d already tried that. Next up is main logic board, which didn’t fix the issue either. Thinking ahead, we also ordered up the display’s cable just in case. Neither fixed the issue.
Curious, I opened up another iMac to see if it had the same issue. It did. I did some creative Googling and found plenty of similar reports, and still believe the issue to be related to the graphics card. We were able to swap parts from one iMac to the other without the need to order service parts as needed, and still the issue remained. Apple was kind enough to offer the customer a replacement machine, and it arrived the other day in perfect working order.