Solving iMac graphics card problems

We’ve been seeing and reading about possible video card failures with some of the iMacs made prior to 2006. The problem shows as static, snow, or lines on the screen that appears briefly, then the screen returns to normal. Reports have been made that a restart is required in many cases, only to have the problem appear sometime later. Is it a heat related problem, a failed video card? Thats what we wanted to find out.

.

We disassembled one of these iMacs to see what we could find. Our subject: a 2007 20″ iMac Core2 Duo @ 2 GHz. It runs the ATI Radeon 2400 XT in a special pci bus called MXM. First off, smart move Apple by making the video bus attached instead of soldered to the logic board. Our intent going into the machine was to replace the video card.

.

What we found after disassembly led us to believe the card was not at fault. In fairness, the machine has only been running for a few hours with no signs of card failure. We’ll let the customer judge the results by running it as they normally do. If our findings are correct, the customer saves $400 for a replacement card and that’ll make her very happy.

.

Here’s the scoop. This machine has never been opened since purchase. Everything looked pretty good inside. The logic board holds the video card on the bottom side requiring its removal to get to the video card. Once the logic board was removed, we noticed a couple of things. First, the temperature sensor wire was touching, even nestled underneath the CPU heat sink. The wires looked good with no signs of burning or cracking, just a little uncomfortably close to the source of tremendous heat. We thought it might be possible this contributed to false readings from the temp. monitor.

.

More importantly, we found two of the screws holding the sink on the GPU were somewhat loose. In addition, once the sink was taken off the GPU, the thermal paste appears to have “missed” the center of the Radeon chip by about a third of its width. The paste was off center and bled off the side of the graphics chip. After cleaning the old paste and re applying diamond thermal paste, it was reassembled and the heat sink tightened.

.

For good measure the 1 Gb memory chip was replaced with a 2 Gb upgrade to eliminate the chance the RAM was in any way responsible. After the machine was put back together, we put SMC Fan control.app on not only to test the fans, but to make sure we didn’t miss any of the numerous plugs, screws or delicate fittings during re assembly. So far, everything is gorgeous, a bright screen, fast response and even tone across the LCD. Sound and Airport perform as expected. So at this point its up to the customer to tell us if this fixed the problem.

.

We wonder how many graphics cards get replaced in error when the problems actually lays in the heat wicking properties or wiring configurations. Only time will tell.

Update: The problem persists. Next action is to replace the video card.

Copyright Denver Macintosh dot com (2011)