My parents 20 inch first generation Intel iMac from early 2006 (A1174) began to show strangely garbled video on random parts of the screen, after a few minutes followed by a ‘frozen’ screen or spontaneous restart few minutes later.
At first these glitches happend only on warm days. The iMac was 7,5 years old then. Now it’s 8,5 years and it crashes more often and sooner then a year ago, so I decided to try and find out whether I can fix the problem myself.
At first I was sceptical about overheating being the problem, because I didn’t even hear the fans and you’d think that they would at least rev up. Still, I did some research about this particular Mac model. I ran Mactracker to determine the exact model of iMac. These kind of video problems are typical for iMacs from that period and are usually caused by heat problems and of course sometimes by bad hardware. SMC Fan Control is often used to keep the fans at a higher speed, to prevent heat problems.
I carefully opened up the iMac and inspected two of the fans. The third one is harder to reach. The bottom left fan had some dust on it, so I cleaned it, but heat sink ‘air tunnel’ on the top was as clean as a whistle. This could not have caused the problem.
A problem with the heat sink is more likely. I found several cases in which the thermal compound needed to be refreshed. Someone suspected the thermal conductor to be leaking. I even found a video demonstrating a method first to resolder the pins of the GPU.
I decided to first put the iMac back together so I could measure the temperature sensors with Hardware Monitor. All sensors indicated normal temperatures, except the one for the video card, or rather: GPU, because only the 24 inch model has a separate video board. The 17 and 20 inch models have a GPU soldered to the logic board, an ATY Radeon X1600 in this case.
After waking the iMac up, the GPU temperature immediately goes up very fast. Within minutes it heats up to around 100ºC, especially if you let it work a bit, for example by playing a video. Not surprisingly, around that temperature the computer becomes unstable. This is way to much. According to youtube user ‘casualtechs’ aka Mark Sicat, anything above 31ºC indicates a problem. He uses SMC Fan Control to set the speed of the central fan to maximum and leave the fans for the hard disk and optical drives as low as they are per default. This way, the iMac can’t overheat without the cooling fans getting uncomfortably loud.
iFixit has some clear pictures and instructions on how to take apart the Intel iMac 2006 that I used. Unfortunately, they stop when the logic board is taken out, while the most important is still to do: removing the old thermal compound, cleaning the surface of the GPU and other chips and heat sinks and applying new thermal compound between the heat sink and chips.
Removing the front heat sink
The aluminium heat sink on the front is held by 4 black plugs that have a spring on the heat sink side and two barbs on the tip that hook behind the surface as soon as they are pressed through the board to the other side. To unhook them is a bit tricky. You don’t want to use too much force and damage them. I managed to get two out by using just my nails to press in both barbs. For the other two I used neelde-nose pliers to carefully press the barbs together while pressing the plug back .
Removing the main heat sink
Cleaning and applying thermal compound
Restoring the heat sinks