I was presented with a late 2006, 24 inch white iMac; it was “broken and old” so I could have it for free to try to fix, and save it from landfill. All the previous owner could tell me was that it had simply stopped booting one day. I naively thought that it would be a simple case of re-installing a corrupted operating system. It turned out not to be that simple at all…
On startup, the machine appeared to behave normally. It ‘chimed’ to say it had passed the self-test routine and proceeded to boot into Mac OS. This was where it failed, presenting the following screen:
This supposedly means “the system has found a bootable installation of Mac OS X, but there is something wrong with it.” Which led me to think that a simple re-install would indeed fix the issue. However, on installing OS X Snow Leopard, the process hung at the screen below, and this time the spinning wheel had stopped; the computer had crashed on boot.
This implied that it was not simply a software issue, as I had originally thought. I tested the hard disk by booting the iMac into FireWire Target Disk Mode, and using Disk Utility on another Mac to test, format and write to the disk. I found no issues with the HDD.
Next, I thought I ought to test the RAM, and the other hardware in the Mac. I downloaded a copy of Apple Hardware Test from THIS LINK and followed the instructions on that page to create a bootable USB drive. I used “iMac6,1 Mac-F4218FC8”, as EveryMac stated that the A1200 model number printed on the base of the iMac related to a model ID of “iMac6,1”.
I ran the extended test 3 times, and each test found no issues, which was not very helpful. Wondering if my OS X install disk was to blame, I tried installing Windows 7 onto the iMac. The install seemed successful, although the video output looked a bit strange. However, when the installer tried to reboot into Windows, a “missing driver” caused the system to crash. As this had shown promising signs, I next tried to install Windows XP (as this seems to run on pretty much anything you give it).
Low and behold, it worked! The install was successful and I was presented with a (pretty much) normal looking XP desktop. The display was a little offset, as seen in the photo below, but I have seen this on PCs before the correct drivers were installed. However, I could tell that this was not the issue.
During the install routine, parts of it should have been plain white text on a black background. Instead, the screen seemed to be littered with commas (still in the monospaced font that was being used on-screen), as demonstrated below. This bizarre phenomenon lead me to think that the graphics card was the source of the problems.
THIS FORUM POST demonstrated that a GPU failure could cause Mac OS X to crash during boot, presumably just as the OS tries to output any advanced graphics. I had never used this iMac while it was alive, so before this point, I did not take any notice of the slight pink mottled effect that was present on any white screen before boot. As the colour cast disappeared when Apple Hardware Test was on-screen, I had presumed that this was just an artifact from an aging display. It turned out, that all the issues I had come across are actually caused by a graphics card failure. A quick eBay search revealed that buying a replacement graphics card was not economical, especially as I wasn’t sure that spending around £120 on a guaranteed working card would actually fix the iMac.
Furthermore, this is a fairly common problem in iMacs of this era; the designers at Apple seem to have decided that it is better for the iMac to run quieter, than to run cooler, so they reduced the speed of the cooling fan that serves the GPU and CPU. This means that during extended use in a warm room, there is not adequate airflow to dissipate heat from the graphics card. If the GPU heats up enough, the soldered joints between it and the graphics card begin to melt, and hence the graphics card fails.
I came across this video on YouTube, demonstrating a method of fixing graphics cards that have developed this fault:
So, with nothing to lose, I decided to give that a go. I followed the disassembly instructions in the service manual for this iMac, which can be found HERE. The same procedure might work for similar iMacs, a Google search should find the service manual for your model.
The graphics card is mounted on the rear of the logic board. In this particular model, the front panel, LCD, right speaker and logic board need to be removed in order to access it. I removed most of the insides of the machine in order to give it a good dusting down in the hope that the fix would work.
After removing the graphics card from the logic board, you need to separate it from the heatsink assembly, by removing the 4 screws shown in the photo below.
I cleaned up the surface of the GPU and set the card over the open side of a spare die-cast box. The foam surround and adhesive gasket need to be removed prior to heating, as well as any residue on the surface of the chip itself.
I used a hot air blower / solder re-flow station to heat up the chip instead of a gas flame, but other that the method was the same as in the video above: heat the whole surface of the GPU chip for several minutes, until you can smell the flux. Keep the heat source moving, trying to heat the chip uniformly, and try to heat only the GPU itself. Be careful not to touch the board, as this could move some of the surface mount resistors on the surface.
Allow the assembly to cool before re-assembling the graphics card and the iMac. Remember to re-apply heat sink compound to the GPU before re-assembly. Unlike in the video, my iMac had heat sink paste applied, rather than thermal pads.
While I had the logic board out of the machine, I used an ESR meter to test the electrolytic capacitors. This is another common cause of failure – often failed capacitors will appear bloated, but an ESR meter provides a more thorough test for failed electrolytics that otherwise look normal.
Amazingly, having re-assembled the iMac, the graphics issues are all fixed and I have managed to install a copy of OS X Snow Leopard, so I now have a fully working iMac, for the grand total of £0, several hours of research and thought, and a few hours to perform the repair.
In essence, an iMac which crashes on startup could well be due to a failed graphics card (which could be a fairly straightforward repair if you have the nerve).This method worked for me and similar techniques have worked for many others. Don’t blame me if it doesn’t go so well for you, though!
The graphics card in this iMac is a form of the NVIDIA GeForce 7300GT. The standard form of these cards were used in Mac Pros, and I am told that this fix can work for those systems too.
To prevent the GPU from overheating again, it is generally recommended to use SMCFanControl to increase the minimum fan speed. Mine is currently set to always be on maximum.
NB: The hot air blower used in this repair was the Atten 858D as reviewed in EEVBlog HERE. It cost a meagre £35 (inc. P&P) via eBay and has already paid for itself with this one job.