"Fixing old iMac GPU
I fix Macs and iPhones as a hobby/part-time job and have done so for the better part of 30 years.
Last week, a musician friend with a maxed-out Core i7, 16GB RAM, 27" iMac from late 2009 started getting the classic signs of a failing GPU (pink vertical stripes and polka dots with boot stalls). He was in the middle of a big music job and en route to a performance in London in a few days so he went straight out and bought a new iMac before calling me over to max-out the RAM with non-Apple SO-DIMMs to save himself $300. As I was leaving, he offered me the old 27" iMac with video problems for nothing (a clever way to avoid Tokyo garbage recycling charges).
I called Apple Japan and they wanted a stupid ¥52,000 (US$430) to replace the old (Radeon) HD4850 MXM GPU; an internet search yielded no great deals other than some greedy people wanting $300+ for a S/H HD6970M from Thailand. So, I took the bull by the horns, removed the GPU and disassembled the heat sink. (By the way Apple design engineers, this model has very poor thermal design because the cooling fan is much too far from the heatsink which accumulates house dust and that is likely why these cards failed; you REALLY should have recalled the GPU for this model too as well as for the 2011 iMac GPU thermal problems and I am surprised no enterprising lawyer started a class action.)
(Info on some Apple's past repair extension programs for Video/GPU problems with some iMacs and MacBook Pros were linked on the video topics page. They expire over time but here's the Apple page of current Apple Exchange and Repair Extension Programs. (Past examples were Early 2011 - Early 2013 MacBook Pro Repair Extension Program for Video/GPU Issues (expired December 31, 2016), and iMac (27-inch): AMD Radeon 6970M Video Card Replacement Program (from Aug. 2013 - program now expired). Repair programs for 15" Mid 2010 MacBook Pro and (2008 era) Macbook Pro Nvidia GeForce 8600M GT have expired.)
I took the bare MXM GPU card to a large professional BGA reballing/reflowing factory I know nestled among Japanese-pear fields in the suburbs of Tokyo. They X-rayed it and found cracks in the solder joints under the BGA so they removed, reballed, and reflowed it for the "princely" sum of ¥6750 (US$56) - a great deal and thanks Kei-All.
A couple of hours later after reassembly using good-quality thermal paste and cleaning out the dust, the old iMac is purring along on a new SSD with El Capitan GM candidate.
With an iFixit guide and a little resourcefulness, almost anyone can save an old iMac with video problems from the landfill for a few more years.
Thanks Robert. Lots of problems over the years like this with BGA chips, many blame on lead-free solder (and thermal cycling). There's an old youtube video on oven baking the 8800GT card (and other devices) as a DIY attempt at BGA solder reflow. IIRC even some old iBooks had similar problems and there was a company (or two) in the US that readers used for the reball service but I can't remember their names now. (Those that didn't have luck with DIY fixes like oven baking or heat guns, often temporary.)