Hi folks, I've an iMac 27" 2011 with the infamous GPU failure. At the same time, I've a known working AMD 5750 pulled from an iMac 27" 2010. I've tried swapping the GPU, which resulted in the machine chiming when boot but a black screen. The third and forth diagnostic LED on the logic board did not lit up, which indicated that the GPU is not communicating with the logic board. I was wondering if there is anyone who has done similar "downgrading" of GPU with success.