Hi! I have a 13 inch Pro, and after 4-5 hours of use, i get an annoying static noise from the machine. I check the...

Led 1 + Led 2 should be turned on when the imac runs. 3rd is for the GPU, but if you remove it, you should still see LED1+2. Can you try it without a HDD and see if it starts and the 2nd led stays on? Maybe its just a corrupted OSX install that reloads on boot. Here is the service manual for this iMac, you might be able to find something useful: http://tim.id.au/laptops/apple/imac/imac...

I had an older iMac with an MXM gpu(so it could be replaced) and i had the same issue, only one led. When i removed the gpu, the second led turned on, so it was a GPU error in my case. Sadly its not possible to test this in your case, but i'm guessing its related. The PSU looks ok, since the logic board gets 12V(this is what the first led means). I would suggest disconnecting everything you can(speakers, ssd, hdd, rams) and basically just leave your logic board connected to the PSU with a single fan and see if it spins when you turn it on, something might be damaged and creates a short on your logic board. But i believe its most likely a GPU issue.