iMac 27 a1419 late 2012 and late 2013

[Salvatore951]

I received many repair request for iMac 27 late 2012 and 2013 with exactly same issues:
-freezing of system
-black screen
-O.S. doesn't complete loading and goes in boot loop

the problem is related to Gpu, but I am quite sure Gpu chip isn't the direct cause of these problems.
symptoms are similar to u8900 failure of retina 15 mid 2012 -early 2013

Can anyone help me? thanks you in advance

 

[dukefawks]

It's the GPUs and pretty hopeless. Reason 1 is finding good GPUs and reason 2 the location of the GPU on the board that makes rework a nightmare. Salvage the LCDs for units with cracked LCDs. These LCDs are fucking $$$$ so you can usually justify scrapping a machine with bad GPU.

 

[Salvatore951]

why are you sure problem is Gpu chip related? have you fixes one of this board by replacing gpu? why does not the problem disappear temporarily reflowing chip? why 650m and 750m of macbook retina 15 doesn't have this kind of problem even if they have same Gpu model?

 

[dukefawks]

I have only replaced GPUs on 21 inch models and that fixed the problem so I assume the same for 27 models.