820-3126-A iMac 21.5 Mid 2011 Unstable

XooP

New member
Just got this iMac. The owner claimed instability which I used quite some time to reproduce. I let the iMac run Heaven 3D benchmark for about two hours before the iMac just cut off. Started the iMac again and started the benchmark again and it lasted for about 10 minutes before cutting off again. After that the iMac would not start at all. Tried it again the next day and I could run the benchmark again for almost two hours before it cut off and would not start at all again.

I have now taken a visual inspection of the power supply, but it seems fine, all caps look okey.

Does anyone have experience with these, should I address the PSU or Logic board?

It never shows any signs of graphics artifacts.
 

dukefawks

Administrator
Could be so many things. I have had 2 of these with similar issues and it was the video card. It was the ISL chip on the video card to be precise. Best is to swap with a known good video card before messing with the original one of course!
 

XooP

New member
Finally got around to this one again and I have now ruled out both power supply and graphics card, so I guess that leaves me with the main logic board.
Do anyone know about any known issues with these boards?
 

XooP

New member
I did i opposite, testet each part in another 21.5 inch iMac. In the other iMac both the graphics card and power supply worked fine, I also testet the RAM to be sure.
The other iMac that I used for testing did however have a a bad graphics card, this is a 4670.
 

dukefawks

Administrator
Not really 100% conclusive I think. Take the 4670 and heat up the GPU, it should then work again and put it in the 2011 machine. Be aware that there are 2 different 4670 cards, one for LVDS and one for eDP LCDs. As long as the imac has the small connecter they are all the same, the 2009 21" models with the big LCD connector were LVDS.
 

XooP

New member
Ok, I'll try, the other iMac is a 2010, and I guess as long as the 6770M works in the 2010, the 4670 should work in the 2011? The connector in both machines are the same.
 

XooP

New member
Had some time to look at this again, roasted the graphics card a bit, but the iMac seems completely dead won't start at all. I don't have schematics for this, do you know what else than the motherboard or PSU that can keep this computer from starting?
 

XooP

New member
No it only worked sometimes, like I said in the first post. I would mostly start up run for a while and then seem dead for a couple of hours or until the next day and then start again. I was fairly gentle when warming the GPU, so I don't think i killed it (or killed it more than it already was). I think the logic board has a problem that keeps it from starting, maybe a bad cap or something.
I have also realized that working on iMacs are very time and space consuming :).

I may take the logic board out again tomorrow if get the time, but it is more hassle to work on this board than MacBook boards.
 

XooP

New member
You are right, this one sucks. I guess I'll just let it stand, and wait until I get a good deal on a logic board.
 
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