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820-3126-A iMac 21.5 Mid 2011 Unstable

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  • 820-3126-A iMac 21.5 Mid 2011 Unstable

    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.

  • #2
    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!

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    • #3
      Hmm, I really don't have another videocard for this.

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      • #4
        I do however own 27" iMac 2011, will this fit?

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        • #5
          Depends if the 27" has a 6770M it can fit. If it has a 6970M then SELL IT NOW, DUMP IT!!!

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          • #6
            Haha , but I love my iMac , I'll have to check which one I have when I get home...

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            • #7
              Phew, I have the 6770M, so I'll try tomorrow.

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              • #8
                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?

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                • #9
                  No known issues with the boards. Did it do the exact same crap with your known good video card?

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                  • #10
                    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.

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                    • #11
                      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.

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                      • #12
                        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.

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                        • #13
                          Yes it should work. Roast the GPU first a little so it will work for testing.

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                          • #14
                            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?

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                            • #15
                              Uhm, it worked before.....what did you do? Did you kill the GPU now?

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