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iMac A1312 2011 27" 820 - 2828 4mem Error Not booting to OS

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    iMac A1312 2011 27" 820 - 2828 4mem Error Not booting to OS

    Hi guys. Hope you are having a great day. This imac iMac A1312 2011 27" with this logic board 820 - 2828. Came to the shop becouse it doesnt boot.I started the apple diagnostic and got 4mem error. I disassembled it, cleaned it and replaced the ram. For a little while it worked normally. But after a while it got to the same state. I tried changing all the ram with a good known working ones. Nothing, i tried a different drive to boot from, nothing. I tried to not boot from sata but i disconnected drive and connected only via usb. nothing. I tried to change the graphic card nothing. I dont have a second processor but my friend tryed it in a different pc motherboard and it worked. Please if you have any idea or advice what could i try post it. Thank you in advance have a great day.

    #2
    Re: iMac A1312 2011 27" 820 - 2828 4mem Error Not booting to OS

    Review the RAM power rail against the schematic. Is it stable?

    What are the voltage readings of the following?

    PM_MEM_PWRGD
    PP1V5_S0_CPU_MEM


    Test with a single known good module.

    Perform a SMC reset and test again.

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      #3
      Re: iMac A1312 2011 27" 820 - 2828 4mem Error Not booting to OS

      The readings are
      PM_MEM_PWRGD 1.491
      PP1V5_S0_CPU_MEM 1.493
      and after smc reset
      PM_MEM_PWRGD 1.490
      PP1V5_S0_CPU_MEM 1.492

      I took the readings without a screen and for some reason the mac booted and from that time it boots normally. Dont know why. I ran some ram test with one and then 3 sticks. all test passed. do you think it could be just a bad connection on one of those circuits?

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