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MacBook Pro A1398 820-3662 CPU VRMs

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    MacBook Pro A1398 820-3662 CPU VRMs

    Hello All,
    I'm having trouble replacing the CPU VRMs on my 820-3662A. This board was crashing randomly and I know the VRMs are the usual suspect.
    I removed all three of the VRMs but after I replaced them, only the Phase-1 VRM was working (it gets very hot, the others are cold). I get 1.29v on the VRM coil outputs, which appears to confirm that only one phase is operating.
    To prove this I removed the VRMs for Phase 2 and Phase 3, then powered the board up. The VRM output is a 1.29 with only U7310 connected. I decided to replace the controller U7200 but it did not change the result. I've tried using FDMF6708 and FDMF3030 FETs and the results are the same. Right now I have only U7310 and 7320 on the board.
    I replaced the VRM FETs on an 820-00138 board and had no problem at all. It's hard to understand. The FETs were purchased from a reputable North American supplier.
    Just curious if anyone has had a similar experience or has any suggestions.

    #2
    Re: MacBook Pro A1398 820-3662 CPU VRMs

    post picture of that area

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      #3
      Re: MacBook Pro A1398 820-3662 CPU VRMs

      Here it is...
      Attached Files

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        #4
        Re: MacBook Pro A1398 820-3662 CPU VRMs

        At both FETs I have the voltage input PPVIN_S5_HS_COMPUTING_HS on the input. With a scope I can see the switching pulses on pin 40, and the output connection is connected to each coil. All of the current sensing resistors measure OK.
        I have made comparisons to parts boards I have and used FETs from those boards with little success, other than for U7310. It doesn't make sense... These FETs worked on my 820-00138 board.

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          #5
          Re: MacBook Pro A1398 820-3662 CPU VRMs

          After replacing all 3 VRM FETs, the computer is running. The FETs I installed were FDMF6823A, which were replacements for the FDMF3030 that I had ordered from Mouser. Not happy about substitution without my consent but the FETs do work.
          After replacing 2 FETs I had the 1.8v that I needed for the CPU. I then needed a way to determine how I would confirm that the 3rd FET was working. I found test points labeled TP[1]CPUVR_BOOT1_RC, TP[1]CPUVR_BOOT2_RC, and TP[1]CPUVR_BOOT3_RC, which monitor the FET activity. I found the working FETS had a switching waveform (scope) of 7-8v. A FET that was not working had a level of 0.5v. This helped me get the last FET working even though it was not obvious that it initially wasn't.
          Looking back, this is the only thing it could be. This group of FETs was the 3rd group I had purchased from reputable resellers. I don't know if it was my fault for not having them down properly or that the other parts were bad. Either way it has been a learning experience.
          I have another issue now where in Big Sur, the screen brightness doesn't work on the F2/F3 keys or in the display preferences. If I boot from a High Sierra flash drive, it works fine. This is the first time that I've had this computer working for an extended period of time so I can't confirm that it's not an issue with the display itself even though the computer recognizes the display. Now another adventure starts.

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