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Need help 820-00840-A no backlight Newbie here

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    #1

    Need help 820-00840-A no backlight Newbie here

    I want to ask what are the components need to be checked and the voltage I should get to have the backlight on. Customer said some water spilled on it but I dont see any corosion which makes a lot difficult for me to find the fault. Thanks
    Last edited by abudo; 02-03-2020, 12:51 AM.
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    #2
    Re: Need help 820-00840-A no backlight Newbie here

    Tried PRAM reset? Image is ok? Check for damage inside LCD connector.
    There are a lot of Louis Rossmann videos on no backlight issue (maybe on A1708 as well), I'd recommend to watch them.
    Also could be affected by flexgate, but if it happened right after liquid damage that should not be the cause.
    OpenBoardView — https://github.com/OpenBoardView/OpenBoardView

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      #3
      Re: Need help 820-00840-A no backlight Newbie here

      Originally posted by piernov View Post
      Tried PRAM reset? Image is ok? Check for damage inside LCD connector.
      There are a lot of Louis Rossmann videos on no backlight issue (maybe on A1708 as well), I'd recommend to watch them.
      Also could be affected by flexgate, but if it happened right after liquid damage that should not be the cause.
      I have not tried PRAM reset. CapsLock is turning on (not brain dead) and yes I see image. The video from Louis with A1708 was problem with the resistance on gate and source(Q8400) was so low which is caused by capacitor (C8400). Now on my case is that mine is ok with those parts and I dont know whats next to test hehehe sorry for bothering you guys

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        #4
        Re: Need help 820-00840-A no backlight Newbie here

        The troubleshooting process for no backlight is always the same. Measure voltage on backlight output, if it's very low measure resistance to ground as well. Also since this is liquid damaged you'll have hints on the board.
        OpenBoardView — https://github.com/OpenBoardView/OpenBoardView

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          #5
          Re: Need help 820-00840-A no backlight Newbie here

          Originally posted by piernov View Post
          The troubleshooting process for no backlight is always the same. Measure voltage on backlight output, if it's very low measure resistance to ground as well. Also since this is liquid damaged you'll have hints on the board.
          yes customer said some water spilled on it, but i dont see any corrosion or any water marks when I checked it. Tomorrow I will be checking it again since it was dayoff yesterday and today. thanks for your response guys it makes and keeps me going. Iam very much open on your suggestions. I will keep you posted and I hope we can get good results. thank you very much.

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            #6
            Re: Need help 820-00840-A no backlight Newbie here

            I already have here an MacBook with serious issues of liquid damage...
            I clean well the board and everything goes fine... But system gets stuck and slow sometimes with fan spin very high...
            I check well again and it was a little cable with a little corrosion that passed away by my eyes... Try to check if any part have any liquid residue or corrosion clean well all connectors as well and flat cables.
            Look for decolored, corrode points at board.
            Hope this simple info could help you to figure out anything.
            Cheers

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              #7
              Re: Need help 820-00840-A no backlight Newbie here

              the shop was busy in the past days and i almost forgot about this macbook so heres the number when i check with a connected screen, charger plugged in and detached battery

              PP5V_S0_BKLT_A (R8445) is 4.9V
              PP5V_S0_BKLT_D (R8444) is 4.9V
              LCDBKLT_FET_DRV_R (R8410) is 0.0V
              PP5V_S0SW_LCD is 4.9


              does this mean that the U8400 is damaged?
              Last edited by abudo; 02-11-2020, 08:18 PM.

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                #8
                Re: Need help 820-00840-A no backlight Newbie here

                please help. im just a newbie Im suspecting the U8400 IC but i need more opinion from you guys

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                  #9
                  Re: Need help 820-00840-A no backlight Newbie here

                  Originally posted by abudo View Post
                  please help. im just a newbie Im suspecting the U8400 IC but i need more opinion from you guys
                  Just check for output. I didn't have this boards yet. But it is always the same. Check output from u8400 , check resistance, if output is present at u8400 -> check the path to the screen. (If u8400 is the bolt boost cuircuit)
                  Maybe R8410 is broken? Check for power on both sides! Don't rush, take your time.

                  I don't have the boardview or schematic so I assume all your voltages what you posted are for the screen sense.

                  Also I remember one case where at screen sense L**** was giving on one side the proper 5v but on the other side only 3V

                  Plz double check everything. Measure on bklt output at lvds with screen and without!

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