820-3787 No LCD_BKLT_EN

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  • kory
    New Member
    • Jan 2020
    • 8
    • United States
    #1

    820-3787 No LCD_BKLT_EN

    This is quite a head-scratcher for me, just received a board yesterday from a customer and it has a shorted LCD cable and connector on the LCD assembly. Backlight circuit resistance without the screen is the nominal >150kOhm. Using a good test screen I still have no image and no backlight.

    This is where it gets weird...

    Prior work on been done on this board, appears that there was liquid around backlight circuit area in the past and the fuse F7700 was replaced with a wire.

    LCD_PWR_EN is good 5V but LCD_BKLT_EN is near 0V. Resistance on LCD_BKLT_EN line is 9.9kOhm (as expected).

    GPU Rails seem good except for one:
    I checked GPU power rails, all enables are 3.3V: P1V35GPUFB_EN, P1V05_S0GPU_EN, P1V8GPU_EN, P3V3GPU_EN, GPUVCORE_EN are all 3.3V going into U9600. Coming out of U8710 is the P1V05_S0GPU_PGOOD and it is good at 3.3V. P1V05_GPU_LL is 1.05V as expected. GPUVCORE_PGOOD is 3.3V and GPUFB_PGOOD is 3.3V as well.

    TP_GPU_PGOOD2 => GPU_PGOOD2 is not good though, it is around 0.1V.

    In the schematics and board I can't figure out exactly what this is for and why it would be low. There is a testpad connecting to pin/ball 110 (L4) on U9600 and in the schematic (page 66) it says "NO PU ON 3V3 and 1V8 PGOODS SINCE THEY ARE SYNTHETIC." There is no pull-up resistor to the 3.3V rail, so is 0.1V normal?

    Another question, do we normally need a screen attached to get LCD_BKLT_EN or can I continue working with only the board to get LCD_BKLT_EN working first before attaching a known-good screen?
    Tags: None
    • piernov
      Super Moderator
      • Jan 2016
      • 4435
      • France
      #2
      Re: 820-3787 No LCD_BKLT_EN

      You have to get image on the internal LCD before worrying about backlight. And similarly machine must POST properly before worrying about image.

      GPU_PGOOD2 is not used.

      You always need LCD to be connected when troubleshooting image/backlight problems.
      OpenBoardView — https://github.com/OpenBoardView/OpenBoardView

        Comment

        • kory
          New Member
          • Jan 2020
          • 8
          • United States
          #3
          Re: 820-3787 No LCD_BKLT_EN

          Originally posted by piernov
          You have to get image on the internal LCD before worrying about backlight. And similarly machine must POST properly before worrying about image.

          GPU_PGOOD2 is not used.

          You always need LCD to be connected when troubleshooting image/backlight problems.
          Thanks Piernov, I appreciate your response. I thought I was testing with a known good screen but I had grabbed the wrong one (I need to read my labels on the back first), sorry about that. So here is the update, blinking folder due to missing SSD but still no backlight and LCD_BKLT_EN is still near 0V. Fuse voltage is 12.5V (but recall prior work on the board replaced it with a wire). So the backlight chip is not being told to turn on by U9600. Everything else seems fine, could it be that this new short in the screen burned out U9600?

          Thanks!

            Comment

            • kory
              New Member
              • Jan 2020
              • 8
              • United States
              #4
              Re: 820-3787 No LCD_BKLT_EN

              Geez, the prior work and liquid damage is making me forget the basics, SMC_LID is 0V.

                Comment

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