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Odd 820-4924 Backlight Behavior

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  • Odd 820-4924 Backlight Behavior

    Working on an 820-4924 board that has a backlight issue; it's minor, and one I normally would not worry about, but the customer sent it in so I'm going to try:

    1. 820-4924 with backlight issue: Backlight does not turn on until board reaches OSX login screen. Behavior is 100% consistent, every time.
    2. Have tried several displays.
    3. No liquid damage at all.
    4. Tried a new BIOS and LED driver to be safe, no change.
    5. Continuity everywhere and all is well from what I can tell, but the board will not output backlight until it reaches login screen. So if you want to boot from external drive, you can't see anything without shining a light at screen, etc.
    6. Once booted, boards works perfectly fine. Sleep/wake is fine.
    7. Passes AHT fine.
    8. Diode readings across all pins of eDP are fine.

    Any clue where to start with this one? I was thinking maybe a backlight clock/data/smbus issue, but everything checks out. Behavior is consistent across multiple panels. New BIOS and BIOS update via HS (.B00) results in no change. No idea where to go with this one.

    As always, any and all help appreciated.

  • #2
    Did you even try a PRAM reset????

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    • #3
      Of course, I "even" tried a PRAM reset ("????"--condescension noted). I tried an entirely new BIOS, which I'd hope would lead you to assume something as trivial as a PRAM reset did not resolve the issue. If it helps, I also tried turning the unit on and off again.

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      • #4
        Lol was just asking because it was not mentioned in the above post...Any history liquid drop etc?

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        • #5
          Got this one resolved; U8310 had a BGA pad (pin 2) loose/detached on it. It measured fine and reflowed fine, but once replaced board is giving backlight properly/immediately. Pad came right off when removed it. Thanks!

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