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[SOLVED]820-4924 no backlight

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  • [SOLVED]820-4924 no backlight

    I have an 820-4924 with no backlight, computer does get an image backlight is just not present. im getting the 12.5v all the way up to Q7700 but not after. No short to ground after the transistor .901 in diode mode both r7702 and r7701 are reading correct resistance also I am getting 5v on pin 5 of u7700

  • #2
    Any history?? Check R7750-R7751, also check pull up resistors...

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    • #3
      R7750 and r7751 are both good, also which resistors are the pull ups, sorry for being an idiot. computer was liquid damaged, but looks as though someone ultrasoniced it already

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      • #4
        r7752/3 are both giving me 1.58k ohms and i get .535 in diode mode on both data lines

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        • #5
          Try a known good LCD assembly. The LCD talks to the backlight chip to enable Q7700. Assuming you checked SMC_LID.

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          • #6
            i did check smc lid, 3.4v but i did try to get continuity from the data lines to the lcd connector, and i have an open line and what looks to be destroyed traces, would that cause no backlight

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            • #7
              I2C lines on pins 3 and 4 of LCD connector need to be good for Q7700 to open, this is communication bus with backlight chip to LCD.

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              • #8
                okay yeah theres no continuity so ill have to run a couple wires

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                • #9
                  I didnt have any insulated wire left, so i ordered some, just wondering if the caps are important in that line or if i can just run a wire from r7752/3 directly to pins 3-4 of the lcd connector

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                  • #10
                    If you mean C8358/59 they are not critical. Why not just fix the corroded via near the LCD connector by poking a strand of wire through it. That is the common failure point in these lines.

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                    • #11
                      oh i didnt realize you could do that. the whole pad where it goes through the board is pretty much gone, just a slight strand is left

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                      • #12
                        i got the wires ran, no short to ground on them both to the right pins and have continuity still no backlight, 12.8v still isnt getting through q7700 anything else i can check or is the screen toast?

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                        • #13
                          the sda line stays at 5v constantly but the scl line jumps around like a data line should

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                          • #14
                            Try a known good LCD assembly first, make sure 2015 model is used!

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                            • #15
                              Display is working now, the scl line had intermittent connection with the pin

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