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820-2915 - Backlight dim and flickering

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  • 820-2915 - Backlight dim and flickering

    Board came in for no backlight. Found broken pins on J9000 connector. Replaced J9000 but now the backlight is very dim and flickering. Here are some measurements taken

    PPVOUT_S0_LCDBKLT - 27v (so replaced U9701). Still 27v
    LCD_BKLT_PWM 2.5v (i'm assuming this is suppose to be 3.4v, so replaced u9600 after checking for blown resistors or shorts).
    PP3V3_SW_LCD - 3.3v
    PP3V3_S0 - 3.3

  • #2
    "LCD_BKLT_PWM 2.5v (i'm assuming this is suppose to be 3.4v"
    Why do you suppose that?
    There is PWM signal and varies pressing backlight control keys; looks good as DC value.

    U9600 has firmware, I hope you have used one from same board model.

    Did you try known good compatible LCD assembly?

    Check for corroded traces and resistors.


    • #3
      Yeah, i had research the chip a little and found that it can come either programmed or not, so replaced it with one from the same version board.

      Tested the display with another board and confirmed that it works.

      Had also checked for corrosion and replaced C9010 which had corrosion on it, but no shorted. But will will recheck again.


      • #4
        Rechecked, no corrosion found


        • #5
          Check continuity from A5/U9701 to K/D9701.
          Replace C9797/99, just in case.
          Be sure all return resistors (R9717-22) are good and post diode mode to ground on the 6 return lines.
          Try to reflow and (very) gently tap on U9701...


          • #6
            Continuity is good
            Replaced C9796/97/99
            Return resistors all checkout good and diode mode reading on all is 0.65 at the resistor and at J9000.
            Reflowed U9701 and also replaced again.

            Backlight is still dim and flickering.


            • #7
              Check if flickers more on low, or high backlight.
              Adjust keys should modify backlight output around 25V.

              Change LCD connector, if you've find corrosion near it.


              • #8
                I've replaced the LCD connector twice

                I can get the backlight to work up to about the 5th bar in the backlight intensity level, above that is when it starts to flicker.
                Backlight voltage is about 29v when the brightness intensity is at the 5th bar, once i increase the brightness into the 6th bar, the backlight starts the flicker and the backlight voltage drops to 27v.

                LED_Return are 2.4 when backlight level is set to the 5th bar, and the display is not flickering. Once i increase the backlight to max, the LED_RETURN drop to 1.8v..


                • #9
                  Ooops 15" display, so backlight voltage should be higher; forget about 25V I mentioned before.

                  Do you have stable BKL_EN?
                  Try changing D9701.
                  Be sure R9714 has correct value; also check R9716.


                  • #10
                    So started looking BKL_EN and found something interesting.

                    LCD_BKLT_EN - 3.3
                    LCDBKLT_EN_L - 0
                    LCDBKLT_EN_DIV - 11.9

                    PPBUS_SW_LCDBKLT_PWR - 3.2 at max lcd brightness
                    PPBUS_SW_LCDBKLT_PWR - 10.8 at 5 bars lcd brightness level

                    BKL_EN - 0.05 at max lcd brightness
                    BKL_EN - 2.68 at 5 bars lcd brightness level

                    On R9788, it should read about 301k (confirmed on a good board), but it's reading 8.9k. I pulled C9782, R9788, R9789, Q9707 and even R0901. With all of them pulled, i'm still reading 8.9k across R9788 pads.


                    • #11
                      Q9706 leakage, change it...


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