Missing LCD Panel Voltages VGH, VGL, VCOM (TCL 55P6US Sound & Backlight but NO Picture)

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  • maneatingmonkey
    Member
    • Feb 2020
    • 36
    • Australia

    #1

    Missing LCD Panel Voltages VGH, VGL, VCOM (TCL 55P6US Sound & Backlight but NO Picture)

    I have a TCL 55P6US with sound and backlight but no picture. This model does not have a T-CON board, and the right (rear perspective) panel board is fed from the left panel board. I have not been able to induce any change by disconnecting the secondary panel board or the tape cut-off method on the primary board.

    There are a bunch of voltages missing on the panel boards. I can't find any shorted caps of the panel boards, and all of the missing voltage rails read in the kiloohms, which leads me to suspect the issue may lie on the circuit board, rather than a short in the panel.

    Below are test point readings:
    VIN 11.90V
    VDD33 3.29V
    VDDD12 1.24V
    VDD18 1.80V
    VGHF 11.67V
    VGL 0.19V
    VAA 1.07V
    HVAA -0.17V
    VSS 0.57V
    VREF 1.07V
    VCOM 0V
    RST 3.25V
    SPI_EN 3.25V
    I've been using this site as a reference https://pcbartists.com/design/power-...matic-tft-lcd/ (please suggest a better resource if you know of one).

    I can't find a test point for VGH, but VGHF is not getting boosted at all from 12V VIN. And VGL is obviously missing too.

    There is no rail labelled AVDD on this board. Is that the same as VAA? If so, I have no idea why HVAA is negative.

    I think chip U2 (photo #4) is what is responsible for generating these voltages. It's labelled 17500G 2510S, but I can't find any datasheet for it.

    The main BGA chip is labelled CSQ13-C4N K6952 1801-AB. It gets quite hot, but it came with thermal pads on both the top and bottom of the board, so I am not sure if this temperature is abnormal. The RST signal next to the BGA is stuck at 3.25V, which I find somewhat concerning. The other side of the resistors driving this signal (LOCKN and HTPDN) read Low, so it seems RST is being pulled High from the chip itself.

    Does anyone have any ideas? I really don't know much about these panel boards, but I'd love to learn more. There are so few resources available though.
  • lotas
    Badcaps Legend
    • Jan 2016
    • 4505
    • Russia

    #2
    Check the resistance of these test points CK1-CK8,LC1,2, ST) with respect to ground and between them (that is, 1 between ground, between 2, between 3... and so on with all the others), then 2 with gnd, with 3 , 4.5...., if there is low resistance between any of them, the protection in the controller will work and the voltage at its output will be blocked.
    Attached Files

    Comment

    • maneatingmonkey
      Member
      • Feb 2020
      • 36
      • Australia

      #3
      Thanks Iotas!

      Resistances to ground:
      ST >20kΩ
      LC1 0.26kΩ
      LC2 0.25kΩ
      CK1 0.25kΩ
      CK2 >20kΩ
      CK3 >20kΩ
      CK4 >20kΩ
      CK5 >20kΩ
      CK6 >20kΩ
      CK7 >20kΩ
      CK8 >20kΩ
      Resistance between points:
      LC1 LC2 53Ω
      LC1 CK1 66Ω
      LC2 CK1 46Ω
      So it looks like there is definitely a problem with CK1, but are LC1 and LC2 busted as well? CK is obviously clock, what does LC mean?

      As expected, these low resistances do not appear on the secondary panel board if I disconnect it from the primary.

      I will try cutting the jumpers below the test points and see what happens.

      Comment

      • maneatingmonkey
        Member
        • Feb 2020
        • 36
        • Australia

        #4
        Update: VGL came back after cutting CK1. I then cut LC1 and LC2, but still no VGH. I proceeded to cut all the other clock lines one by one, then finally after cutting ST I finally got VGH and a picture.

        Immediately I noticed two things. 1) A vertical line down the entire screen, but this is on the secondary panel board side of the TV so probably unrelated. 2) A rounded corner near the shorted CK and LC lines. Zooming in reveals a tiny bubble in the bezel of the panel. I suspect the short lies beneath this point and melted the plastic.

        Either way, this panel is toast

        Comment

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