LG 37LC5DC - 'snow' for video/audio but no control of buttons/menu/etc...

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  • neex
    Member
    • Jan 2017
    • 27
    • Canada

    #1

    LG 37LC5DC - 'snow' for video/audio but no control of buttons/menu/etc...

    All voltages to my knowledge check out fine. The set comes on and I am unable to power it off with the power button.. volume control doesn't work and neither do source selections.... I only get fuzz and 'white noise' audio. I didn't know if this was something common - I have searched the forum so I apologize if I missed something... I will be replacing every primary/filter cap but something tells me this isn't the issue as the rails seem to be putting out voltage within/above spec....

    Here are my readings from P201 and P202

    202
    (1)19V - 20.6
    (2)19V - 20.6
    (3) GND
    (4) GND
    (5) 6V - 6.09
    (7) GND
    (8) 3.4V - 3.46
    (9) GND
    (10) 12V - 12.8
    (11) GND

    P201
    (1) 3.4V (1.94 with connector unplugged and 3.46 with it plugged in)
    (2) GND
    (3) 6V - 5.96
    (4) DIM -
    (5) GND
    (6) ST/5V - 4.97
    (7) I.B. - 1.69
    (8) ERR - .013
    (9) IC - 4.8
    (10) GND
    (11) ST/5V - 4.96
    (12) ON - 2.69
    (13) ACD - 4.21

    measuring from one side of the ground plane to the ground screws is pretty consistent at .036V...

    Any ideas? I feel like I'm pretty close here....

    Thank you,
    A.
  • dskall
    Badcaps Legend
    • Oct 2016
    • 2905
    • usa

    #2
    Re: LG 37LC5DC - 'snow' for video/audio but no control of buttons/menu/etc...

    I would be checking for voltages at the regulators on main board
    I assume no responsibility for any stupid suggestions I might post.

    Comment

    • dskall
      Badcaps Legend
      • Oct 2016
      • 2905
      • usa

      #3
      Re: LG 37LC5DC - 'snow' for video/audio but no control of buttons/menu/etc...

      Also check for voltage at cable to the buttons. There should be power there and one or more pins should change voltage when you press different buttons.
      I assume no responsibility for any stupid suggestions I might post.

      Comment

      • neex
        Member
        • Jan 2017
        • 27
        • Canada

        #4
        Re: LG 37LC5DC - 'snow' for video/audio but no control of buttons/menu/etc...

        I pulled it on the bench again and the original statement about 3.4 changing states was false. The 3.4 ON pin reads 1.8V even when powered with backlights on. I did check the pinout from the power button and it changed voltage from 1.94V. All I can think of now is that a regulator somewhere on the main board is outputting the same or very close. I've also been probing transistors to see if any of them pass along the button press.

        I think that once the 3.4V rail is sorted out this should be fixed. Time to check the backside of the main board I guess....

        P1300 shows 3.3 high to 2.17 at KEY2 when power button is pressed. The very first cap to the right of the circuit tests as open. I hope I'm getting close... now to find a 8b 47 25J compatible cap in by parts bin....

        How do I interpret this capacitor? I found B to mean +/- 0.1pF and I can guess the 25 represents voltage? Is this a 470uf @ 25V cap? Anyone??

        Thanks!
        Last edited by neex; 04-08-2017, 06:25 PM. Reason: need help interpreting SMD v type capacitor coding....

        Comment

        • neex
          Member
          • Jan 2017
          • 27
          • Canada

          #5
          Re: LG 37LC5DC - 'snow' for video/audio but no control of buttons/menu/etc...

          Does 47nF sound right? @5% Tolerance? Not sure of the voltage though... guess I'll aim high...

          Comment

          • neex
            Member
            • Jan 2017
            • 27
            • Canada

            #6
            Re: LG 37LC5DC - 'snow' for video/audio but no control of buttons/menu/etc...

            nevermind - figured out the cap after testing a good one out of circuit... only problem left is no RL ON being sent from mainboard... I think I checked every regulator but might have missed one... getting very close....

            Comment

            • dskall
              Badcaps Legend
              • Oct 2016
              • 2905
              • usa

              #7
              Re: LG 37LC5DC - 'snow' for video/audio but no control of buttons/menu/etc...

              If your getting voltage changes from key press you may have an eeprom problem.
              I assume no responsibility for any stupid suggestions I might post.

              Comment

              • neex
                Member
                • Jan 2017
                • 27
                • Canada

                #8
                Re: LG 37LC5DC - 'snow' for video/audio but no control of buttons/menu/etc...

                that would be more interesting than I bargained for.... the key presses don't manifest any voltage fluctuation at the SMPS however.... only on the lower section of the main board by KEY2 outputs...

                Comment

                • dskall
                  Badcaps Legend
                  • Oct 2016
                  • 2905
                  • usa

                  #9
                  Re: LG 37LC5DC - 'snow' for video/audio but no control of buttons/menu/etc...

                  Key presses go through the main processor.
                  I assume no responsibility for any stupid suggestions I might post.

                  Comment

                  • neex
                    Member
                    • Jan 2017
                    • 27
                    • Canada

                    #10
                    Re: LG 37LC5DC - 'snow' for video/audio but no control of buttons/menu/etc...

                    maybe I'll get a scope on the outputs on the processor then... thanks.

                    Comment

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