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LG 55LD520 - No Backlights, No Image, No Sound (Lot's of Pictures!)

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    LG 55LD520 - No Backlights, No Image, No Sound (Lot's of Pictures!)

    Good evening (or morning) everyone,

    I have an LG 55LD520 that I purchased two years ago and is out of warranty. A few days ago, lightning struck very close to my house. Despite being hooked up to a surge protector, the TV no longer works.

    When I turn on the TV, the red LED indicator at the bottom flashes a few times and then goes to solid blue. However, the backlight does not turn on and there is no image (obviously). The flashlight trick yields nothing. Given that I cannot determine which input I am on, I cannot verify if I have sound.

    I am fairly technologically inclined (built computers, engines, and the like) so I opened up the back panel and started probing with a DVM.

    With the TV on, here is what I found:

    AC Plug to PSB (EL1) (brown wire): 1.3V - 3V continuous fluctuation.
    AC Plug to PSB (EL2) (blue wire): 1.3V - 3V continuous fluctuation.

    C613 +: 320V (This was measured by using the positive lead and a common ground. I didn't see the suggestion on this site to measure the voltage by using both capacitor leads until later.)

    The following results are with the inverter boards unplugged and plugged in (no differences):

    P201: all voltages/grounds matched the printed value. I wasn't sure what value was normal for the non-numerical, non-ground labels.

    P202/P203: all voltages/grounds checked out.

    So my question is this: what else am I to look for? Any fuses on the inverter boards? I saw two fuses on the power supply board (both showed continuity), are there anymore?

    I'm attaching a few pictures of the power supply board and components. There is a suspect area on one of the heat sinks with the black things screwed to them (not sure what they're called). I can't tell if it is heat damage or just assembly dirt.

    (I tried uploading the pictures via this site's attachment function but couldn't get past the "Uploading Files - Please Wait" message. So I'm using Photobucket instead.)











    P201


    P201
    Last edited by TheAssassin; 07-17-2015, 07:49 PM.

    #2
    Re: LG 55LD520 - No Backlights, No Image, No Sound (Lot's of Pictures!)

    Here is the potential heat damage I was talking about:



    Comment


      #3
      Re: LG 55LD520 - No Backlights, No Image, No Sound (Lot's of Pictures!)

      How about the Voltage readings of the signal pins, for example PWR, INV-ON, A-DIM, etc, we need to know ALL the voltages of all those pins to see if the main board is sending those signals to the power supply board or not.
      The wires are in the way so we cannot see all the pin names.
      Never stop learning
      Basic LCD TV and Monitor troubleshooting guides.
      http://www.badcaps.net/forum/showthr...956#post305956

      Voltage Regulator (LDO) testing:
      http://www.badcaps.net/forum/showthr...999#post300999

      Inverter testing using old CFL:
      http://www.badcaps.net/forum/showthr...er+testing+cfl

      Tear down pictures : Hit the ">" Show Albums and stories" on the left side
      http://s807.photobucket.com/user/budm/library/

      TV Factory reset codes listing:
      http://www.badcaps.net/forum/showthread.php?t=24809

      Comment


        #4
        Re: LG 55LD520 - No Backlights, No Image, No Sound (Lot's of Pictures!)

        Originally posted by budm View Post
        How about the Voltage readings of the signal pins, for example PWR, INV-ON, A-DIM, etc, we need to know ALL the voltages of all those pins to see if the main board is sending those signals to the power supply board or not.
        The wires are in the way so we cannot see all the pin names.
        Thank you for the quick reply.

        All the pin names are visible. The last two pictures in the first post are the same socket, just different sides. The other ones don't have any labels on the side that's blocked by the wires. Sorry if that wasn't clear. I labeled the last two pictures as P201 just now.

        I will get readings for these now.

        Comment


          #5
          Re: LG 55LD520 - No Backlights, No Image, No Sound (Lot's of Pictures!)

          Ok here are the readings with everything plugged in and the tv on with solid blue LED indicator light:

          P201 (goes from the main board, with all the in/outputs, to the power supply board):

          ERR: 0.0V
          P-DIM #1: 0.1V
          A-DIM: 2.4V
          INV-ON: 0.0V
          ~
          PWR: 24.3V

          P202 (goes from the power supply board to the inverter board on the right (left if looking at the tv from the front)) :

          INV-ON: 0.0V
          ADIM: 2.9V

          P204 (goes from the power supply to the inverter board on the left (right if looking at the tv from the front)) :

          ERR: 0.0V
          PDIM #1: 0.1V
          INV-ON 0.0V
          ADIM: 2.7V

          Let me know if you need more readings.
          Last edited by TheAssassin; 07-17-2015, 08:15 PM.

          Comment


            #6
            Re: LG 55LD520 - No Backlights, No Image, No Sound (Lot's of Pictures!)

            "INV-ON: 0.0V" You are not getting the BL-ON signal to turn on the backlights, The inverter is getting the needed 24V and the A-DIM (Analog Dimming signal to control the backlights brightness) signal. So something is causing the main board not to send the signal to to turn on the backlights. The PS-ON (PWR)is working otherwise you will not have the 24V.
            Try disconnecting the cables between the T-CON board on the panel and the main board to see what happen. You should also try to see if any of the IC on the main board is getting too hot to touch.
            Never stop learning
            Basic LCD TV and Monitor troubleshooting guides.
            http://www.badcaps.net/forum/showthr...956#post305956

            Voltage Regulator (LDO) testing:
            http://www.badcaps.net/forum/showthr...999#post300999

            Inverter testing using old CFL:
            http://www.badcaps.net/forum/showthr...er+testing+cfl

            Tear down pictures : Hit the ">" Show Albums and stories" on the left side
            http://s807.photobucket.com/user/budm/library/

            TV Factory reset codes listing:
            http://www.badcaps.net/forum/showthread.php?t=24809

            Comment


              #7
              Re: LG 55LD520 - No Backlights, No Image, No Sound (Lot's of Pictures!)

              I disconnected the T CON board and measured the INV ON voltage again and still get 0.0V.

              I couldn't discern any noticebly hot IC's on the main board.

              What would cause the main board to not send the signal to turn on the inverters? Or is it not that simple to diagnose yet?

              Comment


                #8
                Re: LG 55LD520 - No Backlights, No Image, No Sound (Lot's of Pictures!)

                What would cause the main board to not send the signal to turn on the inverters?
                The processor, the other failed circuits that cause the processor to hang up, bad caps, bad resistors,, bad connections, bad BL-ON switched transistors that is not toggling when it gets the command from the processor. It will be time consuming without the diagram to do component level troubleshooting.
                Never stop learning
                Basic LCD TV and Monitor troubleshooting guides.
                http://www.badcaps.net/forum/showthr...956#post305956

                Voltage Regulator (LDO) testing:
                http://www.badcaps.net/forum/showthr...999#post300999

                Inverter testing using old CFL:
                http://www.badcaps.net/forum/showthr...er+testing+cfl

                Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                http://s807.photobucket.com/user/budm/library/

                TV Factory reset codes listing:
                http://www.badcaps.net/forum/showthread.php?t=24809

                Comment


                  #9
                  Re: LG 55LD520 - No Backlights, No Image, No Sound (Lot's of Pictures!)

                  Thank you for your response.

                  While I am comfortable checking for voltages and such, I don't think I'm quite on the level of replacing individual parts like capacitors, resistors, transistors, etc. I'd rather purchase a new main board than try my luck at resoldering components. (I was never very good at soldering.)

                  The way I see it, there are several boards that I'd be ok replacing. The power supply board, the main board, the four inverter boards, and the T-CON board. If you think, with the information I've provided, that the faulty board is the main board, then I would go ahead an purchase a new main board.

                  Comment


                    #10
                    Re: LG 55LD520 - No Backlights, No Image, No Sound (Lot's of Pictures!)

                    I found the main board on shopjimmy.com. Do you think this issue is caused by a faulty main board?

                    Comment


                      #11
                      Re: LG 55LD520 - No Backlights, No Image, No Sound (Lot's of Pictures!)

                      The main board is not sending the command signal, so that will be the starting place.
                      You should also contact shopjimmy to see what they think.
                      Never stop learning
                      Basic LCD TV and Monitor troubleshooting guides.
                      http://www.badcaps.net/forum/showthr...956#post305956

                      Voltage Regulator (LDO) testing:
                      http://www.badcaps.net/forum/showthr...999#post300999

                      Inverter testing using old CFL:
                      http://www.badcaps.net/forum/showthr...er+testing+cfl

                      Tear down pictures : Hit the ">" Show Albums and stories" on the left side
                      http://s807.photobucket.com/user/budm/library/

                      TV Factory reset codes listing:
                      http://www.badcaps.net/forum/showthread.php?t=24809

                      Comment


                        #12
                        Re: LG 55LD520 - No Backlights, No Image, No Sound (Lot's of Pictures!)

                        Shop jimmy's site states that they are not answering TV technical questions at this time.

                        I am going to purchase a new mainboard and see if that works. I'll keep you posted.

                        Comment


                          #13
                          Re: LG 55LD520 - No Backlights, No Image, No Sound (Lot's of Pictures!)

                          A new main board from shopjimmy.com solved the problem! Thank you budm!

                          Comment


                            #14
                            Re: LG 55LD520 - No Backlights, No Image, No Sound (Lot's of Pictures!)

                            Hello everyone. I am new to the site and looking for help in troubleshooting the same LG Model. I have narrowed what I think is possibly the main board but wanted to seek everyone's thoughts here first.

                            On the power supply board, connector "P201", I have separated the "ERR" & "INV-ON" wires from the connector. By doing this, I get backlights on the tv, but no logo display. If I connect the "ERR" wire to the connector, I get a strobe effect on the screen. If I connect the "INV-ON" wire to the connector, the screen goes completely dark with no back light.

                            Can anyone help me identify where to go from here ?

                            Comment

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