LG 42la620v Backlight Issue

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  • thejellyfox2
    New Member
    • Feb 2018
    • 8
    • ireland

    #1

    LG 42la620v Backlight Issue

    I have a LG 42la620v tv with a back light issue. The set powers up as normal displaying the splash screen with the back light working properly but the back light cuts out after a second and the power led flashes three times. The logic board is functioning a picture can be seen and functionality confirmed by holding a flashlight up to the screen.

    I have measured voltages from the PSU and the 3.5 12 and 24v rails all measure correctly and have little to no ripple or transients as measured with a scope. The model of the PSU is EAX64905401(1.7) I can post pictures if needed.

    Additionally the PWR_ON and DRV_ON pins both read 3.5v when the set is initialing and running. The PDIM1 and PDIM2 pins both read ~1.2V They are something to do with brightness control. Im not sure what the appropriate reading is for these pins I don't think that an analogue voltage reference would be used for back light control.

    The led driver outputs ~35v for a split second during startup then immediately drops off to nothing this behavior is repeated regardless of whether or not the back light is connected.

    I an unable to isolate the cause of the fault either the back light or back light driver is defective. Im unsure as to how to test the back light I tried hooking it up across my bench psu but coud not reach the necessary voltage. It would be helpful if someone with some experience with these sets could shed some light on my situation.

    Thankyou for your time.
  • Andrew F. Ali
    Badcaps Legend
    • Jan 2014
    • 2450
    • Trinidad & Tobago

    #2
    Re: LG 42la620v Backlight Issue

    From what I read, this appears that the backlights are faulty.

    Comment

    • nomoresonys
      Badcaps Legend
      • Jan 2013
      • 12086
      • U.S.

      #3
      Re: LG 42la620v Backlight Issue

      Try unplugging mainboard from powersupply board, see if backlights come on and stay on.

      Comment

      • sordids999
        Badcaps Legend
        • May 2013
        • 2149
        • uk

        #4
        Re: LG 42la620v Backlight Issue

        Originally posted by andrew f. Ali
        from what i read, this appears that the backlights are faulty.
        +1

        Comment

        • thejellyfox2
          New Member
          • Feb 2018
          • 8
          • ireland

          #5
          Re: LG 42la620v Backlight Issue

          If the back lights are faulty why do they function for a split second on startup is there any general resistance that I should see across the led arrays.

          Also will the back light driver shutdown if it detects no load?

          I cant really disconnect the psu from the logic board as its relying on PWR_ON and DRV_ON pins staying high. I can try manually jumpering these pins too be brought high all the time and see what that does.

          Comment

          • nomoresonys
            Badcaps Legend
            • Jan 2013
            • 12086
            • U.S.

            #6
            Re: LG 42la620v Backlight Issue

            Originally posted by thejellyfox2
            If the back lights are faulty why do they function for a split second on startup is there any general resistance that I should see across the led arrays.

            Also will the back light driver shutdown if it detects no load?

            I cant really disconnect the psu from the logic board as its relying on PWR_ON and DRV_ON pins staying high. I can try manually jumpering these pins too be brought high all the time and see what that does.
            Ok, I think that means no pullup resistors. sounds like it's worth a try, I'm not sure of the proper procedure.
            Last edited by nomoresonys; 02-25-2018, 01:40 PM.

            Comment

            • thejellyfox2
              New Member
              • Feb 2018
              • 8
              • ireland

              #7
              Re: LG 42la620v Backlight Issue

              I tried manually jumpering the PWR_ON and DRV_ON pins to be high. This resulted in the 12 and 24v rails of the psu becoming active but still no output from the back light driver regardless of any load attached.

              Comment

              • nomoresonys
                Badcaps Legend
                • Jan 2013
                • 12086
                • U.S.

                #8
                Re: LG 42la620v Backlight Issue

                Guess it could be a protection circuit shutting it down maybe due to a backlight fault.

                Comment

                • nomoresonys
                  Badcaps Legend
                  • Jan 2013
                  • 12086
                  • U.S.

                  #9
                  Re: LG 42la620v Backlight Issue

                  Is this a late model tv, can't seem to find any parts for it.

                  Comment

                  • thejellyfox2
                    New Member
                    • Feb 2018
                    • 8
                    • ireland

                    #10
                    Re: LG 42la620v Backlight Issue

                    How would I go about diagnosing a fault with the back light led array not the driver?

                    In my experience leds usually fail open circuit and there seems to be very even illumination across the panel when the back light is running.

                    This leads me to believe the driver is at fault and failing under load.

                    I have not disassembled the panel to that point yet but I assume that the diffuser may hide individual led failures.

                    Browsing parts available I cannot see any additional led driver board so I assume that the leds are being driven directly from the psu board.

                    Comment

                    • nomoresonys
                      Badcaps Legend
                      • Jan 2013
                      • 12086
                      • U.S.

                      #11
                      Re: LG 42la620v Backlight Issue

                      Might need a schematic to make sure that power supply is doing everything it should. I can't read them very well but the pros on here can.
                      Last edited by nomoresonys; 02-25-2018, 03:22 PM.

                      Comment

                      • thejellyfox2
                        New Member
                        • Feb 2018
                        • 8
                        • ireland

                        #12
                        Re: LG 42la620v Backlight Issue

                        I got a schematic from:

                        The driver uses the BD9483f IC if anyone knows about it.

                        All Caps tested good on the psu so ill go into more depth with it tomorrow with the schematic.

                        I can buy a good psu board on ebay I can also get a new driver ic if i can isolate it as the cause of the fault.

                        Comment

                        • nomoresonys
                          Badcaps Legend
                          • Jan 2013
                          • 12086
                          • U.S.

                          #13
                          Re: LG 42la620v Backlight Issue

                          Sounds like a solid plan.

                          Comment

                          • sordids999
                            Badcaps Legend
                            • May 2013
                            • 2149
                            • uk

                            #14
                            Re: LG 42la620v Backlight Issue

                            So what voltages do you have on this connector when tv is switched on etc
                            Attached Files

                            Comment

                            • budm
                              Badcaps Legend
                              • Feb 2010
                              • 40746
                              • USA

                              #15
                              Re: LG 42la620v Backlight Issue

                              Can I see good clear straight shot pictures of the whole top and bottom side of the board?
                              You TV has two LED strings, so we need to verify the Voltage of Anode and the Cathode of both LED strings.
                              1) TV OFF, set your meter to higher than 200VDC scale, then keep the black probe of the meter on the chassis.
                              2) Keep the red probe of the meter on pin 1 (LED 1) of connector P801.
                              3) Turn on the TV while keeping an eye on the meter to see how high the Voltage goes up to, record that number, then wait 10 seconds to see how low it goes down to and then turn the TV off.
                              4) Repeat stepS #2 and #3 on pin 3 (LED+), PIN 5 (LED -), and PIN 7 (LED +).
                              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

                              • thejellyfox2
                                New Member
                                • Feb 2018
                                • 8
                                • ireland

                                #16
                                Re: LG 42la620v Backlight Issue

                                Im getting roughly 35 to 50 volts on both led strings the refresh rate of my meter isnt very fast so there is variation between readings. This happens regardless of whether I have the leds plugged into there connector.

                                I know that between 70 and 105 vdc @ 0.313A is nominal for this board.

                                Comment

                                • nomoresonys
                                  Badcaps Legend
                                  • Jan 2013
                                  • 12086
                                  • U.S.

                                  #17
                                  Re: LG 42la620v Backlight Issue

                                  n/a
                                  Last edited by nomoresonys; 02-27-2018, 06:27 AM.

                                  Comment

                                  • nomoresonys
                                    Badcaps Legend
                                    • Jan 2013
                                    • 12086
                                    • U.S.

                                    #18
                                    Re: LG 42la620v Backlight Issue

                                    Any pics? what are the numbers on your inverter board if you have one, can't seem to find much on this model, hardly see any parts or pictures.

                                    Comment

                                    • budm
                                      Badcaps Legend
                                      • Feb 2010
                                      • 40746
                                      • USA

                                      #19
                                      Re: LG 42la620v Backlight Issue

                                      Originally posted by thejellyfox2
                                      Im getting roughly 35 to 50 volts on both led strings the refresh rate of my meter isnt very fast so there is variation between readings. This happens regardless of whether I have the leds plugged into there connector.

                                      I know that between 70 and 105 vdc @ 0.313A is nominal for this board.
                                      So both boost converters are not working at all then.
                                      I need good clear pictures of the whole top and bottom side of the board.
                                      Did you look up the spec sheet of the LED driver IC yet to see if it has the VCC and the Enable signal to turn on the IC?
                                      "The led driver outputs ~35v for a split second during startup then immediately drops off to nothing this behavior is repeated regardless of whether or not the back light is connected."
                                      This does not make sense to me, it should show the input Voltage at the LED connector if the Boost converters are not working, I do not expect it to show 0V
                                      Last edited by budm; 02-27-2018, 10:38 AM.
                                      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

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