Insignia NS-42L780A12, no backlight.

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  • johnboy1313
    Badcaps Legend
    • Nov 2012
    • 1959
    • USA

    #1

    Insignia NS-42L780A12, no backlight.

    I picked this one up tonight. It powers on and off and shows a picture visible with a flashlight to the screen. When first powered on, meaning, the exact moment the LED turns from red to blue, the backlights flash for a split second. I've had two TVs with a bad bulb and they both had a true two seconds to black failure. This one is more like two milliseconds to black. I see nothing out of sorts on the PSU or the inverters. The connector on the PSU shows all voltages that I would expect, 3.3, 5, 12, and 24V.

    I can't find a service manual for this one so I don't know what, if any, voltages should be present at the 5 wire connector running from the upper left corner of the PSU to the lower inverter board.

    Attached Files
  • budm
    Badcaps Legend
    • Feb 2010
    • 40746
    • USA

    #2
    Re: Insignia NS-42L780A12, no backlight.

    Can I see the whole back side of the TV? I cannot where the 24v connected to on the inverter board.
    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

    • johnboy1313
      Badcaps Legend
      • Nov 2012
      • 1959
      • USA

      #3
      Re: Insignia NS-42L780A12, no backlight.

      Sorry bud, I honestly thought I had attached them. The connector on the PSU is not labeled at all. On the inverter it is.
      Attached Files

      Comment

      • budm
        Badcaps Legend
        • Feb 2010
        • 40746
        • USA

        #4
        Re: Insignia NS-42L780A12, no backlight.

        Ooh, one of those. On the power supply board, there are two 2-pin connectors with black wires close to the High voltage transformer driven by the two MOSFETS (run off 380vdc power supply) mounted on the heat sink to the right of the transformer. Those two connectors carry semi high voltage may around 100~200VAC High frequency, this high voltage is then fed into those small high voltage transformer on the inverter board to step it up to 1000~2000vac.
        At this point we know that the bl_on is working otherwise the backlights will not flash on and go into shutdown. So it can be that power supply is not supplying the PFC Voltage booster, or bad small inverter transformer, or bad lamp.
        So check first to see if we have correct voltage on those two filter caps, they are connected in parallel, has about 370~400VDC when TV is turn on.
        Last edited by budm; 10-03-2013, 09:05 PM.
        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

        • johnboy1313
          Badcaps Legend
          • Nov 2012
          • 1959
          • USA

          #5
          Re: Insignia NS-42L780A12, no backlight.

          Yeah, 165v at standby, 390v when on. The PSU is working to a degree, at least. There is picture and sound, just no backlights. I'm not sure if that five wire connector is supposed to have power on it or not. There isn't any.
          Last edited by johnboy1313; 10-04-2013, 04:51 AM.

          Comment

          • budm
            Badcaps Legend
            • Feb 2010
            • 40746
            • USA

            #6
            Re: Insignia NS-42L780A12, no backlight.

            Post good clear picture of the power supply board both sides, also smaller sections of both side, then I may be able to give you the test points to test. I think the problem is in the high voltage transformer drive circuit.
            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

            • johnboy1313
              Badcaps Legend
              • Nov 2012
              • 1959
              • USA

              #7
              Re: Insignia NS-42L780A12, no backlight.

              Will do. I will try to get to them this afternoon but we've got a big night tonight so it may not be until tomorrow morning. Thanks.

              Comment

              • johnboy1313
                Badcaps Legend
                • Nov 2012
                • 1959
                • USA

                #8
                Re: Insignia NS-42L780A12, no backlight.

                Pictures
                Attached Files

                Comment

                • johnboy1313
                  Badcaps Legend
                  • Nov 2012
                  • 1959
                  • USA

                  #9
                  Re: Insignia NS-42L780A12, no backlight.

                  More pictures
                  Attached Files

                  Comment

                  • budm
                    Badcaps Legend
                    • Feb 2010
                    • 40746
                    • USA

                    #10
                    Re: Insignia NS-42L780A12, no backlight.

                    OK, check out that fusible resistor (0.2 Ohms or so, I cannot read the numbers as printed on the body) which the 380V is fed through it to feed the two MOSFETs that drive the high voltage transformer, if it is open, then you probably have shorted MOSFETs.
                    Attached Files
                    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

                    • johnboy1313
                      Badcaps Legend
                      • Nov 2012
                      • 1959
                      • USA

                      #11
                      Re: Insignia NS-42L780A12, no backlight.

                      It doesn't measure open.

                      Comment

                      • tom66
                        EVs Rule
                        • Apr 2011
                        • 32560
                        • UK

                        #12
                        Re: Insignia NS-42L780A12, no backlight.

                        But does it measure shorted, nearly 0 ohms?
                        Please do not PM me with questions! Questions via PM will not be answered. Post on the forums instead!
                        For service manual, schematic, boardview (board view), datasheet, cad - use our search.

                        Comment

                        • johnboy1313
                          Badcaps Legend
                          • Nov 2012
                          • 1959
                          • USA

                          #13
                          Re: Insignia NS-42L780A12, no backlight.

                          It has 0.1 ohms printed on the side. So, shouldn't it read nearly zero ohms? It read 0.2 on my meter. I chalked it up to the meter not have the ability to read accurately at that low level.
                          Last edited by johnboy1313; 10-05-2013, 08:59 AM.

                          Comment

                          • tom66
                            EVs Rule
                            • Apr 2011
                            • 32560
                            • UK

                            #14
                            Re: Insignia NS-42L780A12, no backlight.

                            That's fine.
                            Please do not PM me with questions! Questions via PM will not be answered. Post on the forums instead!
                            For service manual, schematic, boardview (board view), datasheet, cad - use our search.

                            Comment

                            • johnboy1313
                              Badcaps Legend
                              • Nov 2012
                              • 1959
                              • USA

                              #15
                              Re: Insignia NS-42L780A12, no backlight.

                              Any other ideas?

                              Comment

                              • tom66
                                EVs Rule
                                • Apr 2011
                                • 32560
                                • UK

                                #16
                                Re: Insignia NS-42L780A12, no backlight.

                                Part # of the ~14-16 pin chip just above the MOSFETs in this image?
                                https://www.badcaps.net/forum/attach...7&d=1380931566
                                Please do not PM me with questions! Questions via PM will not be answered. Post on the forums instead!
                                For service manual, schematic, boardview (board view), datasheet, cad - use our search.

                                Comment

                                • budm
                                  Badcaps Legend
                                  • Feb 2010
                                  • 40746
                                  • USA

                                  #17
                                  Re: Insignia NS-42L780A12, no backlight.

                                  That is the IC which drive the primary side of the MOSFET Gate drive transformer.
                                  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

                                  • johnboy1313
                                    Badcaps Legend
                                    • Nov 2012
                                    • 1959
                                    • USA

                                    #18
                                    Re: Insignia NS-42L780A12, no backlight.

                                    Oz9976gn
                                    aa5g19.1n
                                    1123c

                                    Comment

                                    • tom66
                                      EVs Rule
                                      • Apr 2011
                                      • 32560
                                      • UK

                                      #19
                                      Re: Insignia NS-42L780A12, no backlight.

                                      Can't find a datasheet for an OZ9976GN unfortunately. You might be able to figure out the functions of some of the pins but not many.
                                      Please do not PM me with questions! Questions via PM will not be answered. Post on the forums instead!
                                      For service manual, schematic, boardview (board view), datasheet, cad - use our search.

                                      Comment

                                      • lookimback
                                        Badcaps Legend
                                        • Aug 2013
                                        • 1489
                                        • USA

                                        #20
                                        Re: Insignia NS-42L780A12, no backlight.

                                        I just spent over a week, with a lot of help from Bud, working on a Westinghouse which was doing exactly the same thing. About a half second or less and backlights went out. Turned out the guy I bought it from had tried to fix it himself and bent over a pin inside one of the connectors on the inverter board. Original problem was probably the cap on the inverter board.
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