Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

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  • gustagen
    Senior Member
    • May 2011
    • 86

    #1

    Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

    Hello all! I've had great luck in repairing an LCD monitor and a Vizio LCD TV with the generous help and patience of the good folks here on the forum.

    Yesterday I rescued a 46" Samsung LN46D550K1FXZA destined for the landfill. The backlights are not coming on. The display seems to work fine, as I can see the bouncy "Check Input Connection" message by using a flashlight.

    The caps on the power board aren't bulging and I see no signs of leaks. No strange smells on the power board or the inverter board. I have continuity across fuses FS8012S and FS802S on the power board, but I did not remove them from the PCB to test.

    I've attached photos of the power and inverter boards. Please let me know if you'd like more photos of a certain area.

    Any suggestions on where to start? I have a cheap multimeter and VERY basic understanding of electronics. Thanks!!!
    Attached Files
  • sugar_ray
    Senior Member
    • Dec 2012
    • 121
    • Canada

    #2
    Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

    If you can see it using the flashlight? I would look into the tcon board

    Comment

    • budm
      Badcaps Legend
      • Feb 2010
      • 40746
      • USA

      #3
      Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

      Do you have >360VDC (measured right at the two legs) at the main filter caps, the the two 450VDC rating caps laying on the side?
      So the backlights DID NOT even flash on for seconds?
      What DCV do you have on CN802 BLU_ON pin and the P_DIM pin?
      What DCV on CNI801, LD pin?
      Last edited by budm; 03-13-2015, 10:03 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

      • gustagen
        Senior Member
        • May 2011
        • 86

        #4
        Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

        I remounted the boards and powered up the TV. Here are my readings:

        165 V from both large caps at center of power board

        13 V at CN802 BLU-ON pin

        13 V at CN802 P-DIM pin

        12 V at CNI801 LD pin

        I should have explained earlier what happens when I power up the TV:

        - Plug in power cord, LED is solid red.
        - Press power switch on panel.
        - LED flashes red 5 times, then solid red.

        The backlights never flash.

        Comment

        • tom66
          EVs Rule
          • Apr 2011
          • 32560
          • UK

          #5
          Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

          Does the large cap voltage ever reach ~390V? or is it always 160V?
          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

          • gustagen
            Senior Member
            • May 2011
            • 86

            #6
            Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

            I dug up some leads with alligator clips so I could attach my meter and work the controls simultaneously.

            Plug TV in. Both caps read between 165 V and 170 V.

            Power TV on. Both caps read between 165 V and 170 V.

            My meter is cheap and not auto-ranging. I see no significant changes in the meter while leaving the TV powered on for several minutes.

            Comment

            • budm
              Badcaps Legend
              • Feb 2010
              • 40746
              • USA

              #7
              Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

              "13 V at CN802 BLU-ON pin

              13 V at CN802 P-DIM pin

              12 V at CNI801 LD pin"

              What did you use for GND?
              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

              • gustagen
                Senior Member
                • May 2011
                • 86

                #8
                Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

                I used the mounting screws for the power board as a ground for all measurements, including the pin on the inverter board.

                Comment

                • budm
                  Badcaps Legend
                  • Feb 2010
                  • 40746
                  • USA

                  #9
                  Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

                  13V for BL-ON and Dimming control do not look right at all, way too high.
                  The power supply board is also indicating the PFC Voltage booster circuit is not working or the board is not getting the PS-ON (PWR-ON) from the main board, so you need to verify if the PS-ON is present or not.
                  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

                  • Alastair E
                    Chief Womble
                    • Mar 2013
                    • 1963
                    • U.K.

                    #10
                    Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

                    Originally posted by gustagen
                    I used the mounting screws for the power board as a ground for all measurements, including the pin on the inverter board.
                    So--You used the screws as Gnd when checking the big caps for the 390V....?

                    Should measure ACROSS the cap, ie, one lead to one cap wire, other lead to the other (Same) cap--Not to chassis screws for checking the 390V....

                    --This is the HOT side of the PSU so No readings are done to chassis.....
                    TELEFIX

                    How PLASMA SCREENS WORK, X-SUS and Y-SUS what they do--
                    http://www.irf.com/technical-info/appnotes/an-1088.pdf
                    PLEASE DO NOT EMAIL ME PRIVATELY FOR REPAIR ADVICE. QUESTIONS BELONG ON THE FORUM!

                    Comment

                    • gustagen
                      Senior Member
                      • May 2011
                      • 86

                      #11
                      Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

                      I was incorrectly reading the chart for CN802 and attempting to take readings from the front of the board, where only half of the pins are accessible.

                      I have now flipped the power board, maintained all connections, and grounded the screw holes to the chassis with some alligator leads. I am using chassis as ground for the meter. I have kicked myself in the rear to save you guys the trouble. Here are the correct readings:

                      CN802 BLU_ON reads 5V with power on
                      CN802 P_DIM reads 1.67V at power up, fluctuates and hits 1.34 V for a second, then settles at 0.9V
                      CN802 PS_ON reads 3.3V with power on

                      I should also clarify that I took readings of the large caps at the caps' legs. I did not reference the chassis for ground.

                      I also just noticed a very faint high-pitched whine coming from the power board. When the TV is powered on, the whine begins and quickly rises to a high pitch. It maintains this pitch until the TV is powered off, and it quickly falls in pitch and almost dies out. It goes away quickly as soon as the power plug is pulled. It takes about 1/4 second for the sound to rise/fall from low/high pitch.

                      I power cycled the TV while searching for the source of the whine. Using a length of thin vinyl tubing I think I pinpointed the noise to the area of CM809.

                      I am lucky enough to be just a few minutes from a well-stocked electronics supply store. Is it worth throwing a few capacitors at the power board?

                      Sorry for the rookie mistakes and backtracking, and thanks for the help so far!
                      Last edited by gustagen; 03-13-2015, 12:57 PM.

                      Comment

                      • budm
                        Badcaps Legend
                        • Feb 2010
                        • 40746
                        • USA

                        #12
                        Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

                        "CN802 BLU_ON reads 5V with power on
                        CN802 P_DIM reads 1.67V at power up, fluctuates and hits 1.34 V for a second, then settles at 0.9V
                        CN802 PS_ON reads 3.3V with power on

                        I should also clarify that I took readings of the large caps at the caps' legs. I did not reference the chassis for ground."
                        OK, that means you need to get the PFC Voltage booster circuit working first.
                        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

                        • gustagen
                          Senior Member
                          • May 2011
                          • 86

                          #13
                          Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

                          As I was looking over the power board, I noticed a strange mark in the area of CP820. It is not obvious in my previous photos because of a shadow that happens to fall in the same area.

                          Attached is a photo of CP820, as if you were laying under the TV looking up at the board. Looks like I found a bad cap after all.....

                          Unfortunately the nearby electronics shop is closed for the day, so I'm off to the scrap bin to scrounge for a cap!
                          Attached Files

                          Comment

                          • tom66
                            EVs Rule
                            • Apr 2011
                            • 32560
                            • UK

                            #14
                            Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

                            Yes, that could cause this fault, be aware it is possible the diode is also damaged (it is a snubber for the PFC diode)
                            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

                              #15
                              Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

                              The cap is probably showing as shorted circuit right now, so you also need to check the PFC Diode DP802, that will explain why you do not have boosted Voltage right now. Make sure the replacement cap is rated at 2KV or so.
                              What is printed on the body of the cap right now?
                              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

                              • gustagen
                                Senior Member
                                • May 2011
                                • 86

                                #16
                                Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

                                I set my cheap meter to the diode setting to check DP802. It read "000". I swapped the leads and it read "000" again.

                                CP820 has this printed on one side:

                                R
                                221K
                                1KV

                                Comment

                                • budm
                                  Badcaps Legend
                                  • Feb 2010
                                  • 40746
                                  • USA

                                  #17
                                  Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

                                  That is what I expected, shorted.
                                  It is 220pF 1KV, I would go with 1.5KV or 2KV instead since 1KV rating still break down.
                                  Once you remove the cap, you might as well verify that the Diode is not shorted out either.
                                  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

                                  • gustagen
                                    Senior Member
                                    • May 2011
                                    • 86

                                    #18
                                    Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

                                    The electronics shop is closed on weekends, so I'll get a new cap and diode on Monday.

                                    I will report back then. Thanks everyone!

                                    Comment

                                    • gustagen
                                      Senior Member
                                      • May 2011
                                      • 86

                                      #19
                                      Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

                                      SUCCESS!!!

                                      I managed to find a 220pF 2KV cap on a modem PCB in my scrap heap.

                                      I removed the blown cap and the diode seemed to check out fine.

                                      I installed the new cap, held my breath, and powered the TV up. That Samsung splash screen never looked so good!

                                      Thanks to everyone who chimed in!!! This makes 3 LCD devices you guys have helped me put back into service!

                                      See you next time!

                                      Comment

                                      • tom66
                                        EVs Rule
                                        • Apr 2011
                                        • 32560
                                        • UK

                                        #20
                                        Re: Samsung 46" TV LN46D550K1FXZA good picture, no backlight, PCB photos attached

                                        Nice fix! Thanks for sharing....
                                        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

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