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Samsung LN40E550F7F backlight issue

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    #1

    Samsung LN40E550F7F backlight issue

    I purchased this tv broken. It was completely dead. I opened it up and found a blown fuse on the power supply. The new fuse blew immediately. After checking all over the board to find a problem to no avail I simply replaced the power supply board. The new board allowed the tv to turn on. However the backlight comes on only for a couple of seconds and then goes out. The display is there and can be seen with a flash light. I changed out the inverter board as well. The new inverter board did not fix the problem. I disassembled the tv further to inspect the lamps. They all looked good. The screen has no dark spots when the back light is working. I'm not sure what to try next. Any help is very much appreciated.

    Kevin
    Tags: None
    #2
    Re: Samsung LN40E550F7F backlight issue

    just because the lamps "looked" good is not a real test.
    there is a thread here showing using a cfl lamp-bulb to test the invertor.

    Comment

      #3
      Re: Samsung LN40E550F7F backlight issue

      When I power on the unit, all of the bulbs come on fine without any flickering, buzzing, or dark spots at either end of the bulb. Does this rule out the bulbs or could it still be a bulb. I don't think I have the expertise to make the inverter tester.

      Comment

        #4
        Re: Samsung LN40E550F7F backlight issue

        I do not know which boards are used in your version of the TV you have:
        http://www.shopjimmy.com/catalogsear...ng+LN40E550F7F
        But anyway, take the voltage readings on the connector of the power supply board that goes to the main board after the backlights go out.
        Report the reading in the format that we can easily undertand.
        There are labels for each pin, I.E. PS_ON = XXX Vdc.
        We need to know if the all the signals from the main board are present when the backlights blank out.
        Also show us the pictures of the back side of the TV so we can see all the boards.
        Last edited by budm; 05-24-2014, 12:17 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

          #5
          Re: Samsung LN40E550F7F backlight issue

          P_Dim is showing 5.19 volts
          NC is giving a negative reading
          BLU_On is showing 44 volts
          PS_On is showing 3 volts

          everything else checks out fine

          I'll get those pictures posted in a bit thanks

          Comment

            #6
            Re: Samsung LN40E550F7F backlight issue

            44v
            that cant be right - are you sure about that!

            Comment

              #7
              Re: Samsung LN40E550F7F backlight issue

              I didn't think it was right either. I checked it three times. I'll check again.

              Comment

                #8
                Re: Samsung LN40E550F7F backlight issue

                Now it is showing 4.13 volts. My meter was probably not grounded properly or something.

                Comment

                  #9
                  Re: Samsung LN40E550F7F backlight issue

                  I just went down the line and unhooked each bulb individually. It still shuts off after about one second. Does the main board have to be reset after a fault code?

                  Comment

                    #10
                    Re: Samsung LN40E550F7F backlight issue

                    Here is a pic of the main board.
                    Attached Files

                    Comment

                      #11
                      Re: Samsung LN40E550F7F backlight issue

                      Does anyone know where I can find a schematic for a bn44-00440b power supply board? I need to know how to bypass the protection circuit.

                      Comment

                        #12
                        Re: Samsung LN40E550F7F backlight issue

                        So here is a recap. The unit has a new power supply board and a new inverter board. With the main board unplugged I am still getting the two seconds to black problem. I felt that the only problem left to check was for a bad lamp. I made one of the testers out of an old bulb, grounded one lead to the chassis and soldered the other lead to the inverter output. I tried it on each of the eight outputs. The two seconds to black issues persisted. I'm lost.

                        Comment

                          #13
                          Re: Samsung LN40E550F7F backlight issue

                          That Manual is available at servicemanuals.mobi.

                          http://www.servicemanuals.mobi/samsu...pply-sch-.html

                          you can register for free to obtain it

                          Comment

                            #14
                            Re: Samsung LN40E550F7F backlight issue

                            I face similar issues with a Samsung lns3241d. It powers on, the back light flashes then goes off. I have image for a split second. I tested the plug to find I have max 4.8 v going to the inverter board and zero's out.
                            suggestions? caps are new

                            Comment

                              #15
                              Re: Samsung LN40E550F7F backlight issue

                              Originally posted by KevinCro View Post
                              I purchased this tv broken. It was completely dead. I opened it up and found a blown fuse on the power supply. The new fuse blew immediately. After checking all over the board to find a problem to no avail I simply replaced the power supply board. The new board allowed the tv to turn on. However the backlight comes on only for a couple of seconds and then goes out. The display is there and can be seen with a flash light. I changed out the inverter board as well. The new inverter board did not fix the problem. I disassembled the tv further to inspect the lamps. They all looked good. The screen has no dark spots when the back light is working. I'm not sure what to try next. Any help is very much appreciated.

                              Kevin
                              got the same did u finaly found the probleme ?

                              Comment

                                #16
                                Re: Samsung LN40E550F7F backlight issue

                                got the same did u finaly found the probleme ?

                                Comment

                                  #17
                                  Re: Samsung LN40E550F7F backlight issue

                                  This problem would be the fault of bad CCFL tubes. If you can't get CCFL tubes you could probably convert your TV to an LED TV.

                                  Comment

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