Samsung LN40E550F7F backlight issue

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  • KevinCro
    Senior Member
    • May 2014
    • 104
    • United States
    #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
    • stj
      Great Sage 齊天大聖
      • Dec 2009
      • 30941
      • Albion
      #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

        • KevinCro
          Senior Member
          • May 2014
          • 104
          • United States
          #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

            • budm
              Badcaps Legend
              • Feb 2010
              • 40746
              • USA
              #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

                • KevinCro
                  Senior Member
                  • May 2014
                  • 104
                  • United States
                  #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

                    • stj
                      Great Sage 齊天大聖
                      • Dec 2009
                      • 30941
                      • Albion
                      #6
                      Re: Samsung LN40E550F7F backlight issue

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

                        Comment

                        • KevinCro
                          Senior Member
                          • May 2014
                          • 104
                          • United States
                          #7
                          Re: Samsung LN40E550F7F backlight issue

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

                            Comment

                            • KevinCro
                              Senior Member
                              • May 2014
                              • 104
                              • United States
                              #8
                              Re: Samsung LN40E550F7F backlight issue

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

                                Comment

                                • KevinCro
                                  Senior Member
                                  • May 2014
                                  • 104
                                  • United States
                                  #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

                                    • KevinCro
                                      Senior Member
                                      • May 2014
                                      • 104
                                      • United States
                                      #10
                                      Re: Samsung LN40E550F7F backlight issue

                                      Here is a pic of the main board.
                                      Attached Files

                                        Comment

                                        • KevinCro
                                          Senior Member
                                          • May 2014
                                          • 104
                                          • United States
                                          #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

                                            • KevinCro
                                              Senior Member
                                              • May 2014
                                              • 104
                                              • United States
                                              #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

                                                • jeisentrager
                                                  New Member
                                                  • Aug 2015
                                                  • 1
                                                  • USA
                                                  #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

                                                    • stormyt
                                                      New Member
                                                      • Nov 2014
                                                      • 5
                                                      • usa
                                                      #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

                                                        • mickt6
                                                          New Member
                                                          • Oct 2017
                                                          • 2
                                                          • canada
                                                          #15
                                                          Re: Samsung LN40E550F7F backlight issue

                                                          Originally posted by KevinCro
                                                          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

                                                            • mickt6
                                                              New Member
                                                              • Oct 2017
                                                              • 2
                                                              • canada
                                                              #16
                                                              Re: Samsung LN40E550F7F backlight issue

                                                              got the same did u finaly found the probleme ?

                                                                Comment

                                                                • Andrew F. Ali
                                                                  Badcaps Legend
                                                                  • Jan 2014
                                                                  • 2450
                                                                  • Trinidad & Tobago
                                                                  #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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