Samsung LN40E550 Dim Screen

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  • browserjoe
    Member
    • Aug 2016
    • 22
    • USA

    #1

    Samsung LN40E550 Dim Screen

    Few weeks ago my tv screen became dim, the whole screen is dim with part of it being a little darker than the rest. I'm trying to figure out if the dim screen is a bad backlight inverter or power supply.

    I am a beginner with electronics so I'm not too sure if I was testing the inverter transformers correctly, but the resistance values I measured were pretty much the same on both inverters (around 2k ohms, and I couldn't find specs on the transformer) so I decided to test supply voltage.

    These are the part #'s for the 2 boards
    Power supply BN44-00440A
    Inverter LJ97-00207A

    On the power supply board there is a chart that says:
    High (Fet): 280v ~
    Low (Fet): 195v ACDC

    The voltages I measured with tv on:
    High (Fet): 37v
    Low (Fet): 16v

    Is this low? Should it be closer to the 280v and 195v the board says?

    I also noticed the voltage going to the transformers was different to each one, not sure if this is normal. Picture below.

    T1
    Pin 1 = 36v
    Pin 2 = 23v

    T2
    Pin 1 = 23v
    Pin 2 = 16v

    I also don't see any swollen or leaking caps.
    Attached Files
    Last edited by browserjoe; 09-05-2016, 03:45 PM. Reason: Forgot something
  • budm
    Badcaps Legend
    • Feb 2010
    • 40746
    • USA

    #2
    Re: Samsung LN40E550 Dim Screen

    Your meter more likely not to have high frequency band with to be able to measure high frequency (>40 KHz) AC Voltage.
    It sounds like the inverter transformers are not getting enough drive Voltage.
    What DCV do you have between the two legs of the main filter cap?
    Check the capacitance of cap CI826 to see if it still has the correct capacitance.
    Pictures provided by shopjimy.

    Samsung BN44-00440A (I40F1_BDM) Power Supply Unit
    http://www.shopjimmy.com/samsung-bn4...upply-unit.htm
    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

    • CapLeaker
      Leaking Member
      • Dec 2014
      • 8134
      • Canada

      #3
      Re: Samsung LN40E550 Dim Screen

      post a picture of your "dim" screen, so we can see what's going on

      Comment

      • browserjoe
        Member
        • Aug 2016
        • 22
        • USA

        #4
        Re: Samsung LN40E550 Dim Screen

        Are the 2 large 450v caps the main filter caps? I see CI826 off to the side.

        Before I try testing anything I just want to make sure, the caps have to be removed before doing any testing correct? What about discharging them? From my research I see some things that say discharge with the cap removed, and others show discharging with it still on the board with a 10w 10k ohm resistor.

        Is there a way to identify if the problem is on the power supply board or inverter board without removing any components? I'm not too confident in my circuit board soldering skills at the moment, and was thinking of replacing the whole board with one from shopjimmy for now, then I can take my time and learn on the old one and try to fix it.

        If the only way to find out where the problem lies is to remove components, then please bear with me, I am new to this so may take me awhile.

        As far as a picture of the tv goes, I tried but the screen looks normal in pictures. Just imagine turning the brightness way down, with the right side of the screen being a little dimmer. Adjusting the backlight setting on the tv makes no change, setting 0-8 look the same.

        Comment

        • budm
          Badcaps Legend
          • Feb 2010
          • 40746
          • USA

          #5
          Re: Samsung LN40E550 Dim Screen

          No need to remove the cap to check the DCV to find out if it has correct Voltage or not, but you will be working in the deadly side of the circuit so make sure not to touch anything on the board with your hands and do not let the meter probes slipped on you.
          BTW, I would expect the unit to go into protection shutdown mode if the inverter, lamps are bad.
          May be need to go to service mode to do the reset.
          Last edited by budm; 09-06-2016, 01:04 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

          • browserjoe
            Member
            • Aug 2016
            • 22
            • USA

            #6
            Re: Samsung LN40E550 Dim Screen

            The 2 big caps measured 164v on each one with tv plugged in power off.

            Comment

            • budm
              Badcaps Legend
              • Feb 2010
              • 40746
              • USA

              #7
              Re: Samsung LN40E550 Dim Screen

              what do you get when the TV is on with dim screen, 380Vdc?
              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

              • browserjoe
                Member
                • Aug 2016
                • 22
                • USA

                #8
                Re: Samsung LN40E550 Dim Screen

                158v tv on and video showing on screen, 160v tv on with no video just black, and goes back to 164v when tv is turned off
                Last edited by browserjoe; 09-06-2016, 07:56 PM.

                Comment

                • budm
                  Badcaps Legend
                  • Feb 2010
                  • 40746
                  • USA

                  #9
                  Re: Samsung LN40E550 Dim Screen

                  Originally posted by browserjoe
                  158v tv on and video showing on screen, 160v tv on with no video just black, and goes back to 164v when tv is turned off
                  OK, the PFC Voltage booster circuit is not working, the Voltage on the cap should be around 380VDC or more when the TV is ON.
                  The power supply board will need more in-depth troubleshooting in the PFC section to get the Voltage backup at this point.
                  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

                  • browserjoe
                    Member
                    • Aug 2016
                    • 22
                    • USA

                    #10
                    Re: Samsung LN40E550 Dim Screen

                    What would need to be done to continue the troubleshooting?

                    Comment

                    • budm
                      Badcaps Legend
                      • Feb 2010
                      • 40746
                      • USA

                      #11
                      Re: Samsung LN40E550 Dim Screen

                      Need to find out if the PFC Diode is shorted out or not, if not then need to find out what the P/N of the PFC Controller IC is and then get the spec sheet so you can test and find out if it is getting the VCC to run the IC or not.
                      Lots works involved.
                      BTW, do you do any repair with SMD parts?
                      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

                      • browserjoe
                        Member
                        • Aug 2016
                        • 22
                        • USA

                        #12
                        Re: Samsung LN40E550 Dim Screen

                        I have no experience with repairing smd parts.

                        I removed the power supply to look at the backside of it. PC801S (not sure if it is part of the voltage booster circuit) looks discolored, was wondering if it could be bad, or if the discoloration is just from when it was soldered to the board?
                        Attached Files

                        Comment

                        • dick_barton
                          Badcaps Legend
                          • Aug 2015
                          • 6642
                          • Wales

                          #13
                          Re: Samsung LN40E550 Dim Screen

                          Can't really tell from the picture. Can you supply a close up. PC801S looks as though it's an opto-isolator.
                          Willing to help but I'm no expert.

                          Comment

                          • budm
                            Badcaps Legend
                            • Feb 2010
                            • 40746
                            • USA

                            #14
                            Re: Samsung LN40E550 Dim Screen

                            Get the P/N of the IC (I cannot read the designator), and also check the PFC diodes.

                            BTW, PCxxx designator: PC = Photo Coupler.
                            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

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

                              #15
                              Re: Samsung LN40E550 Dim Screen

                              How are you measuring the PFC voltage?--ref to ground (wrong) Or across the big cap(correctly)....?

                              I suspect nothing wrong with PSU or inverters--more likely panel is stuffed, seen this a few times with 40" Samsung...
                              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

                              • browserjoe
                                Member
                                • Aug 2016
                                • 22
                                • USA

                                #16
                                Re: Samsung LN40E550 Dim Screen

                                Originally posted by budm
                                get the p/n of the ic (i cannot read the designator), and also check the pfc diodes.

                                Btw, pcxxx designator: Pc = photo coupler.

                                smva0u
                                sem3040
                                9u1151

                                Comment

                                • budm
                                  Badcaps Legend
                                  • Feb 2010
                                  • 40746
                                  • USA

                                  #17
                                  Re: Samsung LN40E550 Dim Screen

                                  http://www.datasheetcafe.com/sem3040-datasheet/

                                  Did you check the PFC DIODES?
                                  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

                                  • browserjoe
                                    Member
                                    • Aug 2016
                                    • 22
                                    • USA

                                    #18
                                    Re: Samsung LN40E550 Dim Screen

                                    Sorry, I'm going to need help identifying the diodes. I see DP804, and DP805, but I'm not 100% on DP802, DP803, and DP807. Are 802, 803 and 807 the 3 legged surface mount parts circled in the picture?

                                    Diode test on both DP804 and DP805 measure 0.117 in both directions. Will I have to remove them and retest?
                                    Attached Files

                                    Comment

                                    • browserjoe
                                      Member
                                      • Aug 2016
                                      • 22
                                      • USA

                                      #19
                                      Re: Samsung LN40E550 Dim Screen

                                      Voltage at the VCC pin on the IC is 14.56V

                                      Comment

                                      • budm
                                        Badcaps Legend
                                        • Feb 2010
                                        • 40746
                                        • USA

                                        #20
                                        Re: Samsung LN40E550 Dim Screen

                                        PFC Diode DP805 is the big barrel body diode right next to cap CP801S.

                                        https://www.badcaps.net/forum/attach...7&d=1473114608
                                        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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