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    Samsung LE40B750 backlight dies after a while

    Hello dear Badcaps gurus,

    I was hoping someone could help me on my fault finding missing.

    I saved an old, "broken" TV that my parents wanted to throw out from the recycler as I´m a strong believer in fixing stuff instead of throwing it out.

    The patient is a Samsung LE40B750.
    The symptom is "screen going dark after a while". The sound is still fine when it occurs.

    Its not the signal source. First tests show its the backlight that dies. If I use a strong flashlight I can see the picture.

    The time till it turns dark is not constant. It seems to be temperature related. If the TV was off for a while it might run normally for a couple of hours. If it has been on/connected to power for some time it might go dark after a couple of seconds.

    Turning it off and on, without disconnecting the power cable, will solve the issue for a few seconds before it reoccurs.

    Visual inspection of all boards didn´t reveal any blown capacitors, burned spots, or anything else that, at least with my limited knowledge, would look obviously broken.

    I did some research in the forum first of course and apparently some similar issues with this model where fixed by cutting JP852.
    I cant seem to find it on my powerboard however. My powerboard is a H40F2_9HS BN44-00284A in Rev 1.2 so I assume this fix was probably for a different revision.


    I attached pictures of the boards. Maybe someone has a good idea?


    Many thanks in advance for anyone willing to help.
    Attached Files

    #2
    Re: Samsung LE40B750 backlight dies after a while

    Check backlight (cold solders, worn neon tubes).
    Last edited by yokoono; 06-07-2022, 01:20 AM.

    Comment


      #3
      Re: Samsung LE40B750 backlight dies after a while

      Originally posted by yokoono View Post
      Replace the LED strips.
      Hello Yokoono,

      thanks for the suggestion, but as far as I´m aware it has none. I believe it is a CCFL?

      Comment


        #4
        Re: Samsung LE40B750 backlight dies after a while

        As you can see from the pictures, the TV has CCFL. I believe that replacing CCCF is no longer cost effective for an old TV. If you want to experience you can backlight for LEDs. It will be a lot of work.

        Comment


          #5
          Re: Samsung LE40B750 backlight dies after a while

          I couldn´t figure out where the CCFL is getting its power from. Is it through those 2 ribbon cables too?

          Comment


            #6
            Re: Samsung LE40B750 backlight dies after a while

            Red: power / main command
            Green: CCFL power supply.
            Attached Files
            Last edited by yokoono; 06-07-2022, 02:15 AM.

            Comment


              #7
              Re: Samsung LE40B750 backlight dies after a while

              Check the resistance of the ccfl transormers. Three of them are the same and the bottom one is slightly different windings resistance. Check the high voltage blue caps on the right of the BN44-00264 combined psu and inverter to the right of the ccfl transformers as well (examine them carefully), as apparently they can also fail. Also update the firmware on the main board as well as I have found issues with the LE40B450 that would sporadically reboot during operation and I would imagine the boards look to be very similar.

              Comment


                #8
                Re: Samsung LE40B750 backlight dies after a while

                Originally posted by cheeky2 View Post
                Check the resistance of the ccfl transormers. Three of them are the same and the bottom one is slightly different windings resistance. Check the high voltage blue caps on the right of the BN44-00264 combined psu and inverter to the right of the ccfl transformers as well (examine them carefully), as apparently they can also fail. Also update the firmware on the main board as well as I have found issues with the LE40B450 that would sporadically reboot during operation and I would imagine the boards look to be very similar.
                I removed the power board and did the measurements you suggested. Please see the measurements in my picture.
                The transformers behave as you predicted, I couldn´t find any datasheet or something to suggest if these are the values they should have.
                As to the high voltage caps, not sure on how to test them? My DMM isn´t really reporting anything for these. 0 Ohms and a minuscule capacity measurement on all of them.

                All measurements were done with everything still in circuit. Not sure if this is messing things up or not.
                Attached Files

                Comment


                  #9
                  Re: Samsung LE40B750 backlight dies after a while

                  Some additional findings.
                  - The higher I set the backlight brightness, the quicker it dies. E.g. on "10" it takes maybe 5 seconds.
                  - The 4 transformers are making a slight buzzing sound when the backlight is on (not sure if this is normal?) and immediately stop when the backlight dies.
                  - Backlight won´t work only with the powerboard connected, i need all 3 boards hooked up for it to go on.

                  Comment


                    #10
                    Re: Samsung LE40B750 backlight dies after a while

                    Stop working in vain. You have used ccfl tubes. To replace, you must lower the panel. Do you have the courage?

                    Comment


                      #11
                      Re: Samsung LE40B750 backlight dies after a while

                      I´m sorry, what do you mean by "lower the panel"?

                      Comment


                        #12
                        Re: Samsung LE40B750 backlight dies after a while

                        Remove the panel. CCFL is behind the panel.
                        https://www.youtube.com/watch?v=EO2gHBJCvH0
                        Last edited by yokoono; 06-07-2022, 09:44 AM.

                        Comment


                          #13
                          Re: Samsung LE40B750 backlight dies after a while

                          Ah. Ok. Understood. So you believe its the tubes themselves?

                          Comment


                            #14
                            Re: Samsung LE40B750 backlight dies after a while

                            Yes. It is high consumption through tubes and driver protection enters.

                            Comment


                              #15
                              Re: Samsung LE40B750 backlight dies after a while

                              Originally posted by Fusion_de View Post
                              I removed the power board and did the measurements you suggested. Please see the measurements in my picture.
                              The transformers behave as you predicted, I couldn´t find any datasheet or something to suggest if these are the values they should have.
                              As to the high voltage caps, not sure on how to test them? My DMM isn´t really reporting anything for these. 0 Ohms and a minuscule capacity measurement on all of them.

                              All measurements were done with everything still in circuit. Not sure if this is messing things up or not.
                              oh dear! You may well have broken or damaged the ccfl tubes by removing the combined inverter psu! The unit is removed by removing the mounting screws and then sliding unit to the left about 2cm. It then comes out from the plastic insulators that connect to your ccfl tubes. You have likely damaged the ends of the ccfl tubes.
                              The fact that it is taking longer to go off means that is most likely failure of one or more of the ccfl tubes as a result of lowering brightness. Going out so no time to write now.

                              Comment


                                #16
                                Re: Samsung LE40B750 backlight dies after a while

                                Originally posted by cheeky2 View Post
                                The unit is removed by removing the mounting screws and then sliding unit to the left about 2cm.
                                Thanks for your concern. I did it just like you said. The CCFLs are fine (well at least as fine as they were before ).


                                I set the brightness to "0" now and it has been running "normally" for about an hour now. Obviously it´s not as bright as the sun now but it´s at least running.
                                If it really is the CCFLs, and it seems to be from what I can tell, repairing it wont really be an option. I checked and they are rare and hard to get and way to expensive for a 10+ year old TV.
                                Knowning my dad he really got the most out of those CCFLs as this TV was running constantly for probably 10-12h a day, even when he wasn´t in the room...

                                Comment


                                  #17
                                  Re: Samsung LE40B750 backlight dies after a while

                                  led conversion, if you want a project, how to video there and many more on youtube: https://www.youtube.com/watch?v=0hmCZS7q1vk

                                  Comment


                                    #18
                                    Re: Samsung LE40B750 backlight dies after a while

                                    Well I am glad you didn't damage them. I would look out for a cheap or free working damaged screen ccfl TV of a 40inch TV. Then strip out the damaged screen of the TV (getting experience first in carefully taking it apart and use one or more of the ccfl tubes in your own TV). Some can be soldered into the ends of the ccfl tubes but you can normally tell which has failed as has blackened ends on the tube and when ignited by inverter will not necessarily be fully lit.
                                    Personally I wouldn't bother with a led conversion as too much mucking around and long term reliability issues (as leds fail far quicker than ccfl tubes).

                                    Comment


                                      #19
                                      Re: Samsung LE40B750 backlight dies after a while

                                      Originally posted by nomoresonys View Post
                                      led conversion, if you want a project, how to video there and many more on youtube: https://www.youtube.com/watch?v=0hmCZS7q1vk
                                      Thanks for the suggestion. While it would be a good learning experience for sure I won´t put that time and money in a TV that is sold used for about 80€.

                                      I´ll probably just deem it a "not worth to fix".

                                      Comment


                                        #20
                                        Re: Samsung LE40B750 backlight dies after a while

                                        Originally posted by cheeky2 View Post
                                        Well I am glad you didn't damage them. I would look out for a cheap or free working damaged screen ccfl TV of a 40inch TV. Then strip out the damaged screen of the TV (getting experience first in carefully taking it apart and use one or more of the ccfl tubes in your own TV). Some can be soldered into the ends of the ccfl tubes but you can normally tell which has failed as has blackened ends on the tube and when ignited by inverter will not necessarily be fully lit.
                                        Personally I wouldn't bother with a led conversion as too much mucking around and long term reliability issues (as leds fail far quicker than ccfl tubes).
                                        I´ll keep an eye out for a used, broken, cheap one but they are extremely rare now as they are very old already. Also it would have to be close by because the shipping fees alone would kill any chance to fix this economically.

                                        Comment

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