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Sansui 55" LCD T V Model STY0955 (Backlight issue)

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    Sansui 55" LCD T V Model STY0955 (Backlight issue)

    Hi Guys, i posted this message yesterday and now i cant find it anywhere on the forum so now i am going to repost.
    I have added pictures to assist in your understanding.
    Originally this set would come up bright and back lights would switch off intermittently. I opened up the panel and saw that all the LED's do come on but then at a point thy all just fade off. I checked around the PWM controller and at some point while measuring the LED's went dim. I can still see a blueish light emanating from the rear of the TV and am measuring 38.6V on the LED+ line with respect to chassis gnd. On the 6 LED- lines i get 1.6V.
    Would this indicate a failed power supply or can it be the back lights causing this dim light issue?
    I bought myself a LED backlight tester but i dont know if its a quality unit because it only indicates the LED voltage once connected to a LED string.
    I get LED's brighting up somewhat with a voltage indication of about 41V on the tester.
    My question is is it the power supply or the back lights?
    Attached Files

    #2
    Re: Sansui 55" LCD T V Model STY0955 (Backlight issue)

    Is there way to delete this post since i am the author?

    Comment


      #3
      Re: Sansui 55" LCD T V Model STY0955 (Backlight issue)

      If your backlights are blue they need replaced.

      Comment


        #4
        Re: Sansui 55" LCD T V Model STY0955 (Backlight issue)

        They have a blueish color when driven at very low voltage. When the power supply was working correct and i had removed the LCD to investigate this problem they where bright white. I am going to check what Budm asked for and then post the result on the duplicate thread. He wanted an oscillogram of the ADJUST pin. I was busy diagnosing the problem when the customer got all funny and requested me to close the set and he took it. Now after a month he brought it back so now i can continue my diagnosis. Check the other thread for the history.

        Comment


          #5
          Re: Sansui 55" LCD T V Model STY0955 (Backlight issue)

          Please stay in this TV section instead of the power supply thread so I can easily keep track.
          So I copied your paost 1 and my reply here:
          https://www.badcaps.net/forum/showth...887#post882887
          Angelos:
          Hi Guys,
          This tv came in with a back light that used to switch off intermittently. After measuring around the PWM IC and i can confirm that i made NO SHORT CIRCUIT the LED's went quite dark and stayed like that. They barely shine!!!
          This is the readings i have,
          LED+ = 38.6V
          LED+ = 38.6V
          LED+= 38.6V
          LED+= 38.6V
          LED+= 38.6V
          LED+= 38.6V
          LED6- = 1.62V
          LED5- = 1.60V
          LED4- = 1.61V
          LED3- = 1.62V
          LED2- = 1.62V
          LED1- = 1.63V

          also the Adjust pin is 5.1V
          and BL-ON = 1.6V

          If i use a backlight tester and they all come on, not to say that they are not intermittent but the LED tester indicates a voltage of 41V on all six circuits and there is a blueish light emanating from the rear stand offs. I guess being biased slightly.
          Previously before these back lights went from intermittent off to permanently dim i did take off the LCD display and put power to see that all LED's came on nice n bright and then after a few minutes they just faded off. I then started fault finding around the PWM and that's what caused the failure of output voltage.
          Just to give you an idea i disconnected one of the LED- leads and put my Fluke meter in series to monitor the current and when i powered up the TV the backlights flashed and went off immediately. As if it did not like the fluke in series and i cant understand why.

          My question is, is it the back lights or the power supply?

          My reply:
          1) TV OFF, put meter probe on the the two legs of capxon cap C13 (100uf 100V).
          2) Set meter to 200V scale then turn on the TV while watching the meter to see how high it goes up to then wait 10 seconds to see how low it goes down to.
          3) Report the reading.
          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


            #6
            Re: Sansui 55" LCD T V Model STY0955 (Backlight issue)

            Hi Budm
            The test is as follows.
            Set on standby and i press the standby button,
            Initially the voltage goes to 30VDC and then peaks at 38.63V after a few seconds, 10 seconds thereafter the reading remains at 38.6V
            I added additional info you asked for previously.
            The picture DSC0018 is the ADJUST pin as measured on the plug CN1 at about 5VDC
            The picture DSC0019 is the BL-ON pin as measured on CN1 measuring 200HZ at about 5VDC
            Attached Files

            Comment


              #7
              Re: Sansui 55" LCD T V Model STY0955 (Backlight issue)

              Are you sure that you did not get the two control signal backward, the BL-ON should be steady Voltage that either LO (<1V), or HI >2.5v.
              The Dim may be PWM 200 Hz freq.
              Can you read the P/N of the LED power supply IC?
              Last edited by budm; 02-21-2019, 11:17 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


                #8
                Re: Sansui 55" LCD T V Model STY0955 (Backlight issue)

                Originally posted by budm View Post
                Are you sure that you did not get the two control signal backward, the BL-ON should be steady Voltage that either LO (<1V), or HI >2.5v.
                The Dim may be PWM 200 Hz freq.
                Can you read the P/N of the LED power supply IC?
                No, i double checked, what i can do is make a small video of me measuring those two pins and upload it to youtube. Would that be fine?
                The PWM IC is a AP3041. The datasheet is on the other thread i made together with the schematic of that power supply.

                Comment


                  #9
                  Re: Sansui 55" LCD T V Model STY0955 (Backlight issue)

                  Please provide the link to the schematic of the power supply.
                  It is not here: https://www.badcaps.net/forum/showth...887#post882887
                  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


                    #10
                    Re: Sansui 55" LCD T V Model STY0955 (Backlight issue)

                    Originally posted by budm View Post
                    Please provide the link to the schematic of the power supply.
                    It is not here: https://www.badcaps.net/forum/showth...887#post882887
                    Here it is again.
                    Attached Files

                    Comment


                      #11
                      Re: Sansui 55" LCD T V Model STY0955 (Backlight issue)

                      I can confirm that the silk screen printing on CN1 is incorrect. It reads that pin 1 is Adjust and pin 2 is BL-ON but in actual fact they are swapped around. I followed the race and Adjust goes to pin 15 of AP3041 and BL-ON goes to pin 4 of the same IC.
                      I will now take oscillograms on the input pins of the PWM of both these voltages and post an update.

                      Comment


                        #12
                        Re: Sansui 55" LCD T V Model STY0955 (Backlight issue)

                        Originally posted by angelos View Post
                        I can confirm that the silk screen printing on CN1 is incorrect. It reads that pin 1 is Adjust and pin 2 is BL-ON but in actual fact they are swapped around. I followed the race and Adjust goes to pin 15 of AP3041 and BL-ON goes to pin 4 of the same IC.
                        I will now take oscillograms on the input pins of the PWM of both these voltages and post an update.
                        This really confuses me but i traced out a simplistic sketch of the two inputs to the AP3041
                        Picture DSC_0020.jpg measured directly on pin 4 of AP3041
                        Picture DSC_0021.jpg measured directly on pin 15 of AP3041
                        I added a simple sketch of how i traced out the diagram. Hope this helps. I am dumb struck.
                        Attached Files

                        Comment


                          #13
                          Re: Sansui 55" LCD T V Model STY0955 (Backlight issue)

                          What if someone has swapped the individual conductors on CN1's pin 1 and 2 around. This makes sense to me. Could swapping them around to test cause any damage? Since both are inputs to the AP3041 and the datasheet for the PWM IC shows that it can handle a range of voltage. I means the ENABLE pin is to switch on and off the PWM IC. Should it be doing that at 200Hz?

                          Comment


                            #14
                            Re: Sansui 55" LCD T V Model STY0955 (Backlight issue)

                            The EN pin needs to see stable high or stable low, if it is switching HI/LO the the IC will be turning on and off. So did some one work on this TV before you?
                            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


                              #15
                              Re: Sansui 55" LCD T V Model STY0955 (Backlight issue)

                              Yes, the warranty seal was disturbed. Its night time now but in 8 hours i can work on this set. Can i swap those two wires? Have i got anything to lose?

                              Comment


                                #16
                                Re: Sansui 55" LCD T V Model STY0955 (Backlight issue)

                                Yes you can, you need to get EN control signal to be steady HI to enable the IC.
                                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


                                  #17
                                  Re: Sansui 55" LCD T V Model STY0955 (Backlight issue)

                                  Problem sorted out. TV working. Just monitoring to see for possible intermittent faults. Thanks BudM for your guidance.

                                  Comment


                                    #18
                                    Re: Sansui 55" LCD T V Model STY0955 (Backlight issue)

                                    Thanks for the follow-up, I wonder why some decide to swap the control signal wires; it makes no sense to me and made me wonder what the original problem was.
                                    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


                                      #19
                                      Re: Sansui 55" LCD T V Model STY0955 (Backlight issue)

                                      The customer said that the back lights would switch off intermittently. The tv worked for 6 hours today without issue. In the mean time i found a bunch of dry joints on the AP3041 which i soldered. At least its sorted. Off topic i needed to order some speakers for a 55" inch LG TV. The price i got for the set is shocking.Close to $100.

                                      Comment


                                        #20
                                        Re: Sansui 55" LCD T V Model STY0955 (Backlight issue)

                                        Hello, I have a problem with the PSU of the TV Haier 50 "same PSU as the Sansui model STY095, I have replaced all the LEDs, two mosfet aod4286, check resistors, checked filter / inflated capacitors and nothing. Measuring voltage on pin 3 of the ic ap3041 I have 1.65vdc, but I miss that when I place the tip of the fluke the LEDs turn on, thinking that it would be a false contact, I rectified the welding but the problem continues.On the board is present 5, 12 vdc but I have raised the 30vdc to 33vdc. Any other help would be welcome to solve the problem.

                                        Comment

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