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    Toshiba 50L5200U

    I'm working on a Toshiba 50L5200U that after running about 15-20 minutes, the back lights shut off. It uses a 27-D074905 LED driver board, and I found a replacement on Ebay for only $24. But before I order it, I just want to make sure it's the problem. I've verified that the BL_On signal is 3.28V, the Dim signal is 2.422, and the 24V BL voltage is present, when it is working and also after it blacks out. But I want to rule out a couple other possibilities for the blackouts. The LED driver has the main power plug that has the 24V supply and the BL On and Dim signals from the main board. And it has 4 ribbon cables going to the LED strips inside the panel. But there is also a 5th ribbon cable going all the way down to the T-Con board. So I'm trying to figure out if there's any easy way to tell if the T-con is sending a signal to turn off the backlights, or if there are shorted LED's overloading the driver board causing it to shut down, of if the LED driver board itself is bad. Should I just go ahead and order the replacement driver board to eliminate it and hopefully fix the problem, or is there any further troubleshooting steps anyone could recommend? Unlike the main power connector, the pins of the ribbon cable to the T-Con are not labeled on either end.

    #2
    Re: Toshiba 50L5200U

    Try turning down the backlights level (not brightness) down to as low as possible to see if the backlights will stay on longer.
    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


      #3
      Re: Toshiba 50L5200U

      When the backlights fail, do you still have audio and did you try the flashlight test to see if you still have picture?

      Comment


        #4
        Re: Toshiba 50L5200U

        Yes I did still have audio when the screen went dark. And good idea about the flashlight test. That would definitely let me know if my T-Con is shutting down. I'm at work now but I'll have to check that out when I get home tonight. Thanks much for the suggestion, I'll let you know how it looks later on.

        Comment


          #5
          Re: Toshiba 50L5200U

          You have Backlights failure. The T-CON Bd is good.

          Comment


            #6
            Re: Toshiba 50L5200U

            Yes most likely, but the only reason I suspected the T-Con at all is that there is a ribbon cable that runs directly from the LED driver board to the T-Con. So I didn't know if it could be turning off the back lights? (I did verify the BL_On and Dim signal from the main board are still present when it goes dark)
            By the way, this is one of those thin "edge lit" panels that use the thin LED strip along side the panel and not LED strips behind it like most TV's. When it was lit up, the lighting on the screen looked fairly uniform and no dark spots. So I don't suspect the LED strips. I believe the problem is either going to be the LED driver board or a very remote possibility that the T-Con is shutting it off through that ribbon cable. That's the main reason I posted was to see if anyone has ever seen that configuration before and might possibly know what that ribbon cable controls as the pins aren't labeled on either end. When I first opened the TV, I thought I smelled a slight burned smell coming from the T-Con area but I took the cover plate off and didn't see anything that looked burned. I'll do the flashlight test tonight when I get home.

            Comment


              #7
              Re: Toshiba 50L5200U

              The cable from the T-CON Bd. to the LED Driver would be sending PWM (P-DIM) signal to the LED Driver to control the LEDs brightness. BL-ON signal would be from the Main Bd. to the LED Driver.

              Comment


                #8
                Re: Toshiba 50L5200U

                Your TV has Dynamic Contrast (Dynamic Backlights control) that why the dimming control is based on the video scene to give you that 5000:1 fake contrast ratio.
                Disable the Dynamic contrast and then turn down the backlights level to see if Backlights will stay on longer.
                Last edited by budm; 10-22-2018, 07:09 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


                  #9
                  Re: Toshiba 50L5200U

                  I've been busy and haven't gotten around to posting for a couple days. The TV did pass the flashlight test, I still had video after the screen went dark. So I ordered a replacement LED driver board and it came in today. Unfortunately after running almost an hour, it blacked out again with the new board. I remember seeing or hearing about this problem before in these Toshiba's with the edge-lit panels that the LED strips tend to go bad. They're even more difficult to replace than standard LED strips and may not be worth the time and effort on a 6 year old set, plus they don't seem to be available for this model. The backlight brightness was set at 78% and that dynamic thing was turned on. So I turned it off and turned the backlights down to 40% and still achieved a good picture with a little compensating of the brightness and contrast settings. I'm letting it run now to see if it hangs in there this time and how long. I wonder if it would be of any advantage to update the firmware?

                  Comment


                    #10
                    Re: Toshiba 50L5200U

                    Nope. No advantage.

                    Comment


                      #11
                      Re: Toshiba 50L5200U

                      Does anyone know exactly what fails on these LED strips that cause the TV to black out after running awhile? (if the problem even is in the LED strips and not elsewhere? I already changed the driver board.) And will they continue to get worse and eventually act up at a lower setting? And is turning down the backlights a good permanent fix, or should they be replaced? I let it run 2 1/2 hours last night without any blackouts after turning the backlight brightness down to 40%. I did find one used set on Ebay for $23.99 but it's a big job to change them and being used I could end up with the same problem. And the TV is 6 years old so not sure how much time and effort it's worth.

                      Comment


                        #12
                        Re: Toshiba 50L5200U

                        The LED's failed due to being driven at high current close to the limit and with poor thermal management. I would use it as is at lower backights level (running at cooler temp) until the LED finally fail completely.
                        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


                          #13
                          Re: Toshiba 50L5200U

                          Four years later, and I have the same problem as OP. Same model number, same operational symptoms. (That is, no voltage measurements taken.)

                          "Does anyone know exactly what fails on these LED strips that cause the TV to black out after running awhile? If the problem even is in the LED strips and not elsewhere?"

                          That's my question, too. Expressed another way: If the LED strips work for 15 minutes or so, why would they suddenly not work ... and then work again the next day the TV is powered on?

                          A YouTube video has convinced me I can take the set apart (not so sure about putting it back together) to replace LED strips or boards, but I have no idea which strip or board is bad. My home doesn't have a work shop where I can leave the disassembled TV. I need to replace something and the TV works, first time all the time. :-)

                          Summarizing: the TV works perfectly for 10-15 minutes, then video goes black while audio remains working. Flashlight test shows an image. I've reduced everything possible in the menu controls.

                          What would you do?

                          All advice appreciated.

                          Comment


                            #14
                            Re: Toshiba 50L5200U

                            LEDs are wired in series like a string of Christmas lights. It only takes one bad LED to shut down the string or to kick in protection shutdown. All the LEDs have wear and will eventually fail which is why they should be replaced with new. LEDs wear out just like light bulbs and wear faster the brighter and hotter they run. Have bad history with Toshibas. Bad backlights can damage the power supply and vice versa. Avoid them now.

                            Comment


                              #15
                              Re: Toshiba 50L5200U

                              Have you tried the same thing I did above? I turned down the backlight level (not the brightness or contrast) to 40% and mine stayed lit up indefinitely, and is still working today almost 4 years later. Before taking it apart and trying to replace anything, I would try turning the backlight level as low as possible to still have a good picture, and turn off the dynamic contrast. You may get a few more years out of it. Edge lit panels are even more difficult to take apart and work on than the standard type with the LED strips behind the screen. Something I would not recommend attempting for someone without much experience. Or if you do attempt it, try to find a good video on YouTube first. And of course be sure you can get and have a new LED strip before taking it apart. I never found one for mine but haven't looked for a few years either so I don't know if there are any available.
                              And as for having a good place to work on TV's, I'm in a small 1 bedroom rental with no shop or workbench. I take most of my TV's apart on the bed. It's a soft surface that won't damage the screen setting it face down to take the back off. I hope this helps.

                              Comment


                                #16
                                Re: Toshiba 50L5200U

                                "I turned down the backlight level (not the brightness or contrast) to 40% and mine stayed lit up indefinitely, and is still working today almost 4 years later."
                                Exactly the adjustments I made, and my set also ran for hours, longer than I'd ever watch at one time.
                                But the picture seems unacceptably dim.
                                Maybe I'm spoiled by my 16-year-old 42" plasma Panasonic, which though only 720p still looks terrific.
                                "...have a new LED strip before taking it apart. I never found one for mine but haven't looked for a few years either so I don't know if there are any available." Yeah, I can't find any either. And if I found one, it would probably be used and little better than what I have now.
                                You know what, I'm going to sell my set. Ha, the base alone is on Ebay for fifty bucks, though who would ever need a base.
                                I appreciate your sharing your experience and advice, Keshenatech1, and for saving me a lot of unnecessary work.

                                Comment

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