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Toshiba 49L621U - Backlight protection shutdown

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    Toshiba 49L621U - Backlight protection shutdown

    Have common flash of Toshiba logo and backlight shutdown. Set uses power supply / LED driver bd. PK101W1230I. I Measured power supply to main board connector. (at power supply connector P802) and power supply LED driver connector to LEDS. (P803) All measured to chassis ground.
    At logo flash: All 16V lines (16V2, 16VA) = 16.8V & 16.2V, 5V1= 5.1V, ACD=3.34V, PWON= 4.05V, BLON= 3.35V, 5V1= 5.1V, DIM= 5.1V
    Two readings, one at power on, second at logo flash: -VLED 1, = 6V/ 34V, -VLED2 = 6.6V/37V, & -VLED3= 6.6V/ 37V. +VLED (3 COMMON WIRES) = 66V/101V
    Pulled PS led connector (P803). Took a LED tester and injected between –LED 1,2, 3 (one at a time) and appropriate +LED LINE, turn set on and verified ALL 84 LEDS ARE GOOD & VIDEO/ SOUND/GRAPHICS IS OPERATIONAL. Set does NOT shutdown. Tester stabilizes at 75.2V on all three lines.
    I believe problem is in shutdown protection circuitry. Believe ruled out TCONN & MAIN LOGIC BDS. Problem resides on Power Supply?
    Can anyone advise as to components involved in protection/shutdown/comparator operation? What to check, theory of operation? Would purchase Service manual/ Schematics? Thoughts Please! Any response greatly appreciated. Thank You!!

    #2
    Re: Toshiba 49L621U - Backlight protection shutdown

    It could be ONE bad led in the panel (likely -VLED 1) and that will be enough to shut down the circuit, Unless you removed the lcd panel to see allthe led's you cannot tell if one ore more led's are bad.

    Comment


      #3
      Re: Toshiba 49L621U - Backlight protection shutdown

      Those Toshiba models have bad backlights that can potentially damage the LED controller on the power supply. You will likely need to replace both the backlights and the power supply.

      Comment


        #4
        Re: Toshiba 49L621U - Backlight protection shutdown

        So I pulled the panel and found as thought ALL leds are good. They light at about 19.2V for all strips. (7 leds per strip, 4 strips per set, 12 strips, 84 total leds) Have been looking at power supply disconnected. The 3 power mosfets (for leds) all look good in circuit, at least not shorted. I suspect the 28 pin SMD device HAI7207CPC which is I think the shutdown monitoring circuit. Of course I cannot find any datasheet for the device. I am leaning towards getting a power supply board. Anybody have any input. Again thanks for any help offered. Help Please!

        Comment


          #5
          Re: Toshiba 49L621U - Backlight protection shutdown

          In my experience, on this model the Led strips will test good with a tester but fail when powered up with the full current of the power supply. I believe you'll need to replace the backlights. Best to do them all as a set and then turn down the backlight level in settings.

          Comment


            #6
            Re: Toshiba 49L621U - Backlight protection shutdown

            Can you power up set momentarily with power supply board and main board but without TCON and screen, to verify the leds are bad at full current of PS? I would think not! I haven't seen anyone due that before.

            Comment


              #7
              Re: Toshiba 49L621U - Backlight protection shutdown

              I do it all the time, but I'm not sure what that test would confirm as you already know the issue is not with the Tconn or screen.

              Once you've replaced the LEDS I would do that to make sure you've got all the connects tight etc.

              Top Tip: Before testing this way, make sure you disconnect the LVDS cable at the Main Board. If the ribbons from the Tconn touch the metal case while powered up (because you've removed the screen) they can short out the Tconn .

              Comment


                #8
                Re: Toshiba 49L621U - Backlight protection shutdown

                JJanssen7, thanks for input! Plug in just main logic, and PS with LVDS cable off. Did momentary test over and over again watching leds. All light bright on flash. Is this valid to rule out leds or can these bad boys flash and still be bad thus shutting down the fet?? Does anyone have a source with stock on a set of 12 strips of leds (7 per) Toshiba 4708-K49WDR-A1213K11 LED Backlight Strips. Or equivalent ??

                Comment


                  #9
                  Re: Toshiba 49L621U - Backlight protection shutdown

                  Intermittent LEDs can draw higher or lower voltage triggering the shutdown. Without a LED tester, you may be able to probe the voltage going to the strips.

                  Comment


                    #10
                    Re: Toshiba 49L621U - Backlight protection shutdown

                    I have found a LED tester to be less than helpful with these models. I have repaired many of the 43 & 49" versions with the symptoms above. (All strips light up with a tester and flash once when powered up or stay on for between 3 seconds to 15 minutes or more and then shut off, image still on screen.)

                    A new set of strips resolves the problem in 95% of my cases. (I say 95% because no "fix" is 100% of the time).

                    It is usually just one strip that is failing and is not revealed by a tester... I've even tried two different testers, one that is supposedly able to supply more current. The testers simply cannot reveal which strip is failing.

                    My solution (given the failure of LED testers to reveal the problem with this model) is to swap out the strips one by one with a known working strip and power up the TV (with the screen and tconn disconnected of course) and inevitably there is just one strip that has been pulling down the system.

                    But here's the thing. IF you replace just the one strip, the TV will work fine for a month to 3 months and then another strip will fail, requiring the whole procedure again.

                    Comment


                      #11
                      Re: Toshiba 49L621U - Backlight protection shutdown

                      Originally posted by Cardioman View Post
                      So I pulled the panel and found as thought ALL leds are good. They light at about 19.2V for all strips. (7 leds per strip, 4 strips per set, 12 strips, 84 total leds) Have been looking at power supply disconnected. The 3 power mosfets (for leds) all look good in circuit, at least not shorted. I suspect the 28 pin SMD device HAI7207CPC which is I think the shutdown monitoring circuit. Of course I cannot find any datasheet for the device. I am leaning towards getting a power supply board. Anybody have any input. Again thanks for any help offered. Help Please!
                      So how much current did you force through the LED? 100mA? 200mA?
                      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


                        #12
                        Re: Toshiba 49L621U - Backlight protection shutdown

                        budm, dont know right now but i can measure it next. What current does one leg take normally?

                        Comment


                          #13
                          Re: Toshiba 49L621U - Backlight protection shutdown

                          Most TV LED's are driven around 150 ~ 300 mA of constant current..
                          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


                            #14
                            Re: Toshiba 49L621U - Backlight protection shutdown

                            budm, thanks for reply. measured the Dc current, injecting the led tester (with PS led connector disconnected ) into VLED1- and appropriate VLED+ line one at a time. then moving to next leg, (VLED2- & VLED3-) measured 29MA for the 3 legs. (28 leds per leg)

                            Comment


                              #15
                              Re: Toshiba 49L621U - Backlight protection shutdown

                              Need 12 NEW LED strips Toshiba #4708-K49WDR-A1213K11 for 49L621U. Only found in Canada at $120. Set isn't worth it. Anyone have a complete set to sell or have a source? Thanks!

                              Comment


                                #16
                                Re: Toshiba 49L621U - Backlight protection shutdown

                                Originally posted by Cardioman View Post
                                budm, thanks for reply. measured the Dc current, injecting the led tester (with PS led connector disconnected ) into VLED1- and appropriate VLED+ line one at a time. then moving to next leg, (VLED2- & VLED3-) measured 29MA for the 3 legs. (28 leds per leg)
                                Those LED tester only puts out about 30mA, the newer is supposed to be able to put out in hundreds of mA.
                                So if you can get to the user menu, can you lower the backlights level to as low as possible to see what will happen?
                                Last edited by budm; 09-02-2020, 05:37 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


                                  #17
                                  Re: Toshiba 49L621U - Backlight protection shutdown

                                  budm, Down to LEDS no screen at present. Ran each strip with tester while tapping each led. Found an intermittent led.

                                  Comment


                                    #18
                                    Re: Toshiba 49L621U - Backlight protection shutdown

                                    Hey, nice catch. I have one of these sets to look at soon. I'll try that and see if that's a quicker way to identify which strip is bad.

                                    Comment


                                      #19
                                      Re: Toshiba 49L621U - Backlight protection shutdown

                                      Originally posted by Cardioman View Post
                                      budm, Down to LEDS no screen at present. Ran each strip with tester while tapping each led. Found an intermittent led.
                                      If you are able to drive it at high current the bad one will be shown, but tapping works too if all you have is low current driver.
                                      Glad to hear you nail down the problem.
                                      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


                                        #20
                                        Re: Toshiba 49L621U - Backlight protection shutdown

                                        Found out shop jimmy just got led strips back in stock should anyone need them. Thanks for ALL responses!!

                                        Comment

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