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TCL 55C803 - Powers on, good picture, then shuts off

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    TCL 55C803 - Powers on, good picture, then shuts off

    So I received this TV to repair for a friend. Initial symptom the TV would flash backlight then off. Since I've seen this many times, tested the Backlight, and sure enough, 1/4 of the edge lit strip was out.

    Replaced with a good one (can't find new, bought used, but checks out with the LED tester), buttoned up.. when I power it up, I get the pinkish screen, showing TCL Logo, 4K ... stays on for 10 seconds, then back light off, then the same screen comes on for few seconds, then backlight totally off. Image is visible with a flashlight.

    Seems to respond to remote keys (you can hear sound from the TV when you click buttons.) I see the picture with a flash light. I updated software using the flashlight. Pain!

    connection between main board and PS board is not labeled. So I'm not sure what these signals are.

    I'm nervous when I fix one problem to find another- what are the chances? Appreciate the help.
    Last edited by jason123; 04-14-2021, 05:39 PM.

    #2
    Re: TCL 55C803 - Powers on, good picture, then shuts off

    Ok, I'm talking to myself here.. but if anyone has ideas, please let me know.
    I was monitoring the BL_ON pin on the power supply. As it's booting up, whenever the BL_ON comes on, the backlight turns on, except the 3rd time. The BL_ON pin has 3.2 V, but the Backlight turns off. WTH? Help!

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      #3
      Re: TCL 55C803 - Powers on, good picture, then shuts off

      Anyone?? Why when BL_ON is 3.2V would backlight be off (where it was on for 10 sec each time, steady, everything lit?) Is this thread visible
      Attached Files
      Last edited by jason123; 04-15-2021, 09:57 AM.

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        #4
        Re: TCL 55C803 - Powers on, good picture, then shuts off

        Check your solder joints on powerboard, are you getting about 400 volts on your big filter cap/s?

        Comment


          #5
          Re: TCL 55C803 - Powers on, good picture, then shuts off

          Thanks nomoresonys... I did basic connector pin touch up .. but the board doesn't look stressed, so I didn't see bad areas.

          As for the large caps, even when the backlight is on during the boot up cycles, the voltage is ~154 and 160V. It says in that range and never goes up. PFC Circuit? Any help is appreciated.
          Last edited by jason123; 04-15-2021, 01:56 PM.

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            #6
            Re: TCL 55C803 - Powers on, good picture, then shuts off

            Not exactly sure, maybe it is the pfc, could be it doesnt go high so it shuts off after seconds? Did you check the diodes on the pfc circuit? check all the diodes just for good measure.
            Last edited by nomoresonys; 04-15-2021, 02:55 PM.

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              #7
              Re: TCL 55C803 - Powers on, good picture, then shuts off

              Wonder if maybe its just a bad capacitor, could try heating the electrolytic caps with a hairdryer and see if it stays on longer.

              Comment


                #8
                Re: TCL 55C803 - Powers on, good picture, then shuts off

                I found a shorted N-Channel FET (QW1), part number AP50PN520. Also I see R317 is Open, but I can't make out the value. Anyone by any chance has a schematic for this?

                Also, if anyone knows of a replacement for the AP50PN520, as it doesn't seem to be available

                Update:
                I have a TK15A50D (close spec to the AP50PN520), and I have a 30 Ohm resistor that I threw on also. Still no high voltage on the Caps. When I measure reference to HOT GND, I get about 13.7 V on both Gate and Source, and 161V (~same voltage on the large caps) on the Drain.

                I think the FAN7930C is working, but I'm not sure why the FET is not fully on?
                Attached Files
                Last edited by jason123; 04-15-2021, 03:47 PM.

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                  #9
                  Re: TCL 55C803 - Powers on, good picture, then shuts off

                  Is U301 ok, and the diodes in that area and the mlc caps C303, 306, 307, 308?
                  Last edited by nomoresonys; 04-15-2021, 04:32 PM.

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                    #10
                    Re: TCL 55C803 - Powers on, good picture, then shuts off

                    13v on the source would suggest the source resistor is open. It might be R306 (0.15Ω). The gate resistor seems to be 22Ω 220) It is very likely that the ic is bad, when the source resistor opens, it supplies a high voltage via R320 into the ic's CS (current sense) pin.
                    Last edited by R_J; 04-15-2021, 04:41 PM.

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                      #11
                      Re: TCL 55C803 - Powers on, good picture, then shuts off

                      R306 is reading 16.45 kOhm. Would you replace this resistor or the IC (FAN7930C) also?
                      Any thoughts on the transistor substitution and the use of 30 Ohm on the gate?
                      Last edited by jason123; 04-15-2021, 05:00 PM.

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                        #12
                        Re: TCL 55C803 - Powers on, good picture, then shuts off

                        You can try replacing just the resistor, but what can happen is when the source resistor R306 goes open it is because the mosfet shorted D~S and once R306 is open there is 160vdc on the mosfets source, which is way more than the fan7903 can handle on its SC input.

                        Comment


                          #13
                          Re: TCL 55C803 - Powers on, good picture, then shuts off

                          Got you... any chance you'd know the power rating of the 0.15 Ohm?

                          Comment


                            #14
                            Re: TCL 55C803 - Powers on, good picture, then shuts off

                            I don't know the wattage, You should be able to tell by looking at the original one, likely 1/2 watt flame proof
                            Last edited by R_J; 04-15-2021, 06:45 PM.

                            Comment


                              #15
                              Re: TCL 55C803 - Powers on, good picture, then shuts off

                              I would replace the IC too unless you want to take a chance of damaging the new MOSFET.
                              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


                                #16
                                Re: TCL 55C803 - Powers on, good picture, then shuts off

                                it's on order. budm, any thoughts about using TK15A50D in place of the AP50PN520 and 30 Ohm on the gate pin (instead of 22)?
                                Thank you

                                Comment


                                  #17
                                  Re: TCL 55C803 - Powers on, good picture, then shuts off

                                  Hi All.. replaced the PFT IC, FET, the 0.15 Ohm, the 22 Ohm driving the gate of the FET, and now the large caps are now up to 394 V. However, the LEDs (new) simply flash on then off right away. Any further guidance is appreciated.

                                  Comment


                                    #18
                                    Re: TCL 55C803 - Powers on, good picture, then shuts off

                                    It could still be a led problem, the led tester only supplies a low current to the led's under test, under real conditions they can fail.
                                    Monitor the voltage on D615 +(cathode) then turn on the tv and watch the voltage, Then do the same monitoring the voltage on D617.
                                    Another option is to check the voltage on (smd) D630 anode, this is the OVP line.
                                    Last edited by R_J; 04-29-2021, 08:24 PM.

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                                      #19
                                      Re: TCL 55C803 - Powers on, good picture, then shuts off

                                      Thanks R_J. I will look at these components and report back.
                                      The LEDs are ok. I ordered a power board and the work ok. Troubleshooting the power board now.

                                      Comment


                                        #20
                                        Re: TCL 55C803 - Powers on, good picture, then shuts off

                                        So on the Cathode (-) of D615 and D617 is ~0.2 V when off (and keeps going down), when I turn on, I see 2.4x V on my DMM very fast and drops to 0.3 V (and dropping)

                                        D6310, when on, has 1.119 anode (-) and 0.56 on the Cathode (-) when off. When I tur on, I see the Andode go up to 2.8 V (DMM) mementarily, coinciding with the LEDs fast flashing.

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