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    Samsung UN55MU6290 Boot loop

    Hi,

    I have got Samsung UN55MU6290 UHD tv which is not powering on. When plugged in backlight comes on and then goes off in an about 10 seconds and it keeps happening. No video on screen at all. I checked backlights with BL tester and all looks good with BL. When measure voltage on main caps it jumps from 390v to 362v and rises again and keeps cycling. Below are the reading when mainboard connected to power board -

    Pin | Signal Voltage Reading
    1 | Fail Count 3.3 V
    2 | ANA DIM 0.99 V
    3 | A13V 12.8 V
    4 | OD On-OFF 3.3 V
    5 | A13V 12.8 V
    6 | PWM BLU 3.3 V
    7 | A13V 12.8V
    8 | Power ON 3.3 V
    9 | A13V 12.8V
    10 | GND
    11 | GND
    12 | GND

    I suspect the defect with main board as backlight keeps on when mainboard is disconnected. Could you help me to diagnose the exact issue?
    Attached Files

    #2
    Re: Samsung UN55MU6290 Boot loop

    Bad leds 100%

    Comment


      #3
      Re: Samsung UN55MU6290 Boot loop

      My guess is a short on the lower panel driver boards. Check all resistors, diodes, and caps especially on the board all the way on the left facing the back. Unhook one ribbon cable at a time and see if it stops boot looping. If it does you found which side your short is on. The MU series is notorious for having shorts on the panel driver boards. Samsung's quality has gone down hill since 2017.

      Comment


        #4
        Re: Samsung UN55MU6290 Boot loop

        Sorry not 100% my mistake, 50% panel boards, 50% leds, just to clarify, neither BL tester nor mainboard disconnecting are safe testings, cause only when m.b. is connected it starts with full brightness and this built problems that in these new Samsung become a restart...

        Comment


          #5
          Re: Samsung UN55MU6290 Boot loop

          Thanks for the information. After disconnecting right side ribbon to panel driver board I get the stable backlight and no more reboot. However I was expecting half of the picture on the screen but its fully dark/blank. Could thus be panel issue or expected? How should I proceed now?

          Update- It doesn't matter which side I disconnect, reboot stops by disconnecting either of the ribbon. One side has small IC on the driver board while other does not.
          Attached Files
          Last edited by anupkg1985; 09-05-2021, 07:08 PM.

          Comment


            #6
            Re: Samsung UN55MU6290 Boot loop

            So it is a shorted cap on the panel's boards, then test all the bigger ones with the best ohmmeter you have and watch those that have less ohms also decimals, this if it is readable a short, then remove those with less ohms and test them outside..

            Comment


              #7
              Re: Samsung UN55MU6290 Boot loop

              What should be the expected value for good caps and what range of readings should be considered for short caps?

              Comment


                #8
                Re: Samsung UN55MU6290 Boot loop

                not shorted caps is high ohms (maybe kohms) or open, shorted ones around zero.

                Comment


                  #9
                  Re: Samsung UN55MU6290 Boot loop

                  I tested all the caps however most of them are above 5k. Only one near the dc converter IC, C75 reads 1.1k ohm. Doesn't show continuity however if set is turned on then show continuity on this cap. Could this be culprit?

                  Update- Vdd voltage on the driver board is not stable it jumps from 3.3v to 2.6v and back if both sides are connected. If right side ribbon removed then its stable on left side.
                  Attached Files
                  Last edited by anupkg1985; 09-06-2021, 01:57 PM.

                  Comment


                    #10
                    Re: Samsung UN55MU6290 Boot loop

                    sorry i forgot to mention there possibly be some diode packs on the far side of the boards, check them and also the resistors connected to them, i feel the vcc rail doesn't previews possibile problems with caps..

                    Comment


                      #11
                      Re: Samsung UN55MU6290 Boot loop

                      I checked all caps on the right side and none of them falls below 1.5k also the resistors most of them are reading OL or in k ohms.
                      If I block pin 3 on the ribbon which connects to VDD33 on the right side then set stays on but no picture. Could this be panel issue?
                      Attached Files

                      Comment


                        #12
                        Re: Samsung UN55MU6290 Boot loop

                        Never disconnect vcc alone.. possible damages so.. the short can be of the non explicit type so connect all up and test the voltage on AVDD test point or the bigger caps, are there some caps that doesn't present a voltage?

                        Comment


                          #13
                          Re: Samsung UN55MU6290 Boot loop

                          Voltage is not stable on all the VDD test points, it measures from 1.8 v to 0.32v and jumps back.

                          Comment


                            #14
                            Re: Samsung UN55MU6290 Boot loop

                            Not VDD, i say AVDD but in this set it is caled VAA,i see now..

                            Comment


                              #15
                              Re: Samsung UN55MU6290 Boot loop

                              Voltage is fluctuating on VAA test point from 8v to 0.012v, its very hard to measure as it's fluctuating very fast up and down. Also most of the points like HVAA have fluctuating voltage. There is no voltage present on VAA point if I remove right side ribbon.

                              Comment


                                #16
                                Re: Samsung UN55MU6290 Boot loop

                                Now if you have a bench power supply or a dc-dc regulator with current control i tell you what to do..

                                Comment


                                  #17
                                  Re: Samsung UN55MU6290 Boot loop

                                  I don't have current control power supply, only I have is DMM and LED tester. However what is the next step with power supply?

                                  Comment


                                    #18
                                    Re: Samsung UN55MU6290 Boot loop

                                    Don't wait until you find a power supply, so if you have an hot air station remove one by one the condensers on the VAA rail and retest each time until you find the one that blocks the supply rail...
                                    before doing this watch carefully at the magnifier lens each condenser and see if one is slightly different from others, then start remove that..

                                    Comment


                                      #19
                                      Re: Samsung UN55MU6290 Boot loop

                                      Thanks for the your patience and help. I'm not professional so don't have most of the equipments. I have soldering iron and removing smd caps it bit complex with that, however I'll try. I'm curious about the Power Supply stuff, that would be new learning for me.

                                      Comment


                                        #20
                                        Re: Samsung UN55MU6290 Boot loop

                                        the technique with PS is to feed current slowly increasing and feeling the heat on suspected components ( caps)

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