Samsung QLED QN55Q80RAFXZA - No backlights

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  • DennisKal
    Member
    • Jan 2023
    • 18
    • United States

    #1

    Samsung QLED QN55Q80RAFXZA - No backlights

    Hi, I am working on a 2020 Samsung QLED QN55Q80RAFXZA Version FA02. On Christmas Day, the TV made an odd sound while off (in standby). After that it would not turn on. There was no red standby light.

    Based on what I read on the net, I ordered and replaced the Power Board, the Main Board and the LED Driver board with a kit from TVPartsToday. After these were installed, the TV would start up and play the startup tone but the screen was still dark. By shining a flashlight onto the screen, I could see the faint image that was on the screen as it was in the startup mode looking for me to select a source. When I turned it off, the red standby light was now on and working. Progress but not enough.

    To test for the backlights: I unplugged the unit. Unplugged the Main board power cable coming from the Power board. Plugged the unit in again….still a dark screen, no backlights.

    I was advised that the LEDs were probably blown so ordered the LED strip kit from ShopJimmy. I got the LED strips installed after a long painstaking disassemble. I did not replace the board in the center of the chassis that all of the strips plug into. It didn't seem to have any electronics other than wiring and connectors.

    After reassembly of the unit but with the back off, I fired it up again. Same result. Unit starts up, sounds a tone, screen is dark except for what you can see with a flashlight as before. Red standby light on when powered down.

    Sorry for the long novel, but I am hopeful there is something that I overlooked that someone can advise. I am not a technician but have sufficient knowledge and experience to be dangerous.

    Any advice on what to do next? Might the Main or LED driver board that I installed be defective? (I did try using the original LED driver board in place of the one from TVPartsToday and got the same results).

    If it can't be fixed, any advice on brands that are better quality that might last longer? Less than three years for a premium set is not ok. No more Samsung's at my house.

    Thanks, Dennis
  • Storyteller12
    Badcaps Veteran
    • Jun 2020
    • 624
    • USA

    #2
    Re: Samsung QLED QN55Q80RAFXZA - No backlights

    Do you have digital multimeter to test output of backlight to see if any power when turn on? And unplug main board again and perform same test just to make sure.

    If there is power then might need to replace that middle piece you mentioned if that’s not making good contact.

    Comment

    • DennisKal
      Member
      • Jan 2023
      • 18
      • United States

      #3
      Re: Samsung QLED QN55Q80RAFXZA - No backlights

      I do have a multimeter. Can you advise on where to test for power and what values I am looking for?

      Also, the part numbers I installed are
      - Main: BN94-14058G
      - Power Supply: BN44-00987A
      - LED Driver: BN44-00985A

      Comment

      • DennisKal
        Member
        • Jan 2023
        • 18
        • United States

        #4
        Re: Samsung QLED QN55Q80RAFXZA - No backlights

        I took these measurements on the Power board connector that leads to the LED board on the pins that were accessible.

        All connectors in place. Powered up.
        - 18-pin connector on Power Board: measured between Pin X and Pin 17
        - Pin 1: GND = 0V
        - Pin 3: VDRV = 33.36V
        - Pin 5: VDRV = 33.36V
        - Pin 7: VDRV = 33.36V
        - Pin 9: VDRV = 33.36V
        - Pin 11: GND = 0V
        - Pin 13: B13V = 2.6 mV
        - Pin 15: FB = 0.39V
        - Pin 17: GND

        Removed connector to Main board. Powered up. Result: No backlights on.
        - 18-pin connector on Power Board - measured between Pin X and Pin 17
        - Pin 1: GND = 0V
        - Pin 3: VDRV = 33.36V
        - Pin 5: VDRV = 33.36V
        - Pin 7: VDRV = 33.36V
        - Pin 9: VDRV = 33.36V
        - Pin 11: GND = 0V
        - Pin 13: B13V = 1 mV
        - Pin 15: FB = 0.517V
        - Pin 17: GND

        Comment

        • DennisKal
          Member
          • Jan 2023
          • 18
          • United States

          #5
          Re: Samsung QLED QN55Q80RAFXZA - No backlights

          Pictures of the three boards in question
          Attached Files

          Comment

          • Davi.p
            Hobbist
            • Sep 2009
            • 4264
            • Italy - Milan

            #6
            Re: Samsung QLED QN55Q80RAFXZA - No backlights

            It lacks of B13v, the original leds are probably good, all this repair job is made without any technical expertise, wasn't it better to consult a technician? Before giving a cross to a certain brand it would be helpful to know why it brokes and if is by a certain scarceness of the product or an electrical problem occured in your house in that moment, anyway there are not much better piece of hardware out there.. most of the brands are Chinese controlled scarce hardware now..

            check 6. fault and on CN9001A 5. blu on/off
            Last edited by Davi.p; 01-06-2023, 02:53 PM.

            Comment

            • Storyteller12
              Badcaps Veteran
              • Jun 2020
              • 624
              • USA

              #7
              Re: Samsung QLED QN55Q80RAFXZA - No backlights

              Davi.P is right. You can troubleshoot it but you bought new boards. Kinda defeats the purpose. Hard to believe you got the exact same problem on 2 boards. Try putting your old power board in and see if you have power on that pin 13.

              Comment

              • Diah
                Badcaps Legend
                • Feb 2013
                • 6343
                • Germany

                #8
                Re: Samsung QLED QN55Q80RAFXZA - No backlights

                did you replaced LEDs strip with new or used... i think you still have the first problem LED BL problem..
                please show us the old one of the strips how they are and the able of them.

                Comment

                • DennisKal
                  Member
                  • Jan 2023
                  • 18
                  • United States

                  #9
                  Re: Samsung QLED QN55Q80RAFXZA - No backlights

                  Thanks Davi.p.

                  When all connectors in place:
                  CNV901A 6.Fault = 0.69V
                  CN9001A 5.Blu On/off = 2.95V

                  With power connector to Main board removed:
                  CNV901A 6.Fault = 0V
                  CN9001A 5.Blu On/off = 0V

                  Comment

                  • DennisKal
                    Member
                    • Jan 2023
                    • 18
                    • United States

                    #10
                    Re: Samsung QLED QN55Q80RAFXZA - No backlights

                    Storyteller12, with the old power board, Pin 13: B13V = 0V. All zero's on the other pins as well.

                    Comment

                    • Davi.p
                      Hobbist
                      • Sep 2009
                      • 4264
                      • Italy - Milan

                      #11
                      Re: Samsung QLED QN55Q80RAFXZA - No backlights

                      well, i don't know the exact threshold level for those control pins, i suppose is about 2v for the high level, so the BL ON signal is activated and no fault is returning to power board, now i need a very clear sunlight picture of back of power board, we need to find the B13v rail circuitry and where it stops
                      Last edited by Davi.p; 01-06-2023, 08:36 PM.

                      Comment

                      • DennisKal
                        Member
                        • Jan 2023
                        • 18
                        • United States

                        #12
                        Re: Samsung QLED QN55Q80RAFXZA - No backlights

                        Storyteller12, also on the old board, the fuse next to the power cord connecter, FP801S, is blown. This was surely the original problem but I suspect it had other effects downstream.

                        When I installed the boards, I didn't make progress until I changed both the Power and Main boards. I tried them one at a time in different order and did not get to where the unit powered on at all.

                        Comment

                        • Storyteller12
                          Badcaps Veteran
                          • Jun 2020
                          • 624
                          • USA

                          #13
                          Re: Samsung QLED QN55Q80RAFXZA - No backlights

                          Unplug the backlights from the power board and the mainboard from the power board.
                          Then plug in the tv and test that pin 13.

                          Comment

                          • Storyteller12
                            Badcaps Veteran
                            • Jun 2020
                            • 624
                            • USA

                            #14
                            Re: Samsung QLED QN55Q80RAFXZA - No backlights

                            Do this of course with the power board that has a good fuse.

                            Comment

                            • DennisKal
                              Member
                              • Jan 2023
                              • 18
                              • United States

                              #15
                              Re: Samsung QLED QN55Q80RAFXZA - No backlights

                              Davi.p, here is the picture of the back side of the power board. Not much sunshine here today so let me know if I need to take another with better lighting.
                              Attached Files

                              Comment

                              • DennisKal
                                Member
                                • Jan 2023
                                • 18
                                • United States

                                #16
                                Re: Samsung QLED QN55Q80RAFXZA - No backlights

                                Storyteller12, with the power board powered up, but the Main and LED Driver board connectors disconnected, Pin 13 measured 4.9mV.

                                Comment

                                • Storyteller12
                                  Badcaps Veteran
                                  • Jun 2020
                                  • 624
                                  • USA

                                  #17
                                  Re: Samsung QLED QN55Q80RAFXZA - No backlights

                                  Ok, it’s definitely something wrong with the power board you received.
                                  If you can return for a replacement I would. Otherwise we are troubleshoot the board on the component level

                                  Comment

                                  • Davi.p
                                    Hobbist
                                    • Sep 2009
                                    • 4264
                                    • Italy - Milan

                                    #18
                                    Re: Samsung QLED QN55Q80RAFXZA - No backlights

                                    All cables in their place, tv on, read tensions on CNM801, A13v must be connected to B13v through the medium size transistor QM884.. (center pin B13v, lower pin A13v).. A13v feeds most of the main board comprehensive the CPU so it must be there.. whatching bottom side of board, if ps on/off is more than 0,7v you better remove if possible the glue around the QM884 and surrounding smd components, but also we could try to make a jumper soldering on bottom/lower pins of QM884 so the led board be energized, only for test..
                                    Last edited by Davi.p; 01-07-2023, 12:59 PM.

                                    Comment

                                    • DennisKal
                                      Member
                                      • Jan 2023
                                      • 18
                                      • United States

                                      #19
                                      Re: Samsung QLED QN55Q80RAFXZA - No backlights

                                      Storyteller12, Davi.p, for a test what would you think of jumpering the failed fuse on the original power board to see what we see? Maybe that is all that is wrong with that original board.

                                      Comment

                                      • Storyteller12
                                        Badcaps Veteran
                                        • Jun 2020
                                        • 624
                                        • USA

                                        #20
                                        Re: Samsung QLED QN55Q80RAFXZA - No backlights

                                        Usually of a fuse blows a transistor or mosfet is bad also. I wouldnt attempt it. Fuses just don’t fail. That’s a symptom of the real problem

                                        Comment

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