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UN55D6900 BN44-00428B partial led driver circuit failure

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    UN55D6900 BN44-00428B partial led driver circuit failure

    Hello TV experts! I have a UN55D6900. The successfully TV turns on, and the backlights light up, and I get a picture, but there is a section that is dim. I took it apart and 1 out of 4 segments of backlight LEDs are not lighting on both sides of the TV (in other words, the same segment is out on both LED strips).

    I tested the LED strips with my LED tester and all of the LEDs light properly, so I believe the problem is with the PSU/LED driver board (BN44-00428B). The voltages at CNM810 (between PSU and main board) look ok to me. The voltages at CNL801A and CNL801B look off for one of the 4 LED driver circuits. I've bolded the voltages below that seem wrong to me.

    CNM810
    PWM_DIM4 0.265v
    PWM_DIM3 0.265v
    PWM_DIM2 0.265v
    PWM_DIM1 0.265v
    B13V 12.75v
    BLU-ON 4.71v
    GND 0
    GND 0
    ASV 5.40v
    PS-ON 3.3v
    NC 0
    ---------------------
    OD_Level 3.253v
    OD_ON 3.278v
    GND 0
    B13V 12.75v
    B13V 12.75v
    Vamp 1.789v
    Vamp 1.789v
    GND 0
    B5V 5.39v
    B5V 5.39v
    NC 0

    CNL801A
    L2+ 99.7v
    L6+ 101.3v
    L1- 40.8v
    L5- 42.1v
    L8+ 100.8v
    L4+ 68.0v ???
    L7- 41.6v
    L3- 14.69v ???
    IF1 2.97v
    IF2 2.97v

    CNL801B
    R7+ 161.8v
    R3+ 126.8v
    R8- 100.8v
    R4- 68.0v
    R1+ 160.8v
    R5+ 162.3v
    R2- 99.8v
    R6- 101.3v

    I also checked the voltages on either side of each diode in the 4 LED driver circuits (top voltage, bottom voltage)

    D9451 161.6v / 126.8v
    D9351 162.0v / 126.8v
    D9251 126.8v / 126.8v (I think this driver circuit is bad)
    D9151 160.5v / 126.8v

    I've included images of both the front and the back of the LED driver circuits on the board. Looking for help on what I should test next to pinpoint what part of the driver circuit is bad and which component(s) need to be replaced?

    THANKS!!

    P.S. I did find this thread, but it focused on relay clicking/lack of power on, not faulty driver circuits. There are some schematics in there though.
    Attached Files

    #2
    Re: UN55D6900 BN44-00428B partial led driver circuit failure

    bump. help needed, please

    Comment


      #3
      Re: UN55D6900 BN44-00428B partial led driver circuit failure

      I would not rule out bad LEDs. Your LED tester only puts out 10-20mA and the circuit likely runs at several hundred mA. I have replaced LEDs on a few models with symptoms of bad backlights but all the LEDs tested good with a tester (LG UH series with logo flash & Sony with 6 blinks). I've done so many that I typically don't even test any more and just replace them all with new.
      Last edited by neilc6; 07-28-2021, 03:05 PM.

      Comment


        #4
        Re: UN55D6900 BN44-00428B partial led driver circuit failure

        Originally posted by neilc6 View Post
        I would not rule out bad LEDs
        Ok, thanks! In that case I think my next test will be to swap the wires in the connector so that the suspect LED driver circuit is driving a different subset of LEDs. If the original subset of LEDs are still not lit (the new driver circuit being used now appears affected/lower voltage) then the problem is the LEDs. If the problem moves to the new subset of LEDs then the problem is the original LED driver circuit.

        I will report back my findings.

        Comment


          #5
          Re: UN55D6900 BN44-00428B partial led driver circuit failure

          Originally posted by Daveleone View Post
          bump. help needed, please
          I was trying to help but those hookups for the LED's just don't make any sense. I have a UN60D6000 (Man Feb 2012) opened and it is crystal clear. There are 8 LED segments, 4 left and 4 right each measuring about 143VDC with LED tester. Each pin has a wire as depicted in the legend on the board - see attached.

          How were you testing with the LED tester - between what pins? How can you test L1, L2 when L1+ (5) has no wire and L2- (3) has no wire?

          And I got nowhere trying to relate all those wires to the schematic that is in the thread you referenced - there just are not enough wires on that drawing for CNL801A & B.

          Maybe I am just confused
          Attached Files

          Comment


            #6
            Re: UN55D6900 BN44-00428B partial led driver circuit failure

            Originally posted by RDC55 View Post
            I was trying to help but those hookups for the LED's just don't make any sense. I have a UN60D6000 (Man Feb 2012) opened and it is crystal clear. There are 8 LED segments, 4 left and 4 right each measuring about 143VDC with LED tester. Each pin has a wire as depicted in the legend on the board - see attached.

            How were you testing with the LED tester - between what pins? How can you test L1, L2 when L1+ (5) has no wire and L2- (3) has no wire?

            And I got nowhere trying to relate all those wires to the schematic that is in the thread you referenced - there just are not enough wires on that drawing for CNL801A & B.

            Maybe I am just confused
            Ok, I did a bunch of testing with my LED tester and my multimeter to check continuity, and here's what I figured out. All 4 LED driver circuits are the same, and the full circuits look like this:

            1) R1+ to L1-
            2) R3+ to L3-
            3) R5+ to L5-
            4) R7+ to L7-

            Here's how the circuits work...
            - There are 2 LED strips in the TV, a left one and a right one.
            - Each LED strip has 4 segments.
            - The segments on the left strip are: L2+ to L1-, L4+ to L3-, L6+ to L5-, and L8+ to L7-
            - The segments on the right strip are: R1+ to R2-, R3+ to R4-, R5+ to R6-, and R7+ to R8-
            - Testing between any single segment (eg L2+ to L1-, or R1+ to R2-) with the LED tester lights up the corresponding segment of LEDs on that particular strip and drives 69v.
            - Testing for continuity shows that the Left and Right strips are tied together at the following points: R2- and L2+, R4- and L4+, R6- and L6+, and R8- and L8+
            - Therefore, this creates 4 full LED driver circuits. For example, R1+ -> (R2-/L2+) -> L1- (so the full circuit is R1+ to L1-)
            - Connecting the positive lead of my LED tester to R1+ and the negative lead to L1- lights up the corresponding 1st segment of LEDs on both the left and right strips and drives 138v across them.

            Make sense? If it's still confusing let me know and I can draw a picture of it.
            Last edited by Daveleone; 08-01-2021, 02:55 PM.

            Comment


              #7
              Re: UN55D6900 BN44-00428B partial led driver circuit failure

              Originally posted by neilc6 View Post
              I would not rule out bad LEDs. Your LED tester only puts out 10-20mA and the circuit likely runs at several hundred mA. I have replaced LEDs on a few models with symptoms of bad backlights but all the LEDs tested good with a tester (LG UH series with logo flash & Sony with 6 blinks). I've done so many that I typically don't even test any more and just replace them all with new.
              Ok, the suspect LED driver circuit or LED segments were in the "R3+ to L3-" circuit. So, I swapped the wires from the "R3+ to L3-" circuit with the wires from the "R7+ to L7-" circuit. When I did that, LEDs previously attached to the "R3+ to L3-" circuit lit up properly, and the LEDs previously attached to the "R7+ to L7-" circuit did not. In other words, the problem is a faulty LED driver circuit, not the LEDs themselves.

              Would appreciate your help with next steps to diagnose the faulty component(s) of that LED driver circuit. What should I test / how to test it?

              Thanks!

              Comment


                #8
                Re: UN55D6900 BN44-00428B partial led driver circuit failure

                Originally posted by RDC55 View Post
                Originally posted by Daveleone View Post
                Make sense? If it's still confusing let me know and I can draw a picture of it.
                I made a little diagram to show the "R1+ to L1-" circuit, in case the text was still confusing.
                Attached Files

                Comment


                  #9
                  Re: UN55D6900 BN44-00428B partial led driver circuit failure

                  Have you checked if D9251 is ok and not shorted? Also check R9253 (0.22Ω) to see if it is ok and not open.
                  Monitor the voltage on D9251 cathode to see if it ever gets above 126v, If it does spike to 160v and drop, the problem is likely not here but on the return side of the led's
                  Last edited by R_J; 08-01-2021, 03:37 PM.

                  Comment


                    #10
                    Re: UN55D6900 BN44-00428B partial led driver circuit failure

                    Originally posted by R_J View Post
                    Have you checked if D9251 is ok and not shorted? Also check R9253 (0.22Ω) to see if it is ok and not open.
                    Monitor the voltage on D9251 cathode to see if it ever gets above 126v, If it does spike to 160v and drop, the problem is likely not here but on the return side of the led's
                    Both D9251 and R9251 check out ok (note: my multimeter only goes down to 0.1Ω, and it showed 0.3-0.4Ω for R9251, but I checked the other R9x51 resistors and they all read the same).

                    I monitored D9251 voltage when I powered on the TV. My multimeter sampling rate isn't the best, but I tried it a few times and did see it go up to 160-170V momentarily and then drop back to 126.8V and stay there.
                    For reference, the other LED driver circuits would jump up to ~160V, then drop back to 126.8V, then go back up to ~160V and stay there (the normal boot sequence of the TV has the LEDs light up, then go dark, then light up and stay on, so this is normal).

                    Next steps? Thanks for the help!

                    Comment


                      #11
                      Re: UN55D6900 BN44-00428B partial led driver circuit failure

                      The voltage going high would mean the boost circuit is working, so the fault would seem to be in the led current monitoring.
                      It looks like current is monitored across R9265 (for dimch2) R9264 would supply the feedback voltage for that channel back to IC9151S.
                      Try and check the voltage across R9265, R9165, R9365, R9465. One of these should be different and may be what is shutting down the circuit.

                      IC9151S pin 19 drives (pwm) Q9253 gate, this fet connects the return of the led string to ground through R9265. If this mosfet is shorted or not being tuned on, the current through R9265 will be wrong and trigger a shutdown.
                      Last edited by R_J; 08-02-2021, 04:46 PM.

                      Comment


                        #12
                        Re: UN55D6900 BN44-00428B partial led driver circuit failure

                        Originally posted by R_J View Post
                        The voltage going high would mean the boost circuit is working, so the fault would seem to be in the led current monitoring.
                        It looks like current is monitored across R9265 (for dimch2) R9264 would supply the feedback voltage for that channel back to IC9151S.
                        Try and check the voltage across R9265, R9165, R9365, R9465. One of these should be different and may be what is shutting down the circuit.

                        IC9151S pin 19 drives (pwm) Q9253 gate, this fet connects the return of the led string to ground through R9265. If this mosfet is shorted or not being tuned on, the current through R9265 will be wrong and trigger a shutdown.
                        Well, I messed up and accidentally shorted that R9265 resistor and blew it. So, I'm going to give you the post-catastrophe numbers below. I rechecked the resistors and voltages around the diodes and those are still ok, so nothing I posted before has changed...
                        • Voltage drop across R9165=45.4mV, R9265=2.3V, R9635=45.5mV, R9645=45.2mV
                        • If I read the voltages at the bottom and top of each resistor as compared to ground, I see:
                          • R9165 -8.7mV (bottom), 36.7mV (top)
                          • R9265 -8.6mV, 2.27V
                          • R9365 -8.0mV, -37.4mV
                          • R9465 -7.5mV, 37.4mV
                        • I checked R9264 and it reads 0.469kΩ. I checked R9464 for comparison, and it also reads 0.469kΩ. So R9264 seems ok.
                        • I checked the resistors around the Q9253 gate. The bottom resistors checked out ok (they were all 10Ω), but the upper resistors read as follows: R9163=1kΩ, R9263=110kΩ, R9363=1kΩ, R9464=1kΩ.
                        • I also checked the Q9253 for shorts and the bottom pin is shorted with the middle pin, so it is blown.

                        So it looks like I need to replace R9265, R9263, and Q9253 at a minimum. Not sure if that will fix the problem or just get me back to where I was before I accidentally shorted the board. *sigh* Anyways, it's going to take a while for me to get parts, but I'll update again once that's done.
                        Last edited by Daveleone; 08-05-2021, 11:07 PM.

                        Comment


                          #13
                          Re: UN55D6900 BN44-00428B partial led driver circuit failure

                          Originally posted by R_J View Post
                          ...
                          Update: R9264 was also blown out, so I went ahead and swapped R9263, R9264, R9265, and Q9253 out with the corresponding components from one of the working LED driver circuits so we can continue the debug of this non-working circuit (and I will replace the components in the known working circuit later once I receive the parts in the mail).

                          Now the voltages in this non-working circuit read as follows:
                          • R9165 - this is the circuit that I cannibalized, so it's disabled.
                          • Voltage drop across R9265=0V, R9635=45.7mV, R9645=46.1mV
                          • If I read the voltages at the bottom and top of each resistor as compared to ground, I see:
                            • R9265 -5.2mV (bottom), -5.2mV (top)
                            • R9365 -5.1mV, 40.4mV
                            • R9465 -5.1mV, 41.2mV


                          NOTE: Since I took R9263, R9264, R9265, and Q9253 from a working circuit, we can be certain that they are all known good components.


                          What to check next? Thanks again for your help!
                          P.S. One other thought I just had - will taking those components from the R9165 circuit affect the ability of the R9265 circuit to function? (in other words, should I have taken them from the R9365 circuit instead)?
                          Last edited by Daveleone; 08-07-2021, 01:46 PM.

                          Comment


                            #14
                            Re: UN55D6900 BN44-00428B partial led driver circuit failure

                            The problem is that when Q9253 shorted and R9265 went open, the voltage on Q9253 source would have been quite high and likely damaged the FBN2 input of IC9151, that's why R9264 also went open. So I suspect the ic is damaged.
                            On an equivalent ic the voltage range on that pin is 0.3 ~ 5.5v volts (5.5v max)
                            Last edited by R_J; 08-07-2021, 08:34 PM.

                            Comment


                              #15
                              Re: UN55D6900 BN44-00428B partial led driver circuit failure

                              Originally posted by R_J View Post
                              The problem is that when Q9253 shorted and R9265 went open, the voltage on Q9253 source would have been quite high and likely damaged the FBN2 input of IC9151, that's why R9264 also went open. So I suspect the ic is damaged.
                              On an equivalent ic the voltage range on that pin is 0.3 ~ 5.5v volts (5.5v max)
                              Ok, I swapped IC9151 with the known working one from the other circuit. No change (the other circuits still work, the non-working circuit still does not work and has the same voltage readings). So, IC9151 was ok.

                              What next?

                              Comment


                                #16
                                Re: UN55D6900 BN44-00428B partial led driver circuit failure

                                The LEDs are not always correct, you can check the tester-led, you need to measure the current consumption on each group (channel) Here is a service manual that can help you figure it out.

                                Comment


                                  #17
                                  Re: UN55D6900 BN44-00428B partial led driver circuit failure

                                  Service BN44-00428B
                                  Attached Files

                                  Comment


                                    #18
                                    Re: UN55D6900 BN44-00428B partial led driver circuit failure

                                    Originally posted by lotas View Post
                                    The LEDs are not always correct, you can check the tester-led, you need to measure the current consumption on each group (channel) Here is a service manual that can help you figure it out.
                                    Thanks for the reply. I have already verified that the LED strips are ok earlier in the thread. I attached these segments of the LED strips to a different driver circuit, and they worked fine, so the LED strips are not the problem here.

                                    Comment


                                      #19
                                      Re: UN55D6900 BN44-00428B partial led driver circuit failure

                                      So no voltage across R9265 suggests an open in the LED circuit, or a problem on the low side as R_J suggested earlier. You already verified boost voltage is working on that section, but perhaps rechecking boost since you swapped IC's might be in order.

                                      Are you working with the TV open and just the Power Supply connected - no main?

                                      If open, does that suspect circuit flash the LED's and subsequently produce a momentary voltage across R9265 when first powered up?

                                      If working with the main connected try it with just the power supply by removing the cable between the main and power supply - perhaps there is an issue with the PWM drive signal.

                                      Comment


                                        #20
                                        Re: UN55D6900 BN44-00428B partial led driver circuit failure

                                        Originally posted by RDC55 View Post
                                        So no voltage across R9265 suggests an open in the LED circuit, or a problem on the low side as R_J suggested earlier. You already verified boost voltage is working on that section, but perhaps rechecking boost since you swapped IC's might be in order.

                                        Are you working with the TV open and just the Power Supply connected - no main?

                                        If open, does that suspect circuit flash the LED's and subsequently produce a momentary voltage across R9265 when first powered up?
                                        Was testing with PSU & main connected.
                                        Re-checked boost voltage and it still jumps up to 170V right after I apply power. But the LEDs don't come on when it's at that voltage. A moment later the voltage drops to 128V on the bad circuit and 160V on the good circuits, and that's when the LEDs on the good circuits turn on for the first time (then they turn off, then turn back on again as is evidently the normal boot cycle). The LEDs on the bad circuit never come on.

                                        If working with the main connected try it with just the power supply by removing the cable between the main and power supply - perhaps there is an issue with the PWM drive signal.
                                        THIS is interesting. Without the main board connected, the LEDs on that circuit DO turn on. However, on all of the other circuits the voltage on the bottom of the diode is 116V and on the top of the diode is 160V, while on the affected circuit the voltage on the bottom of the diode is 116V but on the top of the diode is 222V, and the LEDs appear to be overdriven (too bright) on that strip (although I'm making that assessment from looking through the holes in the back of the TV because I don't have the panel disassembled).

                                        Does that give you any clues?

                                        Comment

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