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samsung un65nu6900f common problem, digging deeper

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    samsung un65nu6900f common problem, digging deeper

    So i have a samsung un65nu6900f in for service. Its doing the power cycle 2 blinks thing. Many other people have worked around this by taping the LVDS cable at the expense of losing some color quality on the affected side of the panel.

    My customer is not super happy about that compromise so i spent all day tracing this thing out today. I can confirm that the issue is not related to the clock signals, but rather the VGH signal right next to them. It is supposed to be 25V, it is 0v while the whole cycling thing is happening. If i remove R13246 from the main board the tv will power on with the slight discoloration. This is breaking the signal path from VGH on the main board to VHG on the panel. Same as taping the cable but easier to play with back and forth.

    No detectable shorts on that line and in fact it measures identically to the VGH on the other side of the panel in terms of resistance, diode mode, etc.

    Here is where it gets even stranger, if i feed the 25VDC into the VGH test pad on the panel PCB from a bench supply the color clears up and it looks literally perfect.

    I can confirm that with that resistor removed the mainboard is putting out the expected 25V, but with the resistor in place that same measurement point on the main board drops to 0V. That screams short circuit to me, but no measurable short exists. One would assume if a short exists on the panel, then my bench supply should reflect that, and it certainly shouldn't just work perfectly.

    Already replaced mainboard just to be safe and issue is exactly the same.


    Anyone have any ideas?

    If it comes down to it im just going to put a step up regulator somewhere inside it for the 25V, but i would love to figure this one out for real since it is a rather common problem.

    #2
    Re: samsung un65nu6900f common problem, digging deeper

    further weirdness. if i feed the 25v (resistor removed) in the mainboard VGH so the 25V needs to travel the full path through the FFC cables as designed it 2 blinks and cycles. If i feed the 25v (resistor removed) into directly into either of the VGH testpoints on the panel PCB it works normally and the screen is perfect.

    if i run a wire from the VGH supply on the main board to the panel test point it works perfectly.



    From VGH supply point (resistor removed) on main board to furthest VGH testpoint on panel = 2.5Ohms - doesn't work and power cycles blinks

    Random wire added from VGH supply point (resistor removed) to end of wire = less than 1OHM - Works perfectly.

    I am without explanation..........
    Last edited by itelite; 06-18-2022, 02:42 PM.

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      #3
      Re: samsung un65nu6900f common problem, digging deeper

      VGH turned to 0 because the MB shuting it down... when the panel pull over currents because of short at (cells / caps ESD diode etc on panel )
      did you check the R vlaue if it low

      EDIT: BTW can you show us the location of the R in circuit !
      Last edited by Diah; 06-18-2022, 02:54 PM.

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        #4
        Re: samsung un65nu6900f common problem, digging deeper

        Ill get some photos on monday when im back to work. Seems like an easy enough solution for others to use as well. Soldering to the test point on the panel is a little advanced, but its doable.

        Resistor value is 10ohm, and measures 10 ohm.

        Yeah i dont know what to make of it, running the wire from the main board to the test point on the panel was indeed successful so it feels like the panel itself is fine, and the main board is good. Maybe 2.5ohm between the board and panel plus the 10ohm resistor on the main board is somehow enough out of spec to shut it down? I suppose if im feeling brave on monday i can put a few ohms of resistance inline with my wire and see what happens.

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          #5
          Re: samsung un65nu6900f common problem, digging deeper

          i think you did missed reading my previous post with words (( currents )) its currents not V. external supply point out side circuit not able to sync the over load.. reducing R value same way mean.. what will happen later who knows.. fire!!! i don't think so cause the shit panel from sami lcd not OLED, but of sure 100% there will be victim

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            #6
            Re: samsung un65nu6900f common problem, digging deeper

            Turns out this is a un70, not un65. Same main board, just didnt notice it until getting it back together.

            Some photos here
            https://imgur.com/a/D0jeeCT

            So this mod is

            ::Remove or adjust the R13246 resistor off the pads to break the 25/30v VGH connection to the panel.

            Solder a wire to the hot side (will read 25V when powered on) of the resistor/pads.

            Solder other end of wire to the test point marked "VGH" on the panel PCB.

            Cut wire in the middle and put a 10k+ resistor in line. (precise value is probably not needed, it was an educated guess on my part)
            ::

            Another observation, direct 0 ohm wire connection fails to start up.I snipped the blue wire and I put a 150K resistor inline, boots perfectly, image perfect. :edit: I also tested a 1K (fail) and a 12k (success)

            You will know you have this problem if you measure very low or no voltage on that R13246 resistor. When the resistor is removed you will measure 25V on the same point that was low in the prior test.

            I have no explanation for why this works, but hopefully it helps someone fix their un70nu6900f that blinks twice and power cycles forever.



            I also have a strong feeling that simply replacing R13246 with a higher value resistor in the 10K range will do effectively the same thing, but my test tv is assembled and should be going home today.
            Last edited by itelite; 06-20-2022, 09:51 AM.

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              #7
              Re: samsung un65nu6900f common problem, digging deeper

              You should attach the photos to your post so they will be here for others to see. links will disappear.

              Comment


                #8
                Re: samsung un65nu6900f common problem, digging deeper

                yes, show us what you did, a picture is worth a thousand words.

                Comment


                  #9
                  Re: samsung un65nu6900f common problem, digging deeper



                  Comment


                    #10
                    Re: samsung un65nu6900f common problem, digging deeper

                    did you check the temperature of added R 10k+??

                    Comment


                      #11
                      Re: samsung un65nu6900f common problem, digging deeper

                      as long as this is safe, it looks like a good solution to the problem?

                      Comment


                        #12
                        Re: samsung un65nu6900f common problem, digging deeper

                        Yes, no different than room temp. Practically no current going through this.

                        Comment


                          #13
                          Re: samsung un65nu6900f common problem, digging deeper

                          so does anyone know what is broken that this solution is fixing? meaning what would be the "official" factory fix?

                          Comment


                            #14
                            Re: samsung un65nu6900f common problem, digging deeper

                            Originally posted by nomoresonys View Post
                            so does anyone know what is broken that this solution is fixing? meaning what would be the "official" factory fix?
                            This is venturing heavily into pure speculation, but there could be some kind of a signal timing aspect to this.

                            For instance, if solder my direct wire with no added resistance, the tv will not boot. it will do the 2 blink power cycle.

                            If i take the same wire, turn on the tv, count arbitrarily to 3, then press the wire on the test point it will boot with a perfect image. Removing the wire will degrade have the image as expected. Pressing it back on the test point will restore the image.

                            As mentioned if i put any resistance into the soldered wire 10K or above (up to tested 150K) it boots and works perfectly as though it never had an issue.

                            My wild speculation is that maybe the added resistance is delaying the signal enough on the initial boot to allow normal function? It would be an incredibly small delay, certainly not my arbitrary count to 3, but there will be a delay.

                            Ive fixed a lot of things over my life, this one stands out as very difficult to explain.

                            Been running tv without issue for around 6 hours now.

                            Comment


                              #15
                              Re: samsung un65nu6900f common problem, digging deeper

                              so this line No.75 STVP_MB4 ???

                              Comment


                                #16
                                Re: samsung un65nu6900f common problem, digging deeper

                                I believe it is listed as VGH30V or something similar (pin 84). It is actually listed as a NC in the 65" service manual. I do not have the manual for the 70.

                                It appears to be another version of pin 11 on the other LVDS connector. I would imagine that would be another place to look if this location in the circuit measures correctly.
                                Last edited by itelite; 06-20-2022, 03:25 PM.

                                Comment


                                  #17
                                  Re: samsung un65nu6900f common problem, digging deeper

                                  Originally posted by itelite View Post
                                  I believe it is listed as VGH30V or something similar (pin 84). It is actually listed as a NC in the 65" service manual. I do not have the manual for the 70.

                                  It appears to be another version of pin 11 on the other LVDS connector. I would imagine that would be another place to look if this location in the circuit measures correctly.
                                  its as i mensioned before Pin 75 STVP_MB4
                                  all TU /NU/MU size share same sockets design & MB



                                  Attached Files

                                  Comment


                                    #18
                                    Re: samsung un65nu6900f common problem, digging deeper

                                    I can speculate that 10k is reducing inrush current. Otherwise the current spike is large enough and is detected as fault.
                                    Did you measure VGH on other side of 10k resistor? Is it stable?
                                    I think you can just install 10k in place of R13246 instead of running wire.

                                    Comment


                                      #19
                                      Re: samsung un65nu6900f common problem, digging deeper

                                      Originally posted by alfatv View Post
                                      I can speculate that 10k is reducing inrush current. Otherwise the current spike is large enough and is detected as fault.
                                      Did you measure VGH on other side of 10k resistor? Is it stable?
                                      I think you can just install 10k in place of R13246 instead of running wire.


                                      Yes all seems stable. Yes i thought of that after it was all back together. 10k in place of the 10 ohm PCB resistor should effectively accomplish the same thing. Im sure i will see another one with this problem soon enough to give it a try.

                                      Comment


                                        #20
                                        Re: samsung un65nu6900f common problem, digging deeper

                                        Just to clear this up, it is in fact pin 84. It is hard to tell from my photo because of my black marker, but that trace does wrap around and go up to the resistor.

                                        The pin name should be VGHD_30V_PW, but is shown as a NC in the manual. I can confirm that on both the old board and the new board a 10ohm resistor is present in the R13246 position.

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