Samsung UN75NU6950 no backlight but leds works when checked individually!

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  • Anooc7
    Member
    • Jun 2021
    • 26
    • Canada
    #1

    Samsung UN75NU6950 no backlight but leds works when checked individually!

    Hi, This model has bottom mounted leds . Got it with right ride 1/3 of the display black. So I checked the 6 + and - wire that powers 6 different zones and found that the 5th zone was open, which opens 6th zone too makes the right 1/3 of the display dark. Opened it and found one blown led on the 5th zone and shorted that path using solder. So now all the zones works when I check it using the bl tester , but when I put everything back together there is no backlight! The voltage I am getting when neg to chassis is 387v. How does that happen? Still individual zones working with bl tester .
    Any inputs would be helpful, thanks
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    Last edited by Anooc7; 09-16-2021, 06:20 PM. Reason: Photo upload error
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    • R_J
      Badcaps Legend
      • Jun 2012
      • 9535
      • Canada
      #2
      Re: Samsung UN75NU6950 no backlight but leds works when checked individually!

      When you short out one led in the string, the current for that string changes, the led circuit detects that and shuts down. You don't say where you measured the 387v but I suspect your meter is being fooled by the high frequency component in that circuit.
      Last edited by R_J; 09-16-2021, 06:35 PM.

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        • DXseekerMO
          Badcaps Legend
          • Feb 2018
          • 1119
          • US
          #3
          Re: Samsung UN75NU6950 no backlight but leds works when checked individually!

          I have seen a bad LED work with a backlight tester and not work with the Power Supply, probably due to an LED breaking down at a higher current than what is being provided by the tester.

          Was the bottom of the main diffuser melted over the LEDs that didn't work?
          They call me......."threadkiller"

            Comment

            • Anooc7
              Member
              • Jun 2021
              • 26
              • Canada
              #4
              Re: Samsung UN75NU6950 no backlight but leds works when checked individually!

              Originally posted by R_J
              When you short out one led in the string, the current for that string changes, the led circuit detects that and shuts down. You don't say where you measured the 387v but I suspect your meter is being fooled by the high frequency component in that circuit.
              I am getting the same voltage of 387 in 2+ 3 + ——….. to 4+ . So if it shuts down this has to be zero' volts right?

                Comment

                • Anooc7
                  Member
                  • Jun 2021
                  • 26
                  • Canada
                  #5
                  Re: Samsung UN75NU6950 no backlight but leds works when checked individually!

                  Originally posted by DXseekerMO
                  I have seen a bad LED work with a backlight tester and not work with the Power Supply, probably due to an LED breaking down at a higher current than what is being provided by the tester.

                  Was the bottom of the main diffuser melted over the LEDs that didn't work?
                  It wasn't melted but the faulty led came off from the strip and I found it near the strip.

                    Comment

                    • R_J
                      Badcaps Legend
                      • Jun 2012
                      • 9535
                      • Canada
                      #6
                      Re: Samsung UN75NU6950 no backlight but leds works when checked individually!

                      Originally posted by Anooc7
                      I am getting the same voltage of 387 in 2+ 3 + ——….. to 4+ . So if it shuts down this has to be zero’ volts right?
                      Check between the 2+ and 2- pins not to chassis ground. The raw supply voltage for that circuit is between the Discharge - and Discharge + points.
                      It looks like there are 3 drive circuits so I suspect 2 of the strings are in series
                      Last edited by R_J; 09-16-2021, 08:07 PM.

                        Comment

                        • Anooc7
                          Member
                          • Jun 2021
                          • 26
                          • Canada
                          #7
                          Re: Samsung UN75NU6950 no backlight but leds works when checked individually!

                          Originally posted by R_J
                          Check between the 2+ and 2- pins not to chassis ground
                          The tv is not here atm but it was few milli volts to 1v I remember.

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