Best Buy / Sharp LC-32LB261U backlight failure

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  • asel51
    New Member
    • Jan 2019
    • 5
    • USA

    #1

    Best Buy / Sharp LC-32LB261U backlight failure

    Hi, I picked up a free Best Buy / Sharp LC-32LB261U LED TV from Craigslist. The LED back light had failed. I found that the set uses 3 LED strips which have 7 LEDs per strip, and the 3 strips are driven in parallel by the Power Forest PF7903B boost converter. During troubleshooting, I found that the boost converter would startup but be shut down by its over voltage protection circuit as the output voltage spiked to around 42V.

    From the schematics and product brief, the feedback voltage on the PF7903B is 240mV and the 0.192489 ohm current sense resistance in the LED return path (5 resistors in parallel; 2 @ 0.91 ohm & 3 @ 1.0 ohm in parallel = 0.192389 ohms) indicated that the full load current should be 1.247 amps.

    To test the converter, I used a 27.5 ohm 100 watt resistive load and that allowed the converter to run and produce 33.3 volts across the load and 1.246 amps through the load. That confirmed that the boost converter / current regulator was operating correctly.

    Next, I used a bench power supply to test and make sure the LED back light assembly was bad before removing the panel. With 35 volts and the current limit set to 500 mA, I could get no current flow through either of the VLED+ to LED- connections. That confirmed that the LED back light strips were bad.

    After disassembly, I found 3 strips with 7 LED per strip, labeled as Evertop LBM320P0701-FC-2. I tested each LED individually with bench supply @ 2.7 volts and current limit set to 400 mA. I found a total of 5 failed LEDs; 3 on one strip and 1 on each of the other 2 strips. Replacement strips are available on EBay, but before ordering, I wanted to try and replace the bad LEDs on each strip. (As an aside, the reflector in the set is paper and glued to the chassis which makes removing it without damage almost impossible. Very poor design with regard to serviceability.)

    Does anyone know what the LED vendor and part number used on the LBM320P0701-FC-2 strips might be? These appear to be around 1.25 watts based on the 1.247A full load current into the parallel connected strips. This means each strip should have a current flow of 416mA and assuming a forward voltage drop of 3V, that would be 1.247 watts per LED.

    If you know what vendor & part number the LEDs might be, please let me know.

    Thanks,
  • budm
    Badcaps Legend
    • Feb 2010
    • 40746
    • USA

    #2
    Re: Best Buy / Sharp LC-32LB261U backlight failure

    You should check the dimension, and also remove the LED to find out what kind of foot print it has.
    BTW, do you have the schematic?
    https://www.shopjimmy.com/sharp-pltv...-32lb261u.htm#
    That is crappy design using LED strings in parallel so if one string goes open circuit the other string will have the total current flowing through it and that will cause the failure of the remaining string since it is constant current drive setup.
    Last edited by budm; 02-21-2019, 02:26 PM.
    Never stop learning
    Basic LCD TV and Monitor troubleshooting guides.
    http://www.badcaps.net/forum/showthr...956#post305956

    Voltage Regulator (LDO) testing:
    http://www.badcaps.net/forum/showthr...999#post300999

    Inverter testing using old CFL:
    http://www.badcaps.net/forum/showthr...er+testing+cfl

    Tear down pictures : Hit the ">" Show Albums and stories" on the left side
    http://s807.photobucket.com/user/budm/library/

    TV Factory reset codes listing:
    http://www.badcaps.net/forum/showthread.php?t=24809

    Comment

    • Unspun01
      Badcaps Veteran
      • Jun 2011
      • 333
      • Canada

      #3
      Re: Best Buy / Sharp LC-32LB261U backlight failure

      Your math looks good.

      But 416mA sounds really high for an LED? Maybe that's why they burned out? I thought something around 200mA is good enough? Any comments? Maybe these need to have some resistance in-line to drop the current and protect the LEDs?

      Comment

      • budm
        Badcaps Legend
        • Feb 2010
        • 40746
        • USA

        #4
        Re: Best Buy / Sharp LC-32LB261U backlight failure

        They like to drive the LED at the Max and beyond the spec so they will fail so you will buy another TV.
        You can reduce the current by turning down the backlights level or modify the circuit by changing the current sensing resistor.
        Never stop learning
        Basic LCD TV and Monitor troubleshooting guides.
        http://www.badcaps.net/forum/showthr...956#post305956

        Voltage Regulator (LDO) testing:
        http://www.badcaps.net/forum/showthr...999#post300999

        Inverter testing using old CFL:
        http://www.badcaps.net/forum/showthr...er+testing+cfl

        Tear down pictures : Hit the ">" Show Albums and stories" on the left side
        http://s807.photobucket.com/user/budm/library/

        TV Factory reset codes listing:
        http://www.badcaps.net/forum/showthread.php?t=24809

        Comment

        • asel51
          New Member
          • Jan 2019
          • 5
          • USA

          #5
          Re: Best Buy / Sharp LC-32LB261U backlight failure

          Yes, I have the schematic. I have removed the strips and the LED lenses and will remove the LED chips and measure the package outline and look at the pin-out (pads). I'm sure I can find suitable replacements at Mouser or DigiKey or via EBay from China. Yes, this is a very poor design with the LED strips driven in parallel. You are right about one failing which increases the current in the remaining strips or strip until they all fail. Too bad designers and manufactures don't take a conservative approach to the design, but a limited lifetime product does force new sales. When I reinstall the LED strips, I may play with some 3M double-sided thermally conductive tape to better heat sink the LEDs to the chassis, or maybe put some thermal paste under the strips.

          Comment

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