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LED TV BACKLIGHT

Discussion in 'Troubleshooting and Repair' started by Y2KEDDIE, Apr 19, 2019.

  1. Y2KEDDIE

    Y2KEDDIE

    245
    15
    Sep 23, 2012
    I’ve been experimenting with a defunct 55” LG LED tv. For which I have no schematic. Initial failure was screen black, no picture.
    I’m trying to learn here. I believe the black plastic panel which connects to the Timing Control Board is a multi array of LED filters. Is this panel the name sake for the tv (LED or LCD)? Is the black light array a preset brightness used as a light source, or is it supposed to vary with the video signal?
    I found several White LED’s shorted and some open.
    I set all the boards up bread board fashion, and have a channel tuned and sound without a display.
    Further investigation I found their are two supplies feeding the LED assemblies connector. Outputs are labeled LED1, + & -, and LED2, + &-. I measure zero volts across the respective +& -, connector terminals. However I measure +80 volts from the respective + to circuit grounds with a resistive load.
    The owners manual indicates there is a black-lite adjustment in software. I’m assuming there is a current limiter circuit in the negative rail of the supplies and this path is open. Is anyone familiar with this circuit, am I on the right path?
     
  2. dave9

    dave9

    649
    144
    Mar 5, 2017
    1) Give it a signal then shine a strong flashlight at the screen to see if it is functional and all that is missing is backlight operation. There are probably youtube videos for TVs and/or computer monitors demonstrating this.

    2) I would simply trace the circuit and observe where power (and ground) stops, except that if you have blown LEDs, they will need replaced if the array of LEDs is configured in such a way (significant # of them in series) that it is not acceptable to have only partial backlighting working.

    You may be on the right track thinking you have a bad ground path since you observe 80V on the positive rails, but a failure of that (alone) should not have blown any LEDs. I would take the TV model # and do a web search to see if this is a common fault. Yes many TVs do reduce the backlighting brightness, "software" to do that is generally just the firmware and the in-menu controls.

    If your choice would be to buy a new (or used pull) LED assembly and it's prone to failure, it might not be the best repair candidate if it is just going to fail again, unless you identify something further you can do to mitigate that... and who knows, it might be something as simple as putting a hole in the back, mounting a little fan on to move a little air to keep it cooler, but being a major brand TV I wouldn't have thought the LEDs needed that as much as the capacitors to get good life out of them, and if you start actively flowing air through it, you may eventually end up with dust interfering with the lighting (depends on design?). I suppose you could use a dust filter on a little larger fan (to keep post-filter flow acceptable without excessive noise by allowing fan RPM to stay low).
     
  3. Y2KEDDIE

    Y2KEDDIE

    245
    15
    Sep 23, 2012
    What I really need is a schematic. I couldn’t find anything on line as how the circuit operates. Since. I found several LED’S shorted , i’m Thinking the current regulator was lost in the catastrophic event. When I place a resistive load to ground there is voltage with little current limiting. I would expect to see on the order of 20 mA.if I measure across the connector I don’t see any voltage as there is no path to ground.
     
  4. 73's de Edd

    73's de Edd

    2,558
    1,045
    Aug 21, 2015
    defunct 55” LG LED tv . . . . . . doesn't cut it for exact referencing . . . the exact LG modlel pls.
    Some open LED's in a series string arrangement . . . . . . .but shorted ones ? ? ? ? ? ?
    Realistically . . . . . . use a more specific dynamic testing by taking a 9VDC batteries and a series
    270-330 ohm series current limiting resistor , affix flying leads on the ens and then unplug the power connector to the LED strings and take the fluying leads and apply the limited power from the test set up to each LED to evaluate its condition and uniformity of light output from each one.
    each and every one has to be good.
     
    Last edited: Apr 20, 2019
  5. Y2KEDDIE

    Y2KEDDIE

    245
    15
    Sep 23, 2012
    The set i’m Playing with is a LG 5600.` i’ve Already determined several LED’s are shorted, and some open. I decided to scrap the set because the best option was to replace all LED’S to assure reliability;not cost effective. At this point i’m Just experimenting/learning.
    The LED’s are wired In series/parallel configuration. AN open LED renders a series string inoperative, and a short makes the others draw too much current causing further damage . I suspect the short/s caused overloading the current limiting devices. I assume there were active current limiting because the owners manual said the back lite.display was dimmable.
    It’s hard , with my eyesight, to trace the surface mount components on the foil side of the board.
    I’m just curious as to what type current limiting is typically used.
     
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