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Backlight problem on lcd tv

Discussion in 'Troubleshooting and Repair' started by phlox, Sep 1, 2012.

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  1. phlox

    phlox

    76
    2
    May 19, 2012
    Hey guys

    I got a small problem with an lcd tv, the thing worked fine, then stopped working and when it gets switched on the light flashes, so i open it up and found a blown transistor or what ever its called, its got 4 leg en small black square body just like a transistor, well anyway i got that fixed, but the backlights wont go on still, there is image on the display as i have connected the ccfl tubes from the tv to another inverter from ccfl pc lights and that works, but the tv inverter self isnt working,i get volts up to the input of the inverter transformer but no volts out to the ccfl tubes, is the inverter transformer probably blown adn how would i test it?
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,386
    2,772
    Jan 21, 2010
    The inverter board is probably an item you can purchase.

    Since it seems you've isolated it as the fault, and because those inverter transformers may be quite hard to source, I'd be investigating a board replacement.
     
  3. phlox

    phlox

    76
    2
    May 19, 2012
    No backlight on lcd tv

    Unfortunately the inverter circuit and power supply are on board, dont know where ill get a replacement board , i was thinking of using a lcd computer monitors inverter transformers if that might work?
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,268
    Nov 28, 2011
    Have you tried lcdparts.net? I got a transformer from them that fixed my inverter board. They also sell replacement boards.
     
  5. phlox

    phlox

    76
    2
    May 19, 2012
    lcd tv backlight problem

    hey guys, so far no luck, got a inverter transformer from another lcd screen and same problem, now ive got a question, might it be the inverter driver mosfet?
    The reason for asking is on the benq lcd pc screen ita has got 4 inverter tramsformers as well as 4 driver mosfets basically right behind the transformers, and well 2 of the backlights didnt go on but 2 did, found out it was the drivers mosfets on those 2, replaced them and all 4 backlight ccfl tubes lit up again....well except it still goes off after about 3 seconds,but thats another prject im working on,any help on that would be greatly appreciated.

    So for the lcd tv which is the actual problem,how would i know if its the driver mosfet and where is the driver mosfet? I've attached 2 images, of what i think is the driver mosfet, i dont know.

    But on the mosfet it states : " FML G22 " with the little diode symbol and 07 09.

    Ive also read about a "pico fuse" but the board doesnt seem to have one on.

    thanks so far for the help
     

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  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,386
    2,772
    Jan 21, 2010
    That device on a heatsink is a diode.

    Have you checked all the DC rails from this power supply.

    Beware that everything inside those thick lines can kill you while the device is switched on (or even plugged in) and even if it's not, it may be able to give you one hell of a boot.

    Can we see the other side of this board?
     
  7. phlox

    phlox

    76
    2
    May 19, 2012
    lcd tv backlight

    how do you mean th DC Rails?...ill see if i can post a pic bit later
     
  8. phlox

    phlox

    76
    2
    May 19, 2012
    i did check the volts going to the transformers, and i get about 24V dc going to the primary input of the transformers, but no volts going out
     
  9. markm6164

    markm6164

    118
    0
    Jan 22, 2012
    Is there a driver ic on the other side of the board for the ccfl tube? This will drive the mosfets to power the inverters. We need a clear of the other side of the board.
     
  10. GreenGiant

    GreenGiant

    830
    6
    Feb 9, 2012
    from past experience try finding the same model TV on craigslist/ebay that someone cracked the screen on, then pull the board from that and put it in yours, you have to make sure that they are compatible though

    A friend of mine scored a 50" LG, 1080P, 240Hz, 3D ready TV for $220 total that way
     
  11. phlox

    phlox

    76
    2
    May 19, 2012
    lcd tv backlight problem

    attached is two pics of the bottom side of the power supply board, i made a circle around the component which i now suspect is the driver mosfet?.....the board had 2 and one was blown and caused the screen to power on and power off again in sequence, removed it and tv came on and got picture but no backlight
     

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  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,386
    2,772
    Jan 21, 2010
    Have you tried replacing it?

    It seems unlikely they placed two of these on the board if it would operate with one.

    What was the part number of the one you removed?

    Looks more like a regulator than a mosfet, but without more information it's hard to be sure.
     
  13. phlox

    phlox

    76
    2
    May 19, 2012
    lcd tv backllight

    both were exactly the same, uhm ill check the part number bit later, dont have it with me, but it did have two exactly the same ones on, and it looks to me like the power for the inverters come from them , or there abouts, i might be wrong, i followed the pcd tracks.
     
  14. phlox

    phlox

    76
    2
    May 19, 2012
    pcb not pcd sorry, oh and i dnt have a spare so didnt try replacing it yet
     
  15. markm6164

    markm6164

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    Jan 22, 2012
    What is the numbers on that ic close to the mosfets?
     
  16. phlox

    phlox

    76
    2
    May 19, 2012
    okay the "mosfets'' or regulators are the following stu 404d(7bd68) and the IC as far as i can make out, the writing is bit faded : u2003,xk 7n1
     
  17. phlox

    phlox

    76
    2
    May 19, 2012
  18. phlox

    phlox

    76
    2
    May 19, 2012
    on page 50 is the power circuit
     
  19. phlox

    phlox

    76
    2
    May 19, 2012
    i just checked the ic and from some of the pins that goes to the output of the inverter transformer(followed the schematic diagram in link above) theres either no volts or only about 2.7v........so most likely the IC?
     
  20. markm6164

    markm6164

    118
    0
    Jan 22, 2012
    You need the check your voltages on the power rails as stated before and also your voltages at the ic first. The driver ic will send a pwm signal to the gate of the mosfets which is best measuring with a scope. The driver ic will have over voltage protection circuits and feed back circuit to regulate the power the the ccfl tube and will shut down if it detects over voltage usually it lights the ccfl for a few seconds then it shuts down. This might be what's happening. I haven't looked at the service manual yet or googled the driver ic because I'm on my phone.
     
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