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LCD monitor inverter/backlight repair

Discussion in 'Electronic Repair' started by [email protected], Sep 5, 2006.

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  1. Guest

    Hi all,
    I am trying to repair a 19" Samsung 191T monitor that appears to have
    been damaged due to a storm-related power surge over the weekend.

    The problem with the monitor is clearly related to the backlight: the
    four backlight lamps appear "burnt" and if I shine a bright light into
    the back of the panel, the image on the screen appears perfectly fine

    The monitor includes two main circuit boards: a big one that the DVI
    input and power connects to, and a smaller LCD inverter board. The LCD
    inverter board is an Ambit BN44-00060A, which appears nearly identical
    to this one:
    There are no fuses on either board, as far as I can tell.

    The LCD inverter board is connected to the main circuit board via a
    12-pin connector. The voltages output by the LCD inverter board are
    very close to zero, so obviously something is wrong with it :-( If I
    understand correctly, the cold cathode backlight bulbs need something
    like 1000V AC, is that right?

    I would like to be able to verify that nothing is wrong with the power
    supply part of the MAIN circuit board as well... does anyone know what
    the voltages should be on the 12 input pins to the inverter board?
    Since the image is apparently fine, there is no problem with the
    display electronics on the main board, but there could be a problem
    with the power supply on the main board... I would like to know if
    there is a way to tell this.

    Can anyone give me any advice on how to test the LCD inverter and main
    power supply boards? There are no fuses. Thanks for any advice!!!

    Dan Lenski
  2. James Sweet

    James Sweet Guest

    These sort of things can be difficult to track down, it could be
    something wrong in the supply to the inverter, as well as many inverters
    will shut down if they detect a lamp fault. It really helps if you have
    access to another identical monitor to swap parts around, though you can
    do some work without. The inverter itself is my primary suspect, if it
    has any electrolytic capacitors check those.
  3. Guest

    Thanks James! From the healthy market for inverters, it does seem like
    the inverter is a likely culprit.

    Is there any way I can safely mimic a working lamp, e.g. by shorting
    the leads with an appropriate resistor or something?

    I will check the electrolytic caps... though I'm not sure how I can
    actually measure their capacitance while they're connected to the
    circuit, any trick to that?

  4. James Sweet

    James Sweet Guest

    You can get CCFL lamps from lots of places, they're not expensive and
    would work for testing.

    You don't measure the capacitance, what you need is an ESR meter, or you
    can try just replacing them. Surface mount lytics are the most suspect.
  5. Guest

    Right... I guess I'll need CCFL lamps anyway to replace the burnt-out
    backlight lamps. Do you know of a source where I can get them cheap?
    The cheapest I can find for 390 mm lamps like this monitor has is $13 /
    each, at ... would it be a terrible idea to use these
    cheap 310 mm ones:
    Gotcha. Don't have an ESR meter, but there's only one electrolytic cap
    on the board, AFAICT, so I'll figure out some way to test it.

  6. Guest

    I've just measured the 12 input pins to the inverter board with a
    voltmeter. As far as I can tell, 8 of the pins are at ground, and the
    other 4 are at 14V. Does this sound reasonable?

    If so, I suppose I could just replace the whole inverter with the
    inverter from the cheap kits at that I mentioned above.

  7. James Sweet

    James Sweet Guest

    How do you know the lamps are burned out? They can and do fail, but they
    normally last quite a long time.
  8. Guest

    The ends have what appear to be burn marks inside them... and when I
    try to measure the resistance across them, it's infinite. Not sure if
    this is a good way to check the lamps :-/

  9. Andy Cuffe

    Andy Cuffe Guest

    The back light inverter is the most common failure on LCD monitors.
    Normally you should see +9 to +12 v going to the inverter. You will
    also probably see +5 and one other voltage to control brightness.

    Chances are, the inverter is the problem since it usually shares its
    power supplies with the LCD. Look for any fuses on the inverter
    board. They will probably be small surface mount components near the
    12 pin connector. That orange component to the left of the big green
    capacitor could be a fuse. There may be some fuses on the bottom of
    the board too. If you find any open fuses, there's a good chance that
    a new fuse will fix it. About 3/4 of the inverters I see with blown
    fuses have no other problems. If a new fuse doesn't fix it, you will
    probably need to replace the inverter board. Schematics are rarely
    available and parts are impossible to find.
    Andy Cuffe

  10. Could be getter?
    I'd expect that.
  11. Guest

    Thank you, Andy! This is exactly the info I'm looking for. On this
    board, the input is most definitely around 14V. The power brick is
    labeled 14V/3A as well. Perhaps the 14V is to add a couple diode drops
    to the 12V?
    I will look more closely for fuses. Is there any particular label used
    for fuses? As in "C3"=capacitor, "R24"=resistor, "U12"=IC?

    If I have to replace the inverter, is there anything wrong with using
    the ones that come with the cheap case-mod kits? The backlight lamps
    certainly do appear to be fried, since they have a dark gray haze
    around one electrode that I don't see in any of the photos at

    Thanks a lot,
  12. Bob Shuman

    Bob Shuman Guest

    Andy's advice is right on. I'd bet that it is your inverter that went South
    .... There is usually at least one surface mount fuse on the inverter board
    and this can be tested with a simple ohmmeter. But, if the fuse blew, it is
    usually due to a fault elsewhere on the inverter board so simply replacing
    it will not likely solve your problem.

  13. ampdoc

    ampdoc Guest

    Fuses are commonly marked with F or FU.
  14. Guest

    There are usually 4 different lines on the power connector:
    1. 12 or 14 volt main power source.
    2. Ground (usually 2 or more wires)
    3. On/Off control line (zero or 5 volts)
    4. Brightness control (zero to 5 volts).

    Every inverter board I have worked on had a fuse on the main
    power source. Usually located right near the input connector
    and the larger electrolytic capacitor. Trace your 14 volt line.
    The fuse us normally marked F1 or something like that.
    They are surface mounted and may look like a black resistor.
    Usually rated about 2-1/2 amps.

    If the fuse is open, check the driver transistors (or FETs) that
    drive the primary winding of the step-up transformers. They
    usually become shorted.

    Hope this helps....
  15. Guest

    Thanks! As far as I can tell, all the lines on the inverter connector
    are at 0 or 14 V :-(
    There was indeed a (tiny) fuse, I finally found it. It is not blown,
    so that wasn't the problem.

    After messing around with this thing some more, and accidentally
    shorting 14 V to ground in the power supply, I've made the whole thing
    worse. Now I can't even get an image to show up by shining a bright
    light into the backlight tubes. Ugh. I think I'd better cut my losses
    and stop messing with this thing. Oh well. It's been a good learning

  16. Guest

    Are the inverter fuses quick action or slo blo?

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