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LCD backlight problems

Discussion in 'Electronic Repair' started by Kirk S., Dec 23, 2005.

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  1. Kirk S.

    Kirk S. Guest


    Got a KOGI L4BX monitor with a picture and no backlight. Can I simply use a
    VOM to test the output from the inverter? Is it typically worth replacing
    the bulbs? The inverters seem to be about half the cost of the unit so
    replacement of that part isn't really financially purdent (or is it?).

    Thanks for any help!

    Kirk S.
  2. Guest

    No, you cannot simply use a VOM, you need an oscilloscope to check the
    Also be aware that the high frequency and relatively high backlight
    voltage will require some additional care and safety when measuring.

    The inverters are usually very easy to repair for a well experienced
    technician with lots of smps type of knowledge.
  3. James Sweet

    James Sweet Guest

    Not really, you'd probably just blow the DMM. You can get CCFL lamps
    very cheaply and just wire one of them to the inverter for testing,
    otherwise the inverters can often be repaired or replaced with something
    cheap and generic.
  4. Guest

    You wil be suprised by how many CCFL inverters I have repaired by
    simply replacing the marginal ESR capacitors that caused excessive
    start up current that blew one of the surface mount fuses.

    Very rarely do I see one with any output device damage unless the fuse
    had been replaced without fixing the capacitor problems first.
  5. Kirk S.

    Kirk S. Guest

    Thanks for the information...

    I do have a scope however I'm not very experienced using it. Can I simply
    use the 1000x probe on the output and check for a waveform? It isn't like
    checking the chopper on the hot side of a SMPS.

    Everything is surface mount on the inverter. Do the ceramic caps go bad?

    Kirk S.
  6. Guest

    Not all inverters have electrolytic capacitors on them, but most do
    have some square capacitors that are not unmarked ceramic surface
    mounts but usually tantalum or electrolytic surface mounts.

    Usually the scope probe does not even need connected, just place it
    near the inverter transformer and if it is running it will be clearly
    visible by a waveform that is not the usualy 60Hz noise waveform. I
    use this to confirm flybacks and sometimes horizontal coils to see if
    something is running or not. It does not tell me if it is running
    correctly though.

    Start by checking the fuses, should be a main power in fuse and
    probably one on each output transformer and transistor feed. If any of
    the fuses are open, check for obvious shorted components. If no shorts
    are found, you can chance replacing the fuse, but there still may be a
    problem. Without a full schematic or lots of experience, it will be
    difficult to determine what caused the fuse to blow before replacing

    If it seems to work with the new fuse, replace the lamps, they might be
    causing some excessive current draw due to age.
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