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LCD flat panel monitor, faulty backlight

Discussion in 'Electronic Repair' started by Don, Oct 19, 2005.

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  1. Don

    Don Guest

    Hi,

    This is a Vison F153 flat panel monitor (1024x768?)

    When power is first applied (with a valid signal source), the screen
    "flashes" momentarily (i.e. to *normal* intensity) and then goes
    dark. During this flash, it is obvious that the correct image is displayed
    on the screen.

    After the screen goes dark, it is still aparent that the display is
    accurately
    displaying the signal provided (e.g. you can see the ghost of the mouse
    cursor moving around in response to mouse actions; you can double click on
    icons and see those applications open, etc) -- but obviously the backlight
    is no longer on (at all!)

    There are two flourescent tubes in the display (?). Both are behaving
    identically.

    Each tube is excited from it's own inverter (i.e. the inverter board appears
    to have two identical circuits on it).

    So, it seems logical to assume that the supply to the inverters is a likely
    suspect. Or, some control signal shared by both inverters (?).

    Any suggestions as to where to start? I'd prefer *not* probing the
    output of the inverters (for obvious reasons) and suspect that
    would reveal very little that I can't already surmise...

    Thanks!

    [email address is bogus]
     
  2. sofie

    sofie Guest

    Don:
    You already "started" in the correct place.... the inverter board and the
    power supply board. I would check any of the larger electrolytics on the
    inverter board first... if you do not have an ESR meter to properly and
    conclusively check them just replace them. If that doesn't solve the
    problem then go back to the power supply board and do the same.
     
  3. Andy Cuffe

    Andy Cuffe Guest

    Double check that all the tubes are lighting. Even though it appears
    to be normal brightness, one of the back light inverters might not be
    working (there may be two tubes at each end). Suspect any
    electrolytic caps on the board since they tend to run hot.
    Andy Cuffe

    <-- Use this address until 12/31/2005

    <-- Use this address after 12/31/2005
     
  4. Jason D.

    Jason D. Guest

    Hi, this symptom of lighting up for a second then poof is sign of bad
    lamp inverter shutting itself down due to shorted winding inside a HV
    transformer. Usually two or four of them depending on number of lamps
    used. You can see a badly degraded waveform on bad transformer (sine
    wave cut down to squiggles.) if the probe's body is laid on the
    transformer's with scope set to .1mV/div and 1usec sweep.

    This trick is useful to confirm you're getting good operation from
    most high frequency transformers, switching power supplies, switching
    regulators, including flyback transformers by judging from quality of
    waveforms.

    This what I had to replace that lamp inverter and is all well now.
    Have to gonna be exact replacement. If you know same models that use
    same design of this inverter that will help. Picture of guts (board,
    inverter board) etc is helpful.

    Cheers, Wizard
     
  5. Alex Bird

    Alex Bird Guest

  6. Leo Meyer

    Leo Meyer Guest

    This is exactly the same symptom I have with a noname flat monitor. I
    took the inverter out and tested it from a known good 12V source while
    having it connected to the lamps - same result. Interestingly, the
    inverter has another (control?) cable; if I inject up to two volts into
    this cable the lamps stay on but flicker terribly while the inverter
    makes a buzzing noise. If I go up to about 3V the buzz vanishes and
    brightness increases, though the flicker remains. From about 4-5V
    onwards the inverter shuts down again.

    As someone suggested previously, it might well be a broken output
    transformer - but as there are two identical circuits on the inverter
    board, why is the other one shutting down as well?

    I don't see chances for successful repair here. Is there anybody who can
    point me to a supplier of inverters (Europe preferred)? I guess the
    types are quite interchangeable but if anyone can give me a hint on how
    to find an exact replacement, help would be greatly appreciated.

    Kind regards, wishing successful repairs,

    Leo
     
  7. Don

    Don Guest

    This is a Vison F153 flat panel monitor (1024x768?)
    As suspected, the problem was in the power supply (since it would
    be highly improbable for *both* bulbs to fail in the same manner
    *or* both inverters to fail, etc.). An electrolytic in the 12V supply
    to the inverter was lossy and resulting in poor regulation of that
    supply.

    Didn't have a 470uF/25V/105C cap on hand but a 330/25/105 cap
    proved my point -- I'll just have to remember to order some and
    replace that (along with the few others onboard... a cheap prophylactic).

    Thanks!


    [email address is bogus]
     
  8. Jason D.

    Jason D. Guest

    Alex, that's GOOD.

    The PROBLEMS:

    - I'm NOT sure if the guy have the safety knowledge to work with these
    stuff.

    - Not all are same. Some inverters needs digital control, others used
    analog to set the brightness of lamps.

    - Some needs the bias voltage generated and fed back to the LCD panel,
    rare.

    - Need to measure all the pins and identify grounds, the power
    supplies, there are at least two voltages, one for lamp power, one for
    other circuits on inverter itself. Measure for lamp brightness
    (analog types will be voltage levels that rise or lowers as brightness
    is adjusted) except digital controlled will be train of pulses. There
    will be one signal for turning on or off lamps much like enabling or
    disabling a eeprom.

    This is just the surface to scratch, not just length of lamps.

    Cheers, Wizard
     
  9. Jason D.

    Jason D. Guest

    After the screen goes dark, it is still aparent that the display is
    Excellent job!

    But this is unusual. Sounds like this LCD using low quality caps.
    Most LCD panels tend to use good stuff due to space limitations.

    If you don't have a cap for this value either voltage or uF, match
    either and upsize the other values or both one level or more for
    filter stuff.

    Cheers, Wizard
     
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