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LCD monitor - backlight tubes or invertor?

Discussion in 'Electronic Repair' started by [email protected], Apr 16, 2005.

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  1. Guest

    I have a Compaq 8020 18.1" LCD display (guess 4/5 years old) that
    suddenly shows no picture - was running windows & display blanked for
    a couple of seconds then came back briefly on before going again -
    screen is now black as if power is not on, though LEDs & control panel
    buttons seem to operate as normal.

    Flicking around with with on-off control eventually resulted in the
    COMPAQ splash logo coming up (this happened once & only briefly)
    before going back to darkness

    I have taken the back off the unit - it seems to have 4 backlight
    tube positions, though I can't see any light leaking from the metal
    panel covering the actual LCD unit (I'm loathe at this point to
    dismantle any further as I'm not familiar what might spring out into
    my lap/into the carpet...

    Do backlight tubes fail in this manner (ie suddenly) or do they tend
    to slowly degrade?

    As they seemingly have all gone at the same time, would this rather
    point to the invertor board - do they fail in this way? What about
    that tantilising flash of the Compaq splash logo?

    Thanks for any info
  2. Art

    Art Guest

    Scope probe set near the ballast wil produce a high freq display if the
    ballast is running.
  3. Guest

    Thanks Art - Googling around tonight brings up a similar tip - holding
    a small discharge tube near the inverter board apparently causes it to
    light up if board is working

    Also have gleaned that these inverters tend to have an internal trip
    such that if an HT feed circuit to *one* backlight goes down, it trips
    a cutout & all the others are disabled on the board.

    Guess it all points towards the inverter board - this particular one
    is a "Hitachi invc551" - comes up on Google $99 - cheaper options?

    There seems to be mention of (SMC?) fuses in inverter boards that can
    blow - I would be glad of a pointer to a quick fix - tho I can't
    immediatly see any of these on this board..


    Supplimentary question: there seems to be a small web industry
    building up around the unreliability of these HT boards - what are the
    issues that seemingly make them sooo fragile??

    Thx for info !!
  4. James Sweet

    James Sweet Guest

    They're just cheap, that's the main reason they fail.

    Lots of surplus boards out there though not all will support dimming if you
    need that.
  5. Guest

    Yeah yeah - also 1600V+ floatin arond - spookey things might happen -
    PCB track capacance etc etc - beyond my knowledge (& capacity!)...


    my dear old Dad used to say to me -

    "Never trust an electrician with no eyebrows".

  6. Guest

    How many tubes?
    Typically each bulb has its own inverter transformer and transistor
    drivers, but they all have a common regulator ic. If any of the
    inverters detect a problem, the driver ic goes into shutdown, after the
    initial start attempt. Each inverter output will have its own surface
    mount fuse. If one opens, the unit will just flash during lamp start

    The fuses are ultra super small surface mount jobs. Many times they
    simply open up.
    The backlight bulbs can also cause the shutdown circuit to trip, if one
    bulb does not start, same symptom.
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