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LCD Monitor problem

Discussion in 'Electronic Repair' started by Al, May 4, 2008.

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  1. Al

    Al Guest

    I picked up a used Micron AP150T at our town's recycling center. It cost
    me nothing, so there is no loss if I can't figure out what's wrong.

    When I hook it up to a computer, all works just fine. Then the picture
    starts tearing horizontally after a few moments of on time. When I hit
    the Function key, the function block pops up and looks perfectly fine
    while the picture behind it is tearing.

    So I assume that the sync signal from the computer is somehow being lost.
    It does the same with two different computers, so the computer output is
    not the problem; the computer outputs are fine with other monitors.

    Any ideas? Is there a know problem with this unit syncing to the VGA
    signal?

    Al
     
  2. Does it always happen after the same time period? Also when you don't allow
    time to cool down? Perhaps it's simply a connection somewhere which gets
    interrupted when the monitor warms up.

    Do you have a scope? If so, you could track the input sync signals in the
    circuit, and find out where it gets mixed up. If all VGA signal enters the ADC
    correctly, then the fault will be a bit more difficult to find...
     
  3. James Sweet

    James Sweet Guest

    Bad electrolytic capacitors are fairly common and can cause any number of
    strange symptoms. That's where I'd look first, the surface mount sort used
    in flat panels tend to be especially troublesome.
     
  4. Al

    Al Guest

    OK, I'll do the check with the 'scope. And the time of the start of the
    tearing does vary with the interval between startups; something must be
    temperature sensitive.

    Al
     
  5. That does not rule out capacitor based problems. I've repaired several
    mainboards which had leaky capacitors, which worked fine as soon as they were
    warm. It's the opposite to what you're experiencing, but it illustrates that
    caps are temperature senstive.
     
  6. hr(bob)

    hr(bob) Guest

    If this is a crt monitor, there could be a high-voltage arc that is
    not high enough to be seen or heard, but is generating "noise" that is
    being picked up by some other circuit. A scope would be invaluable.
     
  7. James Sweet

    James Sweet Guest

    Capacitors are very temperature sensitive, more so than any other part I can
    think of, aside from those meant to react to temperature.
     
  8. Judging by the subject, I would say it is not a CRT :)
     
  9. Andy Cuffe

    Andy Cuffe Guest

    I second this. About 90% of the bad LCD monitors I see can be fixed
    with new caps. Don't forget about the surface mount electrolytics on
    the main board. I just fixed a Viewsonic that needed to have every
    single cap on the main board replaced.
    Andy Cuffe

     
  10. Al

    Al Guest

    The display generated internally is just fine. Only the externally input
    display tears. So it can't be "every single cap." I imagine it is
    something in the sync input. I'll be checking it out. And like I said, it
    is a "saved" monitor. If a quick fix won't do it, back to the dump.

    Al

    Al
     
  11. James Sweet

    James Sweet Guest


    Sure it could, may only be one or two that are bad enough to cause a
    malfunction, but it's worth checking the rest. Usually when one of these
    surface mount capacitors is bad, the rest are nearly shot too. I usually
    replace them all, then the thing is good for years. In a pinch, one can
    check the ESR of all of them and replace those that check bad. There's
    usually only a dozen or so.
     
  12. Arfa Daily

    Arfa Daily Guest

    Agreed, but be aware that the ESR values of surface mount electros tends to
    be somewhat higher than those of a similar value in 'conventional' leaded
    format, so a cap that you are expecting to read say 1 ohm, may correctly
    read several ohms in surface mount.

    Arfa
     
  13. I'd be interested to know what makes you so sure that the caps
    you bought for replacement are of superior longevity compared
    to those the manufacturer did put in :).

    Regards,
    H.
     
  14. Well, I guess by now that is over, but in the past, the market was flooded with
    bad electrolytes. Some manufacturer copied a formula from another
    manufacturer, but it contained a deliberate mistake to foul anyone who might
    want to steal it. The result was that a lot of electronic equipment contained
    these bad caps. Mainboard from around 2000 are very suspect for example. The
    most likely cause form any mainboard from that period which has become
    unstable, is the caps.

    But the problem wasn't limited to computers. Even airplane electronics
    contained them...
     
  15. Andy Cuffe

    Andy Cuffe Guest

    It's not unusual for bad caps to only affect the video from the
    computer, leaving the OSD looking fine.
    Andy Cuffe

     
  16. James Sweet

    James Sweet Guest

    Well for starters they don't have several years of use on them. All else
    being equal, they're probably not. I hate surface mount lytics.
     
  17. Arfa Daily

    Arfa Daily Guest

    That's interesting. On some Sony servo boards fitted to many of their CD
    players a few years back, there were three sm electros along one edge of the
    board, which used to go faulty, and occasionally leak. This gave the symptom
    of either totally refusing to play, or very poor playability. If you tried
    to sub regular electros for them, the board would never work again, even if
    it worked, allbeit poorly, before. However, fit the correct sm types, and it
    would immediately work again to full original spec.

    Arfa
     
  18. James Sweet

    James Sweet Guest



    Sony has a history of building things which are very particular about
    the components used. I've had issues with some of the TVs not working
    with subs that were fine in everything else.
     
  19. Arfa Daily

    Arfa Daily Guest

    Well, if ever you get one of those Sony players - and there was lots of 'em
    that used variations of the same board, mounted directly to the underside of
    the deck - if it 'feels' like you've got a bad laser, look to see if it's
    got those three caps along the edge of the board. If it has, you better
    start looking for a supplier ... !! I never have figured out why 'standard'
    electros won't work in that position. Like you, I have subbed leaded for
    sm's in other places, without problem. About the only difference that I've
    found is that the leaded types tend to have a lower ESR than the sm's, which
    under normal circumstances, you'd say was a good thing.

    Arfa
     
  20. Doesn't it have to do with higher stray inductance (more inductance, less
    high-frequency shunt capability)? SMD parts are usually the only usable kind
    of part in sensitive equipment, like PWM amplifiers.
     
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