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LCD Monitor problem

Discussion in 'Electronic Repair' started by [email protected], Mar 4, 2006.

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  1. Guest

    I've got a problem with an LCD. It powers on, but I can't get a
    picture from it no matter which of the 4 inputs is plugged in. I
    opened it up to look around, check connections etc.

    As I was testing voltages at several points, I think I must have
    shorted two points at some point, because I noticed that the monitor
    had come on and I could see the built in on-screen display. So I know
    that the monitor itself and the backlight work.

    But I still can't figure out how to get it to actually detect input and
    turn on.

    Any thoughts on what the problem could be?
     
  2. Jerry G.

    Jerry G. Guest

    You will need the service manual, which I doubt you would be able to
    obtain, and the proper set-up and spare parts to service your monitor.
    If it is a very expensive monitor, you should send it back to the
    manufacture's service people to have an estimate. You may find it more
    feasible to replace the monitor.

    --

    Jerry G.


    I've got a problem with an LCD. It powers on, but I can't get a
    picture from it no matter which of the 4 inputs is plugged in. I
    opened it up to look around, check connections etc.

    As I was testing voltages at several points, I think I must have
    shorted two points at some point, because I noticed that the monitor
    had come on and I could see the built in on-screen display. So I know
    that the monitor itself and the backlight work.

    But I still can't figure out how to get it to actually detect input and
    turn on.

    Any thoughts on what the problem could be?
     
  3. Guest

    Well, I could get the service manual for about $25 online for this
    model, so I may just do that.

    I've managed to get it to turn on semi-consistently by briefly shorting
    two pins on the Analog Devices AD9884A chip. When I short them there's
    a high pitched buzzing that seems to be coming from the power supply,
    then I release the short and the signal light goes green and I can
    access the on-screen display. If I then do a Reset from the menu, it
    correctly locates the input signal and shows it. However, I still
    can't switch to other signals through the buttons, and if I try to use
    the +/- buttons to switch display, I can no longer even get into the
    on-screen menu.

    Additionally, shorting those two pins doesn't work EVERY time, just
    regularly.

    I suppose it probably is best just to buy the service manual, though
    I'm not sure it's going to offer any better suggestions.
     
  4. Insomniac

    Insomniac Guest

    I really wouldn't recommend shorting IC pins... you can easily destro
    them, if that hasn't already happened. The IC AD9884A is an analogu
    to digital converter

    I've come across many odd faults with LCDs caused by low value and/o
    bad ESR electrolytic capacitors. Visually inspect all of them on th
    mainboard for bulging tops or leakage. Failing that, remove and tes
    them

    You can sometimes discover if an electrolytic capacitor is the troubl
    by heating them with a hairdryer (don't roast them though!) and see i
    the fault clears or alters in some way

    Otherwise you may be looking at a faulty IC somewhere

    Incidentally, has the OSD failed to come up since shorting the I
    pins? What are the numbers of the pins you shorted
     
  5. Why not tell us what brand and model set you have? Without some specific
    information it is hard to know if we can help.

    Leonard
     
  6. Guest

    It's an Iiyama AS4636D 18inch LCD.

    The OSD didn't come up even before I shorted the pins, so that's not
    the problem. I did visually inspect the board for any visible problems
    before doing anything and couldn't see any problems.

    How would you suggest heating the capacitors without heating up
    everything else on the board also?

    I'm not sure exactly the number of the pins, so I'll have to get the
    spec for the chip and compare that, I'll let you know.
     
  7. Guest

    I believe it was pins 6/7 or 7/8... not sure exactly with the small
    leads.

    Also, upon closer inspection under a heat sink in the power supply
    there are two electrolytic capacitors that appear VERY SLIGHTLY bulged
    out at the top. Is there some way I can test these short of just
    replacing them and trying again? The bulge is very slight as I said...
    nothing I would have noticed had you not specifically mentioned that.
     
  8. Dave D

    Dave D Guest

    If they're definitely bulging they need replacement, it's as simple as that.
    This could well be the source of the original problem.

    Dave
     
  9. Insomniac

    Insomniac Guest

    I use a hot-air station with small nozzle, whereas some just stick th
    tip of their soldering iron on the metal top of the cap for a fe
    seconds, trouble with that is you can cook the cap quickly. One thin
    you could do is get some thick card and cut a small circle in it fo
    the cap to go through, so it acts as a barrier to most of the heat
    If heating the caps does nothing, heating the whole board can tel
    you if you have a bad joint somewhere... even surface-mount ICs ca
    suffer dry joints, often invisible to the eye

    Here is the ICs datasheet, it's PDF and if you scroll through it
    you'll find an image of it with all pins numbered
    http://www.datasheetcatalog.com/datasheets_pdf/A/D/9/8/AD9884A.shtm

    You need a capacitance/ESR meter to test those slightly bulging cap
    really. If you don't have one, substitution is the only way, an
    probably a good idea if there is bulging present. Many bad caps don'
    bulge or leak though, just so you can bear that in mind with the res
    of them
     
  10. Jesse

    Jesse Guest

    Yes I do, I've built and sold many, very light, very inexpensive,
    utility racking but not for display in your kitchen. Post your Email
    and I'll send you a picture if you wish. jesse
     
  11. Jesse

    Jesse Guest

     
  12. Guest

    Well, I replaced the two bulging caps with new ones, and unfortunately,
    nothing seems to have changed. Still can't get a picture or on-screen
    menu.
     
  13. Insomniac

    Insomniac Guest

    It'll be very interesting to know which pins you've been shorting t
    get it partly functioning..
     
  14. Guest

    Well, like I said, I believe it was pins 7/8 which is one of the color
    inputs and a voltage pin I believe.

    I'm not exactly rushing to try repeatedly :)
     
  15. Insomniac

    Insomniac Guest

    Understandable

    Measure the voltage on pin 8, should have 3.3V there
     
  16. Guest

    Yeah, it's 3.3V at pin 8.

    Assuming the chip was getting input though, shouldn't the R,G,B in pins
    be at something (looks like between .5 and 1v from the spec). Because
    none of those pins have a voltage, so I'm guessing the problem is in
    getting the signal from the input to the chip.
    Although that doesn't explain why I can't get the onscreen display
    either.
     
  17. Dave D

    Dave D Guest

    Heat the top of the cap with the tip of a soldering iron until it gets hot,
    but not so hot it bursts! It will cosmetically damage the top of the cap,
    but don't worry about that.

    Dave
     
  18. Dave D

    Dave D Guest


    Something has indeed changed- you've eliminated two suspect parts and
    prevented further trouble!

    Dave
     
  19. Guest

    Okay... now I'm going to scream...

    I started looking at the monitor again today after work, couldn't
    figure out any more about it. Then about an hour later went back to
    try a few more things... and it just worked when I turned it on.

    I literally didn't do anything that should've changed anything about
    it. This is after not working at work for a month or so. I just took
    it when they gave up and were throwing it out.

    I hate electronics :)
     
  20. Guest

    Well, I'm happy to say that now the monitor appears to be working
    mostly correctly.

    It won't come back from sleep mode, but turning it off and back on
    brings it back.

    Also there's a slight darkening of the picture at three corners, but
    nothing that's going to bother me. (unless someone has an idea of what
    that would be and it's an easy fix). I suspect it was more than likely
    somehow caused by me during the inspection/repair process. Who knows.

    Anyway, thanks for the tips/help.
     
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