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Laptop LCD problem inverter or backlight or video board ?

Discussion in 'Electronic Repair' started by kk, Jan 23, 2005.

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  1. kk

    kk Guest

    I have a Compaq Presario 1700t laptop, the 13.3" LCD is not functioning
    correctly.
    When I turn on the laptop, the LCD is very dark but still can see the
    dim image of the Windows is booting up. Then I research on the problem
    and opened the case and find the inverter is ALPS KUBNKM011A. I ordered
    the inverter from ebay and replaced the old inverter. However the
    problem still exist. I found there is no difference on the dim image
    whether plug in the inverter or unplug it from the montherboard.
    I tested there is 12v dc output from the red wire but nothing from
    white and green in the middle. Also, no voltage (no dc nor ac)from the
    socket where the pink wire supposed to plug in.
    Could anybody help me how to test whether the inverter or backlight or
    video board having problem?
    Thank you!!

    The inverter looks like this
    http://www.impactcomputers.com/198700-001-ib.html#
     
  2. Jerry G.

    Jerry G. Guest

    A common cause is the inverter, and or the backlight failing. These are best
    to replace together. In the case of buying a used one, there is a good
    chance the one you buy is also defective. They only go so many hours and
    they have to be changed.

    With some displays, the backlight is an integral part of the display. In
    this case, both the inverter and complete display should be changed at the
    same time. I would not buy used parts, because they do wear out after a
    number of hours, and used parts have hours put on to them.

    --

    Jerry G.
    =====

    I have a Compaq Presario 1700t laptop, the 13.3" LCD is not functioning
    correctly.
    When I turn on the laptop, the LCD is very dark but still can see the
    dim image of the Windows is booting up. Then I research on the problem
    and opened the case and find the inverter is ALPS KUBNKM011A. I ordered
    the inverter from ebay and replaced the old inverter. However the
    problem still exist. I found there is no difference on the dim image
    whether plug in the inverter or unplug it from the montherboard.
    I tested there is 12v dc output from the red wire but nothing from
    white and green in the middle. Also, no voltage (no dc nor ac)from the
    socket where the pink wire supposed to plug in.
    Could anybody help me how to test whether the inverter or backlight or
    video board having problem?
    Thank you!!

    The inverter looks like this
    http://www.impactcomputers.com/198700-001-ib.html#
     
  3. James Sweet

    James Sweet Guest

    If changing the inverter didn't fix it, the problem is probably that the
    CCFL tube is worn out, you can get replacements if you can get it out and
    measure the length.
     
  4. JW

    JW Guest

    <snip>

    Possibly, but note that the controller outputs a signal to the inverter to
    signal whether to turn on the backlights. On some inverters, the signal is
    active high; on others, it is active low. There is usually a small cable
    that connects between the LCD controller and the inverter that contains
    this signal as well as power and ground (which may also be at fault and
    can be checked with a meter.)
     
  5. kk

    kk Guest

    I just ordered the CCFL at $10 plus $10 for shipping. I will give it a
    try.

    There are four wires from the board to the inverter.
    I only get reading from the red and black wire, 12v dc. No reading for
    the other.
    Is it possible to make it active high in case it is the controller
    problem?
     
  6. JW

    JW Guest

    Yes, it's possible. However, there are two wires for control; most likely,
    one is used for dimming, and the other for switching on and off the
    inverter. I may be able to help you, but I would need the data sheet for
    the inverter - a schematic would be best, but even the pinout would
    probably suffice.

    A search on the part #KUBNKM011A you gave revealed nothing except places
    that are selling them. If you can find a pinout, I could probably help you
    further. As a last resort, you could isolate the two signal wires one at a
    time, and try connecting them to +5 volts one at a time. You've already
    located the +12. I would not assume that the black is ground (I remember
    the good old days when this was almost always true. *sigh*), do a
    continuity test between the three remaining wires to ground - a negative
    lead on a surface mount cap is usually a good point for this, as is a
    shield on a connector housing. This will leave you with the two control
    wires to experiment with.

    Good luck.
     
  7. clatonium

    clatonium Guest

    kk- what size was the lamp?
     
  8. cmdrdata

    cmdrdata Guest

    This is an interesting thread as I am also having trouble with similar
    issue. My problem is with a Thinkpad 600E 13.3 active matrix display (
    a second TP that I am repairing). When I turn the TP on, the backlight
    would come on for a couple of seconds (enough for me to read the memory
    size, and then it would go off. If I let it continue booting, I can see
    that the LCD faintly show system info. So I bought a replacement
    inverter on eBay and I got the same problem. Sent that one back (on an
    RMA) and they replaced it with another one. Same symptom. (I have
    successfully done this on my other Thinkpad, so I might swap the one
    that I know work on the unit I am repairing). In the mean time though,
    I have a question for yall: The inverter is all solid state, so what
    really wears out in it that would cause it to fail? My experience with
    solid state devices is that most of them just worked until something
    catastrophic failed inside the chip.
     
  9. kk

    kk Guest

    I will try locating the +5 v signal line this weekend.

    My laptop lcd is a 13.3". I don't have the dimension of the lamp with
    me now. I order from www.lcdpart.com, this shop seems to have parts for
    standalone LCD monitor, not much for LCD monitor.

    KUBNKM011A was made by ALPS
    I found there is another made from AMBIT U131037.00
    This one is compatible with the ALPS one.
    If I am not wrong the Compaq part number is 198699-001.
    The service & maintenance site for Compaq is
    http://www.compaq.com/athome/support/msgs/
    But there is nothing about the troubleshoot the LCD problem.

    Thanks again for the help!

    P.S.cmdrdata, I have the feeling that the ccfl of your lcd is dying.
    You can check the backlight if it turned very dark at one or both ends.
    It is just my guess.
     
  10. James Sweet

    James Sweet Guest

    It's entirely possible that the lamp is shot so the inverter is shutting
    down due to the excessively high voltage required to maintain an arc. You
    could try plugging in a new CCFL, don't take the time to install it in the
    screen just yet, just pick up a surplus tube and wire it up to see if it
    lights.
     
  11. JW

    JW Guest

    FWIW, I have a 18" desktop LCD (NEC model 1810) that had that exact same
    problem. The fault was several blown picofuses on the inverter board
    (there were four total - one for each tube) after replacing them, the
    monitor came on and stayed on. I'm assuming that there was some type of
    feedback circuit in the inverter that kicked in to shut the unit down.
    Perhaps you have one bad fuse which would allow one of the tubes to light?
     
  12. JW

    JW Guest

    Technically, it's not a signal, but a voltage point that you may need to
    tap one or both of the signals into forcing the inverter on (assuming it
    requires a "high" voltage level to be enabled, since you stated there was
    no voltage on 3 of the wires.) Before you go any further, can you check
    both inverters with an ohm meter to see if there are any blown fuses? They
    are usually very tiny, and at first glance may not even look like a normal
    fuse.
    More importantly, no pinout information either.
     
  13. kk

    kk Guest

    Hi, I still don't have time to install the CCFL. But I found out a
    failed fuse on one of the wire.
    Is it replacable and how? I can only think of the surface mount fuse,
    but it seems too bulky to install in the narrow space of laptop.
    Would it be too dangerous to bypass the fuse? I just don't want the
    machine to catch fire or so.
    Thanks
     
  14. James Sweet

    James Sweet Guest

    Don't bypass the fuse! You may be able to replace it with a single strand of
    steel wool, but surface mount fuses can be bought.
     
  15. kk

    kk Guest

    I just confirmed the fuse is the only problem. While I was trying to
    get the Amp reading for the fuse with a multimeter, the LCD monitor
    just light up and working as normal. The fuse is a tiny retangular
    chip mounted on the board.
    Could you give me the link for the place that sell the fuse?
    Or any substitute & how to get and use it?
    Thanks again.
     
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