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polarity of power cord - gateway fpd1520

Discussion in 'Electronic Repair' started by zirath, Mar 28, 2008.

  1. zirath

    zirath Guest

    The ohm meter showed something like a capacitor on the center pin (it
    went to 0 and then back towards the center). The side showed no resistance.

    The monitor went on briefly and then went black when plugged into the
    power. Seems like it's a dud. Not sure if it's worth trying to repair it
    or just return it.
  2. Tim

    Tim Guest

    Is there no little pic of the connector around where it give the info
    about power? Usually there is a universal icon used that indicates
    polarity, kinda like a big C around a dot. Most, but not all, coaxial
    power connectors have the minus side on the outside, that way if it
    touches a grounded side, it will not destroy the adapter. If the adaptor
    cannot start the monitor because it is not powerful enough, it will
    sometimes just flash the backlight then go off.

    - Tim -
  3. zirath

    zirath Guest

    The adapter symbol shows center + and side -. It is labeled 12vdc 4a.
    The monitor is 12vdc 2.5a so the adapter should have enough power.
  4. Arfa Daily

    Arfa Daily Guest

    Before condemning the monitor, I would feel inclined to just try a different
    power supply (bearing in mind that the one you are using now is not an
    'original'). The reason that I say this is that there can be quite a high
    initial pulse of current demand, as the backlights strike, and it just might
    be that the power supply sags a little when hit with this, even though it
    *says* that it is rated for 4A ...

  5. Arfa Daily

    Arfa Daily Guest

    Thinking about it again, were you trying it with a signal going in ? LCD
    monitors, like their CRT counterparts in many cases, don't just 'idle' when
    there is no signal. They do just like you say - that is power up long enough
    to check if they can detect an input signal, and if they can't, go back to
    an inert condition.

  6. zirath

    zirath Guest

    I tried it in a few different conditions: with the monitor on 1st and
    then the computer - it showed the computer booting for a few seconds and
    then went black - then it came on again after the boot was finished for
    a few seconds and then went black again; with the computer on and then
    the monitor; with the monitor disconnected from the computer (the manual
    says it's supposed to bring up a diagnostic screen in this condition but
    it didn't). The screen stayed black the whole time in subsequent attempts.

    The power led would go from yellow to green when the computer started
    booting but the screen stayed black.

    I also noticed that the screen was not completely flush in the housing -
    like someone had tried to, or did, open it and didn't close it completely.
  7. Arfa Daily

    Arfa Daily Guest

    The fact that the light stays green, is a good sign. What you next need to
    do, is to shine a strong desklamp or hand flashlamp at the screen at an
    angle, after it has gone back to black. If you can then see an image on the
    screen, that indicates that the backlamp tubes are not staying alight. This
    could well be because either the inverter is faulty, or that a worn tube is
    making the inverter shut down. Either is quite a common fault condition.

  8. zirath

    zirath Guest

    I was able to see the image on the screen - but the monitor remained
    black the whole time (it didn't go back to black).
  9. zirath

    zirath Guest

    (I shined a bright light on the monitor at an angle).
  10. Arfa Daily

    Arfa Daily Guest

    OK. Well that tells us that the whole monitor part is working, and the
    failure is in the LCD screen backlighting circuit. It is generally a bit
    fiddly, but it is repairable if you are that way inclined. Unless you are
    able (or want to) just throw it back where you got it from.

  11. zirath

    zirath Guest

    I'm into trying to repair it.

    Is there info somewhere on how to do it?

    Thanks for your help.
  12. Arfa Daily

    Arfa Daily Guest

    The fact that the light stays green, is a good sign. What you next
    If the screen comes apart easily, start by examining the backlight tubes. Be
    careful when extracting them as they are quite fragile. If any are badly
    blackened at the ends, suspect them. It's hard to test the inverter, without
    having known good tubes to connect to it. If you have a 'scope, you can get
    a good idea by looking to see if you have waveform at the primaries of each
    of the tube high voltage drive transformers. If you do get a momentary burst
    that then disappears, this could be because the inverter is detecting a bad
    tube, and shutting down. You might see the tubes initially flash up, and you
    might see one not do so. You can disconnect the tubes one at a time, and see
    if that keeps the inverter running, and if it does, you can cross connect
    the tubes to the opposite inverter outputs, to prove the drive electronics.
    Inverters and tubes are available from a number of sources. See, for

  13. zirath

    zirath Guest

    One thing I started wondering about after you were saying it could be a
    problem with the adapter--

    The picture of the original adapter that came with the monitor says the
    plug is supposed to be 6.5mm o.d. 4.4mm i.d. and 9.5mm long. (Thanks to
    Michael Terrell -

    The measurements of the plug that came with the adapter I got (from
    ebay) are : 5.5x2.5 / 9.5mm.

    Could this be the problem?
  14. Arfa Daily

    Arfa Daily Guest

    As long as the plug is 'making' ok to the centre pin, it shouldn't be a
    problem. However, the way round that you have described it sounds wrong. If
    the original was looking for a plug with an id of 4.4mm, (which is *very*
    big for this type of plug) that would suggest that the monitor has a 'thick'
    pin, which I'm surprised that your replacement at just 2.5mm, will fit over.
    2.5mm is a pretty 'standard' size.

    However, if you can see an image with an external light shone at the screen,
    that says that the plug *is* making ok, and the whole monitor is working,
    less the backlights. So no, I don't think that will be the problem.

  15. zirath

    zirath Guest

    I was able to disassemble the front and back panels.

    Is the inverter the part that has the power button and various setting
    buttons? I was able to disconnect this part.

    I didn't see the lcd tubes. The site didn't have this model
    of gateway listed and the instructions for getting to the tubes for the
    gateway models they had didn't match.
  16. Arfa Daily

    Arfa Daily Guest

    The inverter board is usually long and thin, and has a multipin plug going
    to somewhere around its middle, and thickish high voltage wires coming out
    of each end to go to the backlight tube connections at either side of the
    screen. I seem to recall that there are pictures of inverter boards on at
    least one of those sites. they all look pretty much alike generally, if not
    in detail. Each backlight tube will have two of these high voltage wires at
    either end, that will go back to connectors located at the inverter board

  17. neon


    Oct 21, 2006
    Anytime that there is involved and don't know the polarity just stick a diode in series . That will prevent from damaging the unit. Usualy that is instlaled on expensive units to make it fool proof . one way it will work the other way it will not. simple
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