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LCD monitor internal power supply repair

Discussion in 'Electronic Repair' started by n, Aug 19, 2005.

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  1. n

    n Guest

    Hi,

    I"ve got a Northgate 17" LCD monitor who's warranty expired the 4th and
    just gave out on me! Go figure - 15 days after the warranty dies...

    I have isolated the problem to the internal power supply but need help
    determining the exact problem.

    THe problem is intermittant. When the problem does occur, the 100 uF,
    400 Volt capacitor buzzes. At first I thought it was the capacitor (on
    the ac incoming (primary) side. But I think the capacitor may be just
    reacting to a fluctuating power level - maybe from a bad step down
    transformer. I desoldered the capacitor and plugged the monitor in. It
    buzzed - but only once - as opposed to several times with the capacitor
    in place.

    Any suggestions or ideas to look would be appreciated.

    Much thanks.
     
  2. Ken Weitzel

    Ken Weitzel Guest

    Hi...

    Wouldn't it have been a much wiser test to replace the cap
    instead? :)

    Ken
     
  3. JANA

    JANA Guest

    I would think it would be most intelligent to replace the capacitor!

    --

    JANA
    _____


    Hi,

    I"ve got a Northgate 17" LCD monitor who's warranty expired the 4th and
    just gave out on me! Go figure - 15 days after the warranty dies...

    I have isolated the problem to the internal power supply but need help
    determining the exact problem.

    THe problem is intermittant. When the problem does occur, the 100 uF,
    400 Volt capacitor buzzes. At first I thought it was the capacitor (on
    the ac incoming (primary) side. But I think the capacitor may be just
    reacting to a fluctuating power level - maybe from a bad step down
    transformer. I desoldered the capacitor and plugged the monitor in. It
    buzzed - but only once - as opposed to several times with the capacitor
    in place.

    Any suggestions or ideas to look would be appreciated.

    Much thanks.
     
  4. n

    n Guest

    Hi,

    I've checked the capacitor - and the DMM readout matches the
    specification - it's 100uF and it reads between 100 - 102 uF - perfect.
    No signs of leakage that occurs sometimes in power supply caps over time.

    The problem is intermittant - so it may be an intermittant capacitor - I
    will replace it and let you know what happens. However because of the
    DMM readout matching the specification so perfectly, I am pretty sure
    this isn't the problem.

    Does anyone out there know of a good reference for step down
    transformers - specifications? I've heard of sams facts, but need to
    know where I can purchase a copy.

    TIA,

    --Rob
     
  5. James Sweet

    James Sweet Guest


    You can't test electrolytic caps with a dmm, if at all in doubt, just
    replace it as they're cheap.
     
  6. sofie

    sofie Guest

    n:
    Your model of DMM may read the capacitance value BUT it will not give you an
    indication of it's ESR value which is very important in many high frequency
    circuits like SMPS power supplies.
    If in doubt about the capacitor's condition, without an ESR meter to test
    it, just replace the capacitor.... about $1.00 or less.
     
  7. n

    n Guest

    HI everyone,

    Does anyone have a good web store for components? The capacitor I need
    can't be found at my local electronics store (I"ve checked two places).
    They had a 425V @ 100uF capacitor, but it was too big to fit into the
    small LCD casing - about twice as large as the 400V, 100uF capacitor I
    need. Any suggestions? I've been looking on the web to no avail so far.

    Thanks,

    --Rob
     
  8. Guest

    http://www.digikey.com/
    http://www.mouser.com/

    Also check the temperature rating of the capacitor - it will be printed
    on it. 85 C and 105 C are the most common. Replace with one rated at
    an equal or higher temperature.

    Matt Roberds
     
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