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Thread: Fault finding

  1. #1
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    Default Fault finding

    Its an older 2007 Asus monitor which power cycles/turns on and off constantly.
    The power board is DELTA eadp-50cf
    Anyway the caps look okay-ish neither bulging or leaking but the 12v pin shows 8,9V.
    Since i dont have a cap meter at hand how can i determine on where the fault lies?
    Whats the most intelligent route to finding the problematic part if one has no schematics?
    Could VR-s deteriorate even that hard to drop by a couple of volts?
    Or even caps without having anykind of outwardly visible physical change?

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    Default

    Just replace all capacitors except the primary (big 400v+) capacitor.



    The 2 small capacitors on the bottom left side are part of start up circuitry, if they go bad the psu won't start at all and monitor wouldn't stay in stand-by. They're probably fine but they're so cheap you should just replace them.

    the one at top and probably also the one in center right in the picture are part of the inverter circuitry (backlight)
    The top one could degrade due to heat from the inverter transformers and could be bad even if it's not visually looking bad...

    The group of 4 on bottom right (plus the lonely black cap by the connector) filter the voltages going to the board with the actual processor ... it's probably 12v and 5v .. yeah, i see in another picture it's these voltages printed by the connector.

    Just replace them.

    Also check the fuse , the round dark red/brown thing.


    Though I suspect your problem actually lies with the CFLs (the fluorescent tubes). As they age, the current they use increases and eventually one of the pairs of fluorescent tubes uses so much more current, the inverter turns off the backlight to protect against trasformer overheating.

    Could also be one high voltage wire desoldered from the CFL tube due to repeat heat cycles, and potentially arc-ing against the metal on the back of the lcd screen. When that happens, again the inverter turns off power to the backlight.
    Attached Images Attached Images

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    Default

    Seems the caps are ok and ofcourse the fuse also.

    But the question comes about 12v pin.
    Like i mentioned before its 8,9v the IC with which it is connected is an 8v output regulator.
    So a small confusion here.Is it designed in such a way or is there a problem?

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    Default

    I had some problems on a similar board and used a little silicone grease on the high voltage connectors, but I tend to agree with the suspicion that it may be the tubes.
    Last edited by ashiekh; 11-29-2018 at 01:36 PM.

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