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LCD backlight problem

Discussion in 'Electronic Repair' started by Michael, Oct 12, 2005.

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  1. Michael

    Michael Guest

    I have a dell 1504FP (LCD monitor). The backlight doesnt work for a period
    of time.

    On close examination with a scope on the PSU board, BIT3105 (lamp controller
    IC) is getting confused. It has several pins to do with OLP (open lamp
    indicator)- basically the chip switches itself off if it thinks one of the
    lamps is open circuit. The lamps are definitely all working. I can recreate
    the problem by disconnecting any one lamp.

    The OLP circuit waits for a cap to reach 2.5v before monitoring- if I drain
    that cap, the monitor works.

    Could someone look at the BIT3105 data sheet and tell me how to disable the
    OLP circuitry- I dont care if it shortens my monitor life. Please help.

    Thanks in advance.

  2. James Sweet

    James Sweet Guest

    Perhaps a resistor across that capacitor would make it less sensitive? How
    are you draining it? Just shorting across it?
  3. Michael

    Michael Guest

    The capacitor is on pin 19 of the chip. Its the soft start for the lamps.
    For normal function, I think the cap has to fully charge (goes to around 9v
    i think), but the OLP circuit doesnt cut in till 2.5v. I dont really want to
    tinker permanently with that cap.

    Anyone know where I can get a BIT3105 (in the UK), I might change it out in
    case its faulty.

  4. Michael

    Michael Guest

    Sorted it! I soldered a 47k surface mount resistor over the surface mount
    capacitor (C25, on underside of PCD, near BIT3105- in case anyone has this
    problem in the future) and it works fine. OLP circuit disabled. I think I'll
    notice if any of the lamps die- dont need all that overengineered circuitry.
    Thanks for your help people.

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