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Zeroeth level pc monitor repair

Discussion in 'Electronic Repair' started by N Cook, Dec 21, 2005.

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  1. N Cook

    N Cook Guest

    The only excuse I have is i don't normally look at the innards of pc
    I now have a recent monitor where the biasing of driver for pulse
    transformer leading to base of the LOPT driver , correctly biased, seem
    fine but no line rate oscillator, HT etc.
    With pc monitors is there no free running oscillator that is then pulled
    into sync by external H & V syncs from the pc ?
    Is it the case of no external syncs then no monitor oscillator at any

    Perhaps I'm too cautious but the 3 R,G,B signal lines from the pc go direct
    to the subboard fixed to the CRT neck socket close by to spark gaps and HV
    components - alien territory to me to see TTL level suff there.
    I therefore powered up a win95 era pc , known working (with its monitor ) ,
    15 pin high density plug format, to feed syncs + RGB to this one but it does
    not kick into life.
    Chase the H sync signal lines back to what should be their source on the pc
    to monitor lead and there is none.
    Does a recent pc monitor with DDC control lines need a more recent pc ?
    Is it safe to power up pc with old monitor , unplug that monitor and plug in
    new monitor or is it some control line I have to enable/disable ?
  2. Dave D

    Dave D Guest

    VGA signals are not TTL level, they are analogue 75 Ohm video signals. It's
    very common now for the VGA signals to go straight to the CRT base PCB, and
    perfectly safe provided basic rules are followed. As you are in the EU,
    there's very strict laws on electrical safety approvals.
    Kind of a hot-swap of monitors? Yes, I do it all the time (for many years)
    and have never blown a monitor or video card.

  3. Franc Zabkar

    Franc Zabkar Guest

    This is how "energy saving" CRT monitors respond to missing sync
    (old expired URL)

    If either Vertical Sync or Horizontal Sync coming from the display
    controller are removed, the monitor will enter the Standby/Suspend
    mode. In this mode, the deflection and high voltage are turned off,
    reducing power consumption in the monitor to less than 30 Watts.

    If both sync signals are removed, the CRT filament will be turned off,
    in addition to the deflection and high voltage. This is the Off mode.
    The monitor power consumption is reduced to less than 5 Watts.
    CGA and EGA were TTL, VGA is analogue. IME, it is very common to find
    the monitors's video circuitry, eg an LM1203, on the CRT PCB.
    I've seen the amplitudes of H & V sync signals on some older graphics
    cards, eg S3, collapse under the load of some monitors.
    - Franc Zabkar
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