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NEC Multisync P1250+ low HV, no picture

Discussion in 'Electronic Repair' started by James Sweet, Jun 12, 2004.

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  1. James Sweet

    James Sweet Guest

    Got this monitor in with symptoms I don't come across often. There's no
    picture, and the HV while present, only measures about 8KV, I'm guessing
    this is the reason for no pic. It's quite difficult to get in and measure
    waveforms with the monitor operating, is the flyback a common failure in
    these? It uses a separate MOSFET to drive the HV, I tried a ring test on the
    flyback between ground and pin 1 which I *think* is the drive pin and only
    get 2 lights on the tester, I'm not sure however if this applies to this
    sort of setup. Anything else I should look out for on this thing? Seems to
    be a pretty decent 21" trinitron monitor.
     
  2. Wild Bill

    Wild Bill Guest

    I'm not familiar with this unit, and I haven't done any monitor
    troubleshooting for a couple of years.

    Just for grins, have you measured focus potential at the CRT socket? I'm
    sure that it's highly unlikely that F is draining about 20kV, so it's just a
    WAG.

    The only other suggestion I'd have, that I'm fairly sure you're already
    aware of, would be low B+

    Cheers
    WB
    .....................
     
  3. Jerry G.

    Jerry G. Guest

    The main power supply may be working too low. It is also possible that
    something is loading it down. You will have to properly troubleshoot the
    unit to determine the exact cause. Without a service manual and a reliable
    parts source, this is going to be a difficult one. NEC will not sell any
    parts or service manuals to non NEC service reps.

    --

    Jerry G.
    =====


    Got this monitor in with symptoms I don't come across often. There's no
    picture, and the HV while present, only measures about 8KV, I'm guessing
    this is the reason for no pic. It's quite difficult to get in and measure
    waveforms with the monitor operating, is the flyback a common failure in
    these? It uses a separate MOSFET to drive the HV, I tried a ring test on the
    flyback between ground and pin 1 which I *think* is the drive pin and only
    get 2 lights on the tester, I'm not sure however if this applies to this
    sort of setup. Anything else I should look out for on this thing? Seems to
    be a pretty decent 21" trinitron monitor.
     
  4. James Sweet

    James Sweet Guest

    I'll try to do some probing on the power supply, this is one of those
    monitors where you almost have to tear the whole thing apart to reach the
    PCB at which point a bunch of cables are unplugged so it can't be turned on.
     
  5. Wild Bill

    Wild Bill Guest

    If it's like the 21" Sony/IBM models I had here recently, there are shields
    surrounding everything.
    The PSU is accessible by removing the top shield, then removing the screws
    for the PSU panel, then it swings down. You can open the cable anchor
    twisty-holders and remove the inner shield that covers the PSU board.

    The models I saw had enough cable length for the PSU to lay down flat while
    still connected.
    The sweep/deflection board didn't have as much freedom of movement.

    Cheers
    WB
    ..............
     
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