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Monochrome Fold over problem?

Discussion in 'Electronic Repair' started by SomeBody, Feb 15, 2006.

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  1. SomeBody

    SomeBody Guest

    Hi,

    I'm working on some 9" monochrome monitors and I've run into some problems
    on many of these units.

    This problem is horizon fold over. I am at a loss of what or how to fix
    this problem.

    I have uploaded the schematics and the data sheets for the IC for
    this monitor at:

    http://home.earthlink.net/~zbadone/schematics/

    If anybody would care to look these over and point me in the direction or
    what to look for to correct this problem that I can NOT fix!

    Thank you.
     
  2. carneyke

    carneyke Guest

    Check the bipolar capacitor in the horizontal output ckt. They are
    around 5 UFD and a few hundred volts. They open and cause a horizontal
    line. Their job is to couple signal to the yoke and block DC.
     
  3. carneyke

    carneyke Guest

    I would check ESR on all those electrolytics in the horizontal circuit
    or replace them. I am relatively certain C221 (has an * next to it) is
    the bipolar cap causing you grief.
     
  4. : I'm working on some 9" monochrome monitors and I've run into some problems
    : on many of these units.

    These are basic security monitors, it's really a hunt and peck job.

    As a couple others have suggested, pull and test all the electolytics in the
    horizontal section (areas b1,b2,a1,a2 on the schematic). Anything bipolar
    I'd just replace now.

    If those come up clean, move on to checking the diodes and resistors.

    Usually there is a lot of room to work on these and the card all the
    electronics are on can usually be lifted and twisted to access both sides.

    With horizontal fold over, there is a slight chance that either transformer
    (horz osc and flyback) are bad or shorted. If nothing pops out with the
    cap/diode/resistor check, will just have to subsitute these to see.

    Really though, just pull and replace any electrolytic that is bipolar or
    rated at 160V or over, if it isn't bad now, it will be soon. I'd guess
    nearly 80% of the problems these things develop can be fixed by doing that.

    -bruce
     
  5. Guest

    There is another possibility.

    The description I've seen by the OP doesn't tell us whether it is the
    raster or the video folded over. A bad case of H sync timing can look
    like that description, and could well still be an electrolytic, but in
    the H osc stages, not the sweep.

    If this is the case, you've got overlapping reverse image that is
    stretched horizontally, it might not be a sweep problem at all EVEN IF
    THERE IS A BLACK AREA AT THE SIDE(S). To find out for sure, reduce the
    brightness/constrast and turn up the G2 until you see the vertical
    retrace lines. Does that light up any dark areas ? Be sure to turn it
    back down !

    If the whole screen lights, you have a sync and/or blanking problem,
    most likely caused by something in the dropping/waveshaping of the
    feedback from the sweep to the sync circuit etc. If the whole screen
    doesn't light, and has some bright areas where the beam is actually
    slowed down, turn the G2 down quickly. Don't want to burn the screen.

    This would not be the first time somebody mistook a horiz phase problem
    for a sweep problem. We need to know which it is for sure.

    JURB
     
  6. carneyke

    carneyke Guest

    Jurb,
    Thanks for the tip on the difference between sync and sweep problem.
    I've seen multiple images caused by the horizontal running too fast and
    the sweep doing it's job of filling the width of the CRT. Very Good Tip
    !
     
  7. Have you tried replacing the cap in series with the horizontal yoke
    coil?
    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
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