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Monitor Sync Signal in 80s Arcade Machine

Discussion in 'Beginner Electronics' started by Computer Nerd Kev, Feb 7, 2013.

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  1. Hello, this was going to go to sci.electronics.repair but it
    seems that Aioe users are blocked from there, so I'm posting
    it here instead.

    I recently bought a broken "cocktail" style arcade game
    machine made in 1980. Initial repairs were easy. I got the
    machine to start by playing with the connections to the CPU
    and logic boards and fixed the lack of green on the monitor's
    display by soldering the green lead of the RGB signal back
    onto the monitor board.

    The problem I'm left with is that after about three minutes
    (less if it has been recently used), the picture on the
    monitor is reduced to some coloured vertical lines and a
    whining noise is emitted. Before this, the display bends
    inwards at the bottom (and a bit at the top) of the screen. My
    assumption has been that this is a sync issue so the first
    course of action was to look at the sync signal that goes from
    the CPU board to the monitor with my Oscilloscope. I did this
    expecting to see a slowly worsening waveform, but instead I
    saw what seemed to my eye a quite nice saw tooth.

    However what did catch the attention of my untrained eye was
    that this waveform varied by less than a volt between 4 and
    5V, except for a momentary drop to 0V at the end of the wave.
    Period of the waveform was 90uS.

    I only have theoretical knowledge of monitor sync signals, so
    I didn't know what to expect, but it seems to me that this is
    an oddly small voltage change for such a signal. I tried to
    find something on the web that would tell me if this is the
    case, but failed. Hence I ask here.

    Thank you for reading.
     
  2. Apparently something wrong in your horizontal drive signal. Most likely the
    line output transformer, maybe the horizontal coils but there are still some
    other possibilities. Be aware that the coils are current driven.

    petrus bitbyter
     
  3. OK, so the sync signal going in is fine then?
    Forgive my ignorance, but if a coil or transformer failed, would this be
    possible in such a way as for the monitor to function for the three minutes
    it usually runs before failure. Or did you mean the components associated
    with the the coil or transformer?

    Also, if I go to repair this part of the circuit (and let's assume I'm not
    replacing the Line Output Transformer, I don't suppose I could find a spare
    available), I know I'll need to discharge some capacitors, but should I
    discharge the high voltage output of the transformer as well?

    Thank's for your help.
     
  4. This kind of failure is pretty common especially with coils and
    transformers. In this components insulation tends to degrade partially over
    time, usually depending on the temperature. So a transformer may look liko
    te perform excellent while cold but fail when it grows warmer. Off course,
    other components may fail the same way sometimes. I'm afraid you to lack
    both skills and equipment to find out what component(s) are actually
    failing. Even well trianed and equipped professionals usually dislike this
    type of failures. And, you're right about the line output transformer.
    Hardly a chance to find a good replacement.

    petrus bitbyter
     
  5. OK, I'll pull the monitor unit out again to at least give the main board a
    close visual inspection. Just my luck that the monitor is the only part of
    the machine that I haven't got a schematic for.

    Thank you for your help. It probably saved me a lot of messing about with
    sync signals.
     
  6. Guest

    Quote
    "Even well trianed and equipped professionals usually dislike this type
    of failures."
    End Quote

    "usually"..................I would think "always" would be closer to the
    mark, Petrus!! '-)

    Daniel
     
  7. Thank's, I'll keep that in mind when I finally get around to
    looking at the thing. Unfortunately it's something of a
    process getting the monitor unit into a visible position.
     
  8. I presume I could use an infared temperature sensor to do the
    same job while keeping my fingers out of zapping range?
     
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