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Know Studio 40.ND Television Character Generator chroma issues

Discussion in 'Electronic Repair' started by William R. Walsh, Apr 8, 2013.

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  1. I am trying to troubleshoot chroma issues with a Knox Video Technologies
    Studio 40.ND television character generator. This device is capable of
    accepting input from up to two video sources (composite or S-video can be
    selected by a switch). It appears that all genlocking and synchronization
    functionality is built into the unit--there is no obvious way to input a
    sync reference and the unit will function as a text generator only when no
    video sources are attached.

    All of the character generation functions work correctly and appear to be
    stable. Color of the text and backgrounds appears to work correctly.

    Input from either camera connector (tested with S-video and composite input)
    demonstrates a serious chroma stability problem. The color will shift at
    random times and at different places in one "frame" of video. The chroma
    signal from the cameras goes to a Harris CA3126E chroma processor IC. I have
    removed this IC and tapped the chroma input at its socket, and the signal
    looks good up to that point. I have also tried another IC and careful
    adjustment of the chroma oscillator circuit (after marking the position of
    the trimmer in that circuit). No improvement has resulted.

    Tracing the circuit has not been successful as the board is dark and appears
    to have at least two layers. I called Knox Video and was told the product is
    "ancient", that they threw away all of the documentation and the person who
    worked with these is no longer employed by the company. They cautioned
    against attempting any adjustments on the video board, stating that the
    controls interact.

    I am hoping that someone might have repaired a similar fault or that someone
    may have a user's manual or schematic for this unit. I'm just about to give
    up on it.

    William
     
  2. Responses have been inlined.
    I have a working color bar generator that I've fed into the unit. (I've
    found that it's just barely possible to get stable and correct color with
    the bars by tweaking the trimmer in the color oscillator circuit of the
    character generator.)

    As far as a vectorscope goes, no, I don't have one. It was brought to my
    attention elsewhere that I could use a VirtualDub plugin and video capture
    card to provide a vectorscope in software. I'm not sure if I'd understand
    how to use it. Nor do I know if it would be accurate (enough).

    I may try a new crystal and make the effort to replace electrolyic
    capacitors at some point in the future. Right now I've set it aside as I'm
    tired of fiddling with it and have no pressing need for it to work.

    William
     
  3. nick

    nick Guest

    Software based vector scopes are used for correcting digitized video they are unlikely to be helpful in calibrating equipment.
     
  4. Hi!!
    This is a 3M Mincom unit made sometime around 1974. I've found virutally no
    information about it, but the build quality is outstanding. I'd dare say it
    was professional/broadcast grade equipment when it was new. (It came with
    several other pieces, including a video distribution amplifier and a
    subcarrier distribution amplifier.)

    It has a number of options that appear not to be present on this model,
    including a switch to go between color bars and a black burst.
    For a one-off job such as this, I'm not sure that it's worth the acquisition
    costs, even at pennies on the dollar. I was really hoping that someone might
    have the owner's manual or a schematic buried in a forgotten file cabinet. I
    get the feeling after obtaining this unit, that they weren't common or
    popular. If I better understood what all the circuitry in the unit was
    trying to accomplish, I really feel that I'd have a good shot at repairing
    it.
    I agree with that observation. This was suggested by someone else.

    William
     
  5. Leif Neland

    Leif Neland Guest

    William R. Walsh forklarede den 14/04/2013:
    Would it be possible to use a Raspberry PI, which have both HDMI and
    composite video output, to generate whatever video signals one would
    need?
     
  6. Guest

    A vectorscope consists of an oscilloscope and a special graticule, nothing more.

    Just draw a circle on a piece of clear plastic and mark it with the markings from a protractor or something.

    Also, I doubt this will help solve the problem. This is almost always a PLL problem, probably a cap somewhere, but not for sure.

    Question, it has two of these chips and they're both doing it ?
     
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