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Quasi-interesting CRT puzzler

Discussion in 'Electronic Design' started by Ancient_Hacker, Aug 17, 2006.

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  1. Okay, this is kind of a retarded question, but here goes.

    Let's say you want to convert you basic PC monitor from raster scan to

    You pull off the yoke connector from the main board and find out the it
    still runs okay, well except that you've burnt a little spot of
    phosphor in the center of the screen unless you were prescient enough
    to back off the brightness, contrast, and the three screen controls

    Then you hook up a DC supply to the V and H yoke coils and find out it
    takes about 3 AMPS to do a full screen deflection in the vertical
    direction, about TEN amps in the horizontal direction.

    (I think that's because the horizontal deflection has to happen faster,
    therfore lower inductance in the h coil, therefore fewer gausses per

    So we need some pretty hefty drivers, many amps plus many volts if we
    want fast deflection.

    Not a huge problem so far.

    But now we realize we don't want to burn the screen if we're drawing
    short or slow vectors, and we don't want the lines to be invisible if
    we're drawing them long or quickly.

    So we need some sort of Z-axis brightness modulation, somehow
    proportional to the "writing rate", like in inches per millisecond or

    Anybody have any idea how to compute this on the fly as it were?

    Ideally I'd like to be able to draw 6000 full length vectors per

    A quick web search didnt find anything promising.



  2. Write at a constant speed. Variable speed is trouble and will require manual
    adjustments to compensate for CRT variables.

    Graham H
  3. To paraphrase Gene Kranz "Constant speed is not an option". I need to
    draw some straight lines, arcs, curves, all coming from different
    analog sources, each with its own deliberate and preset and in several
    cases, unchangeable sizes, lengths, and drawing rates.

    I know it's doable as I just got a new $400 crown fitted to my third
    molar. Ouch.

    Explanation: ( My dentist is a "somewhat less than perfect kids
    dentist" )

    Further explanation: (He has an ancient "Asteroids" arcade video game
    juggernaut in his waiting room to keep the kids busy))

    Further Explanation: (Being a bit off-, I had to go back for several
    refittings to get the crown to sit right so I could actually eat
    anything other than chocolate milkshakes (not that I was complaining),
    I spent lots of time in the waiting room.)

    Much Further explanation: ( The game draws many different length but
    seemingly constant intensity vectors and arcs to draw the asteroids )
  4. You can overcome the problems by retaining the raster scan at zero
    brightness and using the vector information to brighten the spot. I
    don't know exactly how it's done (comparators, I suppose), but I have a
    Rohde & Schwartz sweeper that does it that way, and the servicing
    information on it.

    6000 vectors/s doesn't represent a very high video frequency.
  5. Tony

    Tony Guest

    This is sort of an obvious answer, but since the write speed in each
    direction has to do with the rate of change of current, which is
    proportional to the voltage on the coils, couldn't you just use op amps
    to sum the absolutes of the suitably scaled rates of change of voltage
    and use that to modulate spot intensity? I recognize spot velocity
    isn't proportional to the two linear sum of the two coil voltages, but
    it may work well enough as an approximation. You could, I guess, get
    really fancy and via analog methods modulate the spot brightness in
    proportion to the square root of the sums of the squares of the squares
    of the voltages. That would be an entertaining and not too difficult a
    circuit to design.
  6. Thanks, I found a tek patent on exactly such a circuit. Not that I'd
    copy it exactly. But the general method is obvious once you hear it.

    You just differentiate the x and y signals and that gives you a measure
    of the velocity. Ideally you'd square and square root the sum to get
    the actual distance per time info, but just adding them should be good
    enough for starters.
    It does if you're trying to do it with a deflection yoke! To delta the
    current form -10 to +10 amps across a typical 2millihenry deflection
    coil 6000 times per second takes how many volts? Waay out of the SOA
    of most transistors. I wish there were a shortcut.
  7. Al

    Al Guest

    You got a crown for only $400???? Where is your dentist. Here, in
    Taxachuesetts it's at least $1100.

  8. In message <>,
    Indeed; I meant as a video frequency for the brightness modulation of
    the raster.

    True vector displays date from the valve/tube era, with higher impedance
    coils and BIG voltages. The horizontal output valve/tube would typically
    work with 3 kV pulses on the anode.
  9. Eeyore

    Eeyore Guest

    I one worked on a spiral scan CRT system for radar. That was fun !

  10. Tuned deflection coils, more efficient use of power?

    I worked on a radar display in the early 70's that used deflection coils of
    around 10uH total. We used a class A push-pull output stage with
    centre-tapped coils and a series inductor. We got around 5MHz small signal
    bandwidth. The energy stored in the inductor provided the high voltage
    needed for fast transitions of the beam. The system could display a 625 line
    TV picture and write all over it with vectors and characters in the vertical
    flyback period. Problem - 800 watts per axis!

    Graham H

  11. Hello,

    So let's do the math.

    V = L * di/dt

    Now I assume the worst case. The factor two is for forward and
    backward movement so that we are back at the starting point.

    V = +/-(2e-3 * 10*2*6000) = +/-240V

    This requires a +/-300V supply with 10A.

    I would say possible, but mission useless. Ok, it's a hobby project.

    Best regards,
  12. Guest

    Adding the absolute values, I assume.

    I could see a problem with differentiating input that is switched
    from one analog source to another. Unless the transition is smooth
    you'll see big derivatives you may have to control and/or ignore.

  13. On Thu, 17 Aug 2006 12:37:56 GMT,
    Not a very good one it seems.

  14. James Beck

    James Beck Guest

    Get a few schematics for the old WellsGardner Color XY monitors used in
    the likes of Atari Tempest and so on.
    They basically had a deflection coil that was symetrical for the X and
    Y, used identical X and Y deflection circuits and used the amplitude of
    the drive circuit to handle the bigger X than Y (yes it could drive the
    guns off the edge of the phosphor). The guns were driven just like they
    are in a raster scan system just with the addition of a "Spot Killer"
    circuit that would kill the guns if there were no X or Y deflection,
    keeping you from burning a hole or line in the phosphor.
    The X and Y amps look a lot like a beefed up audio amp.

  15. Guest

  16. Joel Kolstad

    Joel Kolstad Guest

  17. Guest

  18. Rich Grise

    Rich Grise Guest

    This is really the only practical way to do it, other than to buy
    an actual vector monitor. Converting one would be a nightmare - for
    one thing, the yoke is dramatically different from a raster scan
    monitor's yoke - both sets of coils are the same (in a vector
    monitor) with about two dozen turns of about #18 wire. And you'd
    have to take out the deflection amps, while somehow keeping the
    HV supply, and replace them with some hefty power amps.

    Having worked on both kinds of monitors back in my video game
    repairman days, I'd say it's a non-starter.

  19. Rich Grise

    Rich Grise Guest

    Next time you're in there, ask him if he'll open up the game and let
    youi look at the monitor yoke. That should give you some idea of what
    you'd be up against.

  20. Yipes! Luckily I overstated the actual situation-- we only need to
    draw about 20 full-screen lines 60 times a second, so we only need 10
    amps at 1/5th of that, or 60 volts I think. In addition we need to
    draw up to 100 1/4 inch high characters at 60Hz, so applying the fudge
    factors, hmmmm.... 60 volts may be plenty. Well within the capability
    of a HV op amp with a couple booster transistors. And a heat sink And
    a fan.

    Crazy project!
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