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television to oscilloscope

Discussion in 'Electronic Repair' started by m kinsler, Jun 23, 2007.

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  1. m kinsler

    m kinsler Guest

    I'd like to try building an oscilloscope from an old CRT monitor or TV
    set. The purpose would be to display audio waveforms at one of the
    science museums I work with; there's no calibration necessary.

    I've tried the various sound-card based 'scope programs available for
    the PC, and none of them show sufficient detail; I really believe I'll
    need an analog device to show things like the difference in the audio
    waveforms of different musical instruments. The big screen of a TV
    set would be helpful for demonstrations.

    I understand that this is a totally novel concept, and that Google
    doesn't yield a single thing on the subject except for the twenty-six
    thousand articles listed under "TV oscilloscope."

    But I must say that those plans seem either oversimplified or more
    theoretical than practical. The problem I keep concerning myself with
    is that the deflection yoke of a CRT is, or at used to be, part of the
    high-voltage circuit.

    Additionally, we run into the problem that a magnetic deflection coil
    is an inductance, and thus won't accurately show, say, a waveform
    that's not pretty darned sinusoidal. I would imagine that any corners
    on a waveform sent into a vertical deflection coil would be converted
    into spikes.

    So I'm lazy. Has anyone actually done this sort of thing and actually
    had it work to any degree? Thanks.

    M Kinsler
  2. James Sweet

    James Sweet Guest

    It can be done, but the performance is pathetic compared to that you can get
    from a $10 Eico or Heathkit oscilloscope. Old basic scopes are SO cheap now,
    why bother trying to convert a TV?
  3. N Cook

    N Cook Guest

    When I worked in a college we did this and also a TV diplay - audio spectrum
    A lot of the functionality of a scope is the front controls of mS/cm ,
    amplitude control for various inputs. Cross mix/gate the sweep out and an
    internal 'y' signal of a basic scope with a sync generator/ramp for feeding
    into a TV / projector TV. For spectrum analyser a series of bandbass filters
    with S&H outputs polled across the bands. I remember how saw-toothy a bowed
    violin, stick-slip, waveform looks and graphically showing the difference
    between white noise and pink noise as a spectrum display.
  4. I built one of these about 20 years ago based on a kit in Electronics
    Australia. Basically a complete time wasting exercise. With a sound card
    and a PC and one of several sound analyzing pieces of software you get
    an out of this world result, compared to even old sound/vibration
    analysis hardware from HP or other makers. If you need a TV screen just
    to show the class then use a video card with TV capability or one of
    them modern plasma/LCD flat screens with VGA in.

    regards Andrew
  5. Right. So you need either to substitute an inductor to keep the HV
    happy, or a separate yoke.
    So you turn the yoke around and use the vertical for the timebase.
    As others have noted, this is certainly not worth doing to obtain
    a useful instrument.

    For a science museum display, what would be the point? Size or just
    showing that it can be done?

    If you need to cover a large area, just get several old scopes. :)

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  6. Bob Urz

    Bob Urz Guest

    As others have said, the direct conversion is futile.
    Now, if you a scavenger on the cheap.

    1) old cheap O scope
    1) old cheap large TV.
    1) old camcorder with tape section broken so you can get it cheap.


    Put signal into the scope as usual
    point video camera at the scope.
    (make a hood to shield outside light)
    use video camera output to feed the tv.

  7. J. T. Laurie

    J. T. Laurie Guest

    that's what I'm trying to do though I'm trying to do it just for the hell of
  8. Jim Yanik

    Jim Yanik Guest

    You could pick up a used TEK T922/932/935 or TEK 442(T935 in a rackmount!)
    for well under $100 on Ebay. That's 15Mhz-35Mhz,simple circuitry,easy to
    repair,no TEK-made ICs.
    For that you get calibrated graticule,switchable calibrated gain,reasonable
    triggered sweep.
  9. CJT

    CJT Guest

    I did it in 1968 +/- as something to liven up parties. I found two TVs
    with the same chassis and added the yoke and vertical section from the
    second one to the first. I hooked the original horizontal section to
    the horizontal windings of the extra yoke so I'd have HV. Then I hooked
    the extra vertical section to the part of the yoke that was originally
    part of the HV section. In my particular case, the horizontal and
    vertical sections of the yokes weren't terribly different, which helped.

    I hope that makes sense. In my application, I didn't need a sweep
    generator -- I was using it as an XY display of the two stereo channels
    against each other. And frequency response wasn't critical. It made a
    pretty psychedelic display.

    Eventually (maybe fairly quickly) the center of the tube will burn
    unless you add some circuitry to blank the beam when it's centered.

    By the way, in 1968 it was black-and-white. You might run afoul of
    the safety circuitry in a color set.

    One question that comes to mind is why you don't just use a real
  10. CJT

    CJT Guest

    You better find a scope with a high persistence phosphor if you want
    to try this way.
  11. Bob Urz

    Bob Urz Guest

    CHeap alternative method #2

    Get cheap old windows 98 machine for next to nothing
    get cheap old 17" VGA monitor or such
    get shareware oscilloscope software on the net.

    Then use the soundcard on the computer to display waveforms in
    the audio range.

  12. m kinsler

    m kinsler Guest

    One question that comes to mind is why you don't just use a real
    Mostly because the screens are quite small.
    I've tried that, and the waveforms are just not detailed enough to be
    very useful. One looks a lot like another, and the response is far
    too slow. I may be using the software incorrectly, but I've had
    little luck over several tries.

    M Kinsler
  13. James Sweet

    James Sweet Guest

    The problem is likely lack of triggered sweep, something you really need a
    real scope for.
  14. Guest

    i have managed to convert an old PC monitor into a very crude
    oscilloscope. it does show quite a good waveform but square is a bit
    pointy. but otherwise is quite good for an old 640x480 15inch CRT

    depending on the intended use it may be sufficient. all you need to do
    is rotate the yoke coils 90° either way so that the vertical
    deflection coils are now horizontal. disconnect the now vertical
    (horizontal) coils and attach to either an amplifier (adding resistors
    to match output load impedance of the amp) or direct input if the
    voltage is high enough and will handle inductive load resistances
    below 1 ohm or so. also the CRT control board may not like having no
    horizontal coil so if you can attach an inductive load around the same
    resistance as the deflection coil (i used a 12volt car battery
    transformer secondary winding) to the old coil connectors should make
    it turn on. you'll need some kind of oscillator (like a NE555 timer)
    to produce a triangle/saw tooth wave into the (now) horizontal coils
    to adjust the timebase frequency. or find a way of adjusting the said
    frequency on the board and depending on what needed adjusting either
    put a rotary switch with various values of the component or an
    adjustable resistor. I'm guessing it will either be a resistor
    capacitor oscillator or something else.
  15. bz

    bz Guest

    You might try a different approach, there are A to D capture devices that
    will turn any computer into a scope.

    Cheap monitor, cheap computer, A to D capture device and you have a scope
    that will show detail and can be seen by the class.

    You can also feed the video from the computer into a modulator and put it
    onto your large screen or projection tv.

    bz 73 de N5BZ k

    please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
    infinite set.

    remove ch100-5 to avoid spam trap
  16. Marra

    Marra Guest

    Buy a cheap scope.
  17. Chris Jones

    Chris Jones Guest

    The Telequipment D83 has a nice (relatively) big screen.

  18. z

    z Guest

    Friend of mine did it in high school for the science fair. Worked OK,
    but very limited as to upper frequency. I have no idea what the actual
    modifications involved were.
  19. Guest

    What does that mean? We can only help if you explain the problem.
    are the waveforms in fact similar, or quite different?
    Please explain. PC sluggish, scope bandwidth too low, what?
  20. Guest

    While I'm here I think you can rule out a converted tv tube on safety
    grounds. With such basic scopes there is nothing to stop deflection
    going off screen and heating a point of glass tll it softens and goes

    FWIW correct waveform was obtained by using current drive, but
    bandwidth is still lmiited. They were just crude tools for tinkerers
    that couldnt possibly afford a real scope.

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