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tv vs. oscilloscope

Discussion in 'Misc Electronics' started by Allan Adler, Jun 17, 2006.

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  1. Allan Adler

    Allan Adler Guest

    A long time ago I asked about what one would have to do to convert
    a TV into an oscilloscope. I can't find my original posting on
    dejanews but my recollection is that people told me one can't really
    do that with a TV. I was thinking about it recently and realized that
    I didn't really understand the reasons which, according to my vague
    recollection, had something to do with the electron beams in oscilloscope
    CRTs being controlled by magnetic fields while those in TV CRTs being
    controlled by electric fields. For one thing, I'm not sure whether this
    refers to a difference in the coils used or whether it refers to a difference
    in the construction of the CRTs themselves.

    One idea I had for converting a TV into a really lousy oscilloscope is
    based on something I read once in an electronics magazine in the late
    1970's and tried out (this isn't safe, e.g. once I did it and inadvertently
    shorted the whole thing out with a rather large bang but no explosion):
    (1) Open the back of the (b&w) TV and unplug the CRT tube.
    (2) Carefully slip the yoke off the tube without cutting any wires.
    (3) Take another TV and clip the wires to its yoke and remove it from
    the TV. Connect its wires to another signal source, such as the
    speaker wires from a record player. Then slip this coil onto the
    yoke of the first tv.
    (4) Plug the CRT tube from the first TV back in.

    Then one turns on the first TV and the record player. When I played
    Rimsky-Korsakov's Scheherazade the white dot in the center of the screen
    spiraled around the screen in interesting ways.

    I think that if one can do that, one can at least do some of the simpler
    things one can do with an oscilloscope by building various circuits and
    connecting them to the lead wires to the coil of the 2nd TV, or maybe by
    programming a computer to supply signals to those leads.

    Since one doesn't have any control over the circuits of the first TV,
    one can't do stuff like turn the electron beam on and off, to keep
    the dot from leaving a trail if one wants it to jump around to various
    places on the screen.
     
  2. Guest

    No you cant do that. There are too many reasons to list, just forget
    the idea.
     
  3. Allan Adler

    Allan Adler Guest

    Since posting this, I did a google search for:
    converting tv oscilloscope
    and found some information, including a description of what one person
    managed to build at: http://www.dansworkshop.com
     
  4. Don Bowey

    Don Bowey Guest

    It is fairly simple to cause interesting patterns to be displayed on a TV
    screen, but it is quite another thing to obtain an oscilloscope type
    display. In the 60s, a lot of circuit diagrams were published for
    displaying games like Pong, and music.

    If you have studied TV at all, you should be aware the TV uses a raster
    scan. Each horizontal line is 62.5 microseconds in duration. If I want to
    place a vertical line in about the center of the screen, on line 250 I will
    put a brief (1 uS) voltage onto the cathode of the picture tube at about 32
    microseconds into the horizontal scan. I will repeat that on subsequent
    lines for as many lines as it takes to make the line as tall as I want it.
    I may use 20 horizontal lines to describe my vertical line. My vertical
    line Is actually a result of *intensity modulating* the cathode of the
    picture tube. The vertical stage of the TV just moves the horizontal lines
    vertically.

    In a basic oscilloscope, I can display a vertical line during a single
    horizontal sweep, because a voltage applied to the vertical channel drives
    the beam up or down depending on the polarity of the voltage. If I set the
    Horizontal sweep to 62.5 uS, and put a 1 uS pulse into the vertical channel,
    I will see the "line" but more importantly I will see it isn't a line at
    all, but a rectangular pulse that goes vertical and stays there for 1 uS,
    and then falls back to the baseline.


    With this basic difference between a scope and a TV, you should see not to
    expect too much from a converted TV.

    Don
     
  5. Don Bowey

    Don Bowey Guest

    Yes, you can do that, it's just complex.

    Don
     
  6. Sjouke Burry

    Sjouke Burry Guest

    Still have the articles:
    RADIO ELEKTRONICA 1973-11 and 12.

    I don t have a scanner,and the language is Dutch.
    For eye candy connect left and right loudspeaker
    output of a stereo amp to the coils.(Disconnect
    coils from main board)
    To avoid burn in you should make sure that you
    always apply signal,and adjust to a rather low
    brightness.
    The coil resistance is vert:~40 ohm, hor: ~4 ohm.
    The article advises to use a spare set of coils,
    connected to the mainboard to ensure proper
    operation of the TV electronics.

    For use as a low frequency scope you have to
    build current output stages,and your frequency
    range is limited.


    Also you can combine a comparator and a video
    source,compare the input signal with a 64 usec
    sawtooth,and inject a small .2 usec pulse or the
    comparator output itself into the video signal.
    In this case no mods to the tv are needed, but
    just a video input(scart or s.video). the
    bandwidth can be rather high as in a sample
    scope.
    If you use a number of comparators/sawtoothes??
    you can have a nice multichannel display.
    A few high gain sensors, and you have your
    own earthquake display system.

    Also have a nice live insurance policy :) ;)

    Have fun and be careful.
     
  7. clfe

    clfe Guest

    I rcall in my days before I could "afford" a scope, articles in magazines
    regarding converting a TV into an O-Scope. I may even still have such
    articles. Question now is - WHY? Half decent and decent scopes are had by
    the dozen on E-Bay or most any Hamfest. Many are cheap enough that if you
    get a year out of it, you've gotten your monies worth. I've picked up some
    really decent scopes at liquidations for $50 a pop.

    clfe
     
  8. Yukio YANO

    Yukio YANO Guest

    There are a number of sites on Google that supply the Software to drive
    a sound card that uses the computer monitor as an output for a Audio
    Frequency range oscilloscope.

    That is unless you really enjoy Smoke and Lightning.

    You could avoid most of the Lightning by using a LCD Display .
    If you did that, you might as well consider going the Sound Card route.

    If all else fails I would sell you an ancient Heathkit IO12
    oscilloscope. for $US 25.00 + S/H (~$25.00) from Saskatchewan, Canada.

    There are any number of used scopes for sale on Ebay for Under $100, so
    building your own is a No-Brainer, especially when you consider you
    really need a Scope to troubleshoot and test your creation.

    Yukio YANO
     
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