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TV for oscilloscope

Discussion in 'Electronic Design' started by Abstract Dissonance, Jan 27, 2006.

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  1. Anyone have any experience converting a TV into an oscilloscope?

    I have found the deflection coils and have tested them but I'm not really
    sure where to go from there. I know that for the horizontal I need a
    trigger(a sawtooth) but I'm not sure at what currents and voltages I
    need(with my variac hooked directly to it about 3 volts gives almost full
    deflection(which I measured about 0.05A).. but I'm not sure if thats right.)


    Is it simply a matter of putting an variable gain opamp on the vertical and
    a variable sawtooth on the horizontal? Seems like it would be kinda hard to
    measure frequency that way since I'd have to convert the resistance of the
    sawtooth into frequency.

    Any ideas?

    Thanks,
    AD
     
  2. Nope, but I've converted an oscilloscope into a TV ... anyway, if you
    mean the old-style CRT TV's, the horizontal deflection circuit is
    resonant at the horizontal line rate, so the only way to make the
    display work for a variable horizontal scan rate - the variable
    sawtooth - like an analog scope, is to store the signal in memory and
    do a scan-rate conversion. The vertical deflection will also have
    limited bandwidth. Here is a discussion on the subject:

    http://www.fieldlines.com/story/2005/12/25/162750/50

    Frank Raffaeli
    http://www.aomwireless.com/
     
  3. GEO

    GEO Guest

    If it is possible to use a circuit like the oscilloscope discussed in
    http://www.fpga4fun.com/digitalscope.html
    along with a special video driver logic ( haven't seen that yet, but
    must be possible) between video input of TV and the logic from
    digitalscope, there shouldn't be any problem for using a TV as a
    digital scope. Or a digital LA, for that matter. Only thing someone has
    to design a video output device, which takes in
    something like RAM data from digitalscope and
    generates the video output in visible format. Digital scope from
    fpga4fun works with sw and PC. Something like the old Atari interface
    is what is required. Resolution won't be good enough though, but it is
    still possible.
     
  4. Deefoo

    Deefoo Guest

    That's old-skool electronics. I have seen several designs for such projects
    in electronics magazines from the 60s and 70s. I don't remember how they did
    it, but I seem to remember that it was quite easy and involved rotating the
    deflection coils 90 degrees. (Why? I ask myself now some 30 years after
    reading those magazines. Maybe I'm mixing things up? Maybe they used
    vertical for time?) Search your local library and antique stores.

    I'm sorry to say that it was one of the big errors in my life to throw away
    a large collection (5 meters) of electronics magazines that I considered
    obsolete at the time. I've missed them often since :eek:(

    --DF
     
  5. I know that this is not really a detailed help, but in the 80s, we
    simply stuck 1:1 transformers between the speaker outputs of our
    amplifiers and the deflection coils and got a nice vibrating diagonal
    line that would sometimes even look remotely like a circle, if the musik
    we played had good stereo... . Ooooh!
     
  6. Tim Shoppa

    Tim Shoppa Guest

    Yeah, I've done it with the $20 5" B/W TV's that you can buy at
    discount and drugstores.

    I generally use them as X/Y scopes (no horizontal timebase to speak of)
    but I have a few hints:
    With modern electronics generating the sawtooth is trivial.

    Remember that a real scope has blanking. If your TV has a baseband
    input jack (the cheapest ones usually don't but sometimes they do!)
    then just putting the right DC voltage in will blank the video driver.

    At low frequencies (hundreds of Hz) that's pretty good. At higher
    frequencies you will get distortion because you're driving the
    inductive coils. What you really want is a current sense resistor, and
    a feedback loop to adjust the drive so that the current (not the
    voltage) is proportional to the signals.
    Well, if you wanted you could phase-lock the horizontal sweep to a
    crystal. But remember that for typical cheap scopes (I'm thinking of
    the ones that cost $30-$50 in the Heathkit or Eico catalog in the
    50's), the frequency accuracy was entirely dependent on the RC time
    constants in the sweep circuit.

    A well-calibrated analog scope typically has a timebase accuracy of a
    percent or so. The cheapies were probably 10% on a good day. Compare
    that to a frequency counter with a crystal timebase (fifty cents) which
    gets accuracies of 0.002% without much difficulty.

    The real value of a scope is not so much in measuring amplitudes or
    times to a fraction of a percent (although you can do that with a
    well-calibrated professional one), it's in being able to see the
    waveform. Don't obsess too much with precision yet, just work on
    getting the picture!

    I encourage you to work on using your new "TV-scope" in X-Y mode, and
    learn about Lissajous figures for measuring frequency.

    The timebase and blanking can come later... there's a lot of fun to be
    had in X-Y mode!

    Tim.
     
  7. Tim Shoppa

    Tim Shoppa Guest

    Typically the vertical deflection coils have substantially larger
    sensitivity, meaning more turns, meaning more inductance than the
    horizontal coils. That's OK in TV-scan use because vertical sweep is
    60Hz (well, with interlace and resultant sawtooth and blanking etc. you
    do need more than 60Hz bandwidth).

    Typically you want more frequency response in the vertical channel of a
    scope than the horizontal channel. Although there's nothing set in
    stone saying what response has to be on what axis, nor whether time has
    to be horizontal or vertical.

    The modern cheapie 5" B/W TV's you find at drugstores for $20 do not
    have as many turns on the vertical deflection coil than the hulking
    sets of the 60's/70's, but even if you wind custom coils for both
    horizontal and vertical you're going to have to struggle to get
    response much above the audio range.

    Tim.
     
  8. Rich Grise

    Rich Grise Guest

    [crossposted to sci.electronics.design,sci.electronics.basics]
    Conceptually, yes. In practice, probably not.
    As others have mentioned, the deflection is proportional to the current
    through the coils.

    Just as a matter of perspective, I've worked on video game monitors, and
    certain games, notably Asteroids and Star Wars, use a "vector display",
    i.e., they paint the image by drawing it with the electron beam - it's
    essentially a Lissajous diagram of some really crazy-looking waveforms! :)

    But the deflection coils are dramatically different from the coils
    on a raster scan display. (which is the same as an ordinary TV or
    computer monitor). The coils have significantly fewer turns, which means
    way less inductance, but you need much heftier deflection amplifiers,
    because you have to drive some pretty substantial current through them.

    So, you could do a very rudimentary one, but I wouldn't expect a large
    frequency range or much precision - well, as much precision as you build
    into your deflection drivers. :)

    Good Luck!
    Rich
     
  9. I don't understand why? I don't have to use the circuitry of the TV to
    generate the sweeps?
    If it has something to do with the coils themselfs not being able to handle
    the range then why can't I just rewrap them?
    Jon
     
  10. Cool, I might try that. I thought about using some A/D and my computer to
    get more bandwidth but since I have no oscilloscope to start with I wanted
    something something to help get me on my way.

    Thanks,
    Jon
     
  11. I've seen that mentioned too. One site specifically says to do this. I think
    now, after someone mentioned it, it has to do with the "light" deflection
    coils or something.
    Heh, I'll take note of that so I don't make the same mistake ;)

    Thanks,
    Jon
     
  12. This TV is recent(maybe 99). It wasn't being used so I tore it apart.


    What do you mean struggle? What would be the potential problems in wiring
    the coils myself(as they seem to be the issue)?

    Do you happen to know about how many turns they use on the coils?

    Thanks,
    Jon
     

  13. yeah, for the most part... I still have not done one but it seems easy
    enough ;)

    Blanking? Only jacks the TV has is a coax and the video in/Audio L and R
    in(whatever its called... the RCA Red, yellow and white type of connects).
    Sure, its not the perecision I want at this point but simply to see whats
    going on. Obviously since I don't have a grid on there then I can't make
    measurements of any kind(except maybe guesses). But I would like to atleast
    be able to see waveforms and such... right now I can't do much since I need
    to figure out how to amplify the signals properly. I guess I have to work
    up slow though.

    but I have to have two signals to do that and I have to have some signals to
    play around with. I suppose I could use my soundcard and an opamp to "play"
    signals through it(and even generate a sawtooth for retriggering.

    Do you happen to know the specifics of driving the coils properly? like
    about how much voltage and current are needed in general(amps, mA's, etc..).

    Could I just make a voltage controlled variable current source and use my
    signals to drive it?

    Thanks,
    Jon
     
  14. heh, which always seems to be the case ;/
    ok. I have the equation in some book but I have failed to look it up.
    For now I'm satisfied with just being able to see anything cool ;) and
    understanding whats going on. I know the basics that I learned in physics
    about 7 years ago but ofcourse we never actually applied it... its cool to
    be able to see whats going on and where those equations came from(or why).

    Right now when I look at the TV I see only about 1% that is familiar so it
    kinda scares me when I start fiddling around in there. (it also has a build
    in VHS player so I assume half the electronics is for that).

    I need some TV electronics book that shows all the innards of a TV and how
    they works then I'd be more comfortable. You happen to know if theres
    anything like this out?
    Thanks,
    Jon
     
  15. Guest


    I've not done it, but have a few things to add.

    PC scopes are free now and do 20Hz-20kHz, and some sound cards will go
    above 20kHz. So theres no point building a scope for that frequency
    range.

    Even the most basic CRT scope can do a few MHz if you use the beam
    modulation to indicate amplitude instead of the scan coils. Let the
    tube scan as normal, couple your signal to the cathode or grid.
    Coupling some signal to the scan oscs might be able to sync them,
    havent tried that.

    An as-is TV gives you 2 timebases to play with: 50Hz and 16kHz. By
    putting signal into the other unused scan coil, you can thus observe
    frequencies of upto around 10x those, ie 500Hz on one axis, 160kHz on
    the other. Thats if you can drive the frame scan coils at 160kHz, which
    I'm not so convinced about.

    You've got inductive current driven scanning, so feeding V into the
    scan coils will give plenty of distortion. As someone mentioned,
    current drive them.

    A computer monitor with faster scan frequencies would give you higher
    frequency limits than a tv plus original scan yoke. A colour tube also
    gives you 3 colour channels to play with. You might want to plug the
    monitor into a computer to get it to play ball without a bundle of
    mods.

    In principle I suppose one could run 2 of the colour channels as a pc
    monitor, and use the 3rd to display waveform by brightness modulation.

    To take frequency limit even higher, use envelope detection. This is
    just a diode, cap, resistor on the input. Thus the tube displays the
    envelope of the waveform, not the wave itself. This can be used to
    extend the working range of a scope greatly, though of course with less
    information displayed.

    While this sort of kit is pretty much obsolete here, it is still of
    value in countries where buying a nice fast scope is out of the
    question.


    NT
     
  16. Rich Grise

    Rich Grise Guest

    This brought to mind the time that I looked at ordinary NTSC color video
    on an actual scope. It was almost spooky. I could turn the sweep way up,
    and see the color burst itself, and then crank the sweep down and see
    the scan lines - averaged over 1/60 second, of course, and since this
    was off-air (I was scoping my TV), the image was continually changing.
    The pattern on the scope screen looked almost like a "top view" of the
    picture, like a graph of light/dark, averaged. Then I cranked the sweep
    down even more, sunc on the vertical, and got a "side view".

    It was way cool! %-}

    Cheers!
    Rich
     
  17. Rich Grise

    Rich Grise Guest

    Video in == baseband. The blanking goes in there. With TV video, it's in
    the signal itself - it just goes black for the retrace.

    I don't know if these are DC coupled.

    It'd be interesting to see what happens when you turn your channel
    selector to "AUX" or whatever - that's what my set calls the yellow
    video in, but when I turn mine there, if there isn't a video signal, it
    blanks the whole screen. So you'd need a sync generator of some kind,
    but ou _should_ still be able to use it for intensity modulation whenever
    you get your deflection drivers going.
    I don't know what this means. Can you rephrase it?

    Thanks!
    Rich
     
  18. Sjouke Burry

    Sjouke Burry Guest

    Hmm... Saved the article in 1973 (Have no scanner and is not
    in english),Title:Audioscoop.Coil resistance between 1.5 and
    5 ohm.
    They advice to put 10-15 ohm in series,to get a more linear
    response.
    You have to disconnect the line sweepback suppresion pulses,
    or you will see a chopped wave.
    I would advice to put your own voltage to that point, or
    you will burnout the screen very quickly(or turn brightness
    back to zero if possible).
    Afer that, connect the coils to your stereo amplifier.
    Other articles:
    1962: Semi conducting diamont.
    1969: Pneumatic logic gates(FF,OR,AND,halfadder,shiftreg.)
    1970: Color tv with rgb lasers and mechanical rotating mirrors.
    1970: Physical parameters measurments with Quartz crystals.
    1966: Fuel cells.
    etc. etc.......
    Anybody have a nice operational timemachine??
     
  19. Rich Grise

    Rich Grise Guest

    Mostly, getting something designed that will drive the yoke windings,
    which are quite inductive, to the frequencies you want to play with. You
    can't use the existing deflection amps, because they're not designed to do
    an arbitrary pattern or Lissajous or anything - they're designed to make a
    raster.
    Well, look at the existing yoke. Visualize what its magnetic field must
    look like while it's whipping the electron beam around.

    Wind some coils, get a current-limited power supply, and poke around
    a bit!
    Hundreds, maybe thousands - except in the vector displays I've seen,
    maybe dozens to a few hundred.

    Good Luck!
    Rich
     

  20. Its a total waste of time unless you can design and build make own
    deflection yoke. The existing yoke is the limiting factor, and has been
    since the first modification plans for old tube type B&W TVs were
    published 45 years ago. Sure, you can rotate the yoke 90 degrees to
    improve the vertical response and remove the resonant capacitors to gain
    a slight bit more, but you'll need a second original yoke to keep the
    flames inside the flyback transformer. Bottom line, you need
    electrostatic deflection, not electromagnetic.

    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
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