# TV for oscilloscope

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

1. ### Abstract DissonanceGuest

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,

2. ### Frank RaffaeliGuest

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. ### GEOGuest

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. ### DeefooGuest

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 (

--DF

5. ### Matthias MelcherGuest

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 ShoppaGuest

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

Tim.

7. ### Tim ShoppaGuest

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 GriseGuest

[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

Good Luck!
Rich

9. ### Abstract DissonanceGuest

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. ### Abstract DissonanceGuest

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. ### Abstract DissonanceGuest

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. ### Abstract DissonanceGuest

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. ### Abstract DissonanceGuest

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. ### Abstract DissonanceGuest

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

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 GriseGuest

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 GriseGuest

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 BurryGuest

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.
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 GriseGuest

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. ### Michael A. TerrellGuest

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