tv vs. oscilloscope

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.

 

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

 

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

 

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

 

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

Don

 

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.

 

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

 

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