project: connect stereo audio out to monitor deflection yoke ?

[[email protected]]

I am trying to find an "artistic" use for a few old computer monitors
(one 9" monochrome and one 14" colour, both svga) that I have lying
around, and am in need of suggestions from those with more experience
of this.

The idea is to perform one of the experiments listed towards the end of
sam's Monitor FAQ, in the section on "turning a monitor into an
oscilloscope". NO, I don't want to do that, but rather make an audio
display, driving the yoke with a stereo amp to move the beam around the
screen.

However, before I disconnect anything from the yoke to attach the
stereo, presumably I would have to connect some sort of dummy loads on
the horizontal and/or vertical lines from the mainboard, as I don't
want to risk running (and blowing up) the monitor with the yoke
effectively disconnected from the scan circuits (wired to the stereo
spkr outputs instead)?

I would be interested to hear from anyone who has successfully hitched
an audio source to the scan coils in this way. Or equally, someone who
has been there, attempted that and found it a waste of time and effort!

Any suggestions?

regards, Ben

 

[CJT]

I am trying to find an "artistic" use for a few old computer monitors
(one 9" monochrome and one 14" colour, both svga) that I have lying
around, and am in need of suggestions from those with more experience
of this.

The idea is to perform one of the experiments listed towards the end of
sam's Monitor FAQ, in the section on "turning a monitor into an
oscilloscope". NO, I don't want to do that, but rather make an audio
display, driving the yoke with a stereo amp to move the beam around the
screen.

However, before I disconnect anything from the yoke to attach the
stereo, presumably I would have to connect some sort of dummy loads on
the horizontal and/or vertical lines from the mainboard, as I don't
want to risk running (and blowing up) the monitor with the yoke
effectively disconnected from the scan circuits (wired to the stereo
spkr outputs instead)?

I would be interested to hear from anyone who has successfully hitched
an audio source to the scan coils in this way. Or equally, someone who
has been there, attempted that and found it a waste of time and effort!

Any suggestions?

regards, Ben

I did it in the psychedelic 60's. It was a big hit in our dorm room.
I used the (tube) vertical amp that was already present, with a
duplicate of it hooked to the horizontal portion of the yoke, and a
spare yoke away from the tube hooked into the original horizontal
circuit so we'd have HV.

We burned the phosphor mid-CRT pretty badly, but the TV was a throwaway
anyway, so it didn't matter.

Of course, the TV was B/W back then.

I used a couple of op-amps to drive the two tube power amps.

Thanks for the memories. <g>

BTW, I'm not convinced a regular stereo amp is suitable to drive
a yoke.

 

[Matt J. McCullar]

Interesting idea, but video monitors aren't designed to handle that sort of
thing. Your typical vertical deflection circuit is designed to work at 60
Hz, and the horizontal deflection circuit wants to run at 15.7 kHz (or even
twice that frequency, if it's a VGA monitor). Going too far beyond those
boundaries creates all kinds of hellacious noise, and/or fried components.

It would probably be easier to use a cheap X/Y oscilloscope instead. They
are designed to handle just about anything, and you can plug your stereo
outputs directly into the 'scope inputs with no trouble at all. Any
adjustments for optimum picture can be done right there from the front panel
of the 'scope.

 

[Harvey]

Interesting idea, but video monitors aren't designed to handle that sort
of
thing. Your typical vertical deflection circuit is designed to work at 60
Hz, and the horizontal deflection circuit wants to run at 15.7 kHz (or
even
twice that frequency, if it's a VGA monitor). Going too far beyond those
boundaries creates all kinds of hellacious noise, and/or fried components.

It would probably be easier to use a cheap X/Y oscilloscope instead. They
are designed to handle just about anything, and you can plug your stereo
outputs directly into the 'scope inputs with no trouble at all. Any
adjustments for optimum picture can be done right there from the front
panel
of the 'scope.

or use a cheap PC with sound card and something like
http://polly.phys.msu.su/~zeld/oscill.html

 

[CJT]

Interesting idea, but video monitors aren't designed to handle that sort of
thing. Your typical vertical deflection circuit is designed to work at 60
Hz, and the horizontal deflection circuit wants to run at 15.7 kHz (or even
twice that frequency, if it's a VGA monitor). Going too far beyond those
boundaries creates all kinds of hellacious noise, and/or fried components.

It would probably be easier to use a cheap X/Y oscilloscope instead. They
are designed to handle just about anything, and you can plug your stereo
outputs directly into the 'scope inputs with no trouble at all. Any
adjustments for optimum picture can be done right there from the front panel
of the 'scope.

Scopes (especially cheap ones) are generally electrostatic deflection,
so have smaller screens.

 

[NSM]

I am trying to find an "artistic" use for a few old computer monitors

oscilloscope". NO, I don't want to do that, but rather make an audio
display, driving the yoke with a stereo amp to move the beam around the

You can't drive a yoke with audio and it makes no sense. You could just
figure out the connections and hook the L and R to two of the color inputs
but it'd be kind of crappy. I'd look for a circuit to do this from maybe
Elektor or Silicon Chip (two magazines).

 

[CJT]

You can't drive a yoke with audio and it makes no sense.

Ever hear of Lissajous figures?

You could just

 

[James Sweet]

I am trying to find an "artistic" use for a few old computer monitors
(one 9" monochrome and one 14" colour, both svga) that I have lying
around, and am in need of suggestions from those with more experience
of this.

The idea is to perform one of the experiments listed towards the end of
sam's Monitor FAQ, in the section on "turning a monitor into an
oscilloscope". NO, I don't want to do that, but rather make an audio
display, driving the yoke with a stereo amp to move the beam around the
screen.

However, before I disconnect anything from the yoke to attach the
stereo, presumably I would have to connect some sort of dummy loads on
the horizontal and/or vertical lines from the mainboard, as I don't
want to risk running (and blowing up) the monitor with the yoke
effectively disconnected from the scan circuits (wired to the stereo
spkr outputs instead)?

I would be interested to hear from anyone who has successfully hitched
an audio source to the scan coils in this way. Or equally, someone who
has been there, attempted that and found it a waste of time and effort!

Any suggestions?

regards, Ben

I did this years ago with an old 9" mono computer monitor, had to put an
inductor where the horizontal yoke connected and a power resistor in series
with the H yoke to match the impedance to what a stereo amp is expecting. It
was entertaining for a little while but it got boring fast, as well as it
burned a large hole in the phosphor in the middle of the screen, not that it
mattered with all the burn that was already on it.

 

[NSM]

Ever hear of Lissajous figures?

Yup. And you can't drive a yoke with an 8 ohm speaker output. It takes some
serious amps. And you can't vary the frequency by very much either.

 

[JW]

Yup. And you can't drive a yoke with an 8 ohm speaker output. It takes some
serious amps. And you can't vary the frequency by very much either.

FWIW, I did exactly that about 20 years ago on a color RCA tube chassis
that had a deflection problem. Disconnected the yoke from the chassis and
used the left channel to drive the horizontal and the right channel to
drive the vertical. The patterns would fill the screen at about 3/4 volume
using a 50W per channel receiver. It was quite the hit at parties.

 

[[email protected]]

Thanks for the responses.

Seems like the older sets were far more forgiving when it comes to
disconnecting the yoke.

I have already tried using the vertical of the 9" mono monitor, which
produced a result, but as JW posted, you need a fair bit of volume to
do it.

I haven't yet tried the horizontal. Will look at the option of an
inductor and series load. and also look out for the magazine circuits
NSM suggested for the colour.-

will post with results at a later stage!

regards, Ben

 

[Ol' Duffer]

We burned the phosphor mid-CRT pretty badly, but the TV was a
throwaway anyway, so it didn't matter.

You could detect the incoming signals and feed them to the
CRT board to modulate beam intensity.

BTW, I'm not convinced a regular stereo amp is suitable to drive
a yoke.

At least not without some extra work. Yoke windings are
predominately inductive and intended to be current driven
(implying high impedance source), while stero amps are
designed to be low impedance voltage sources driving mostly
resistive loads. Add to that the fact that the two windings
are usually optimized for different signal voltage levels
(12~40 Volts vertical and 100~300 Volts horizontal ballpark).
You could try 70 Volt PA speaker transformers with series
resistors to match/step up the amp to yoke.

 

[NSM]

Thanks for the responses.

Seems like the older sets were far more forgiving when it comes to
disconnecting the yoke.

I have already tried using the vertical of the 9" mono monitor, which
produced a result, but as JW posted, you need a fair bit of volume to
do it.

I haven't yet tried the horizontal. Will look at the option of an
inductor and series load. and also look out for the magazine circuits
NSM suggested for the colour.-

will post with results at a later stage!

I recall the Radio Shack Color Computer had an optional ROMPak which did
various displays like those in Media Player software when you hooked it up
to your stereo. They even claimed you could tune your room with it - a bit
dubious I think.

 

[CJT]

Yup. And you can't drive a yoke with an 8 ohm speaker output. It takes some
serious amps. And you can't vary the frequency by very much either.

That's going to depend on the specifics. I agree that an audio amp
might not be ideal. But the other poster says he did it, and I believe
him.

 

[CJT]

You could detect the incoming signals and feed them to the
CRT board to modulate beam intensity.

Yes, I played around with that a bit, but the desired intensity
is a function of beam X-Y velocity, and at the time it was easier
to just let it burn.

 

[JURB6006]

You'll need a spare yoke and stepup transformers to match impedance. Set the
original yoke aside still hooked up and put the spare on the tube. To use a
computer monitor you may need to have an old PC just to get it to turn on.

In fact, to set the purity you'll need to feed the monitor all one color from a
PC, then you can interrupt the video signal and start playing around with that
too, use a low value cap fo example to feed the high frequencies to modulate
the beams.

(yes, as you suspect I have done it, on an old 21" round color TV)

Have fun.

JURB