TV for oscilloscope

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You'll probably need a free soundcard oscilloscope like

to work your way through converting a TV.

Some repetitive waveforms can be converted down to 300-20,000 Hz using a receiver with a BFO (beat-frequency oscillator).

Happy learning,

Steven

TV tube safety has come a long way from then, to such a point that even if the _front_ of a modern tube is broken its unlikely to im/explode in a dangerous way. More to the point, since the risk is not zero, the scope builder should be aware of what causes neck overheating, how it can be a problem, and avoid causing the problem in the first place. When deflection is excessive in a tv, or when the scan coils are too far back, the e beam hits the neck of the tube and heats it up.

When using a tv tube as a scope, the total light output requirement is a small fraction of that of it in tv mode, so the beam current will be turned way down (if you want the tube to survive). This reduces the threat. The OP should avoid using excessive brightness, and avoid overscanning the tube. If your waveform goes off the edge of the screen, turn the gain down or disconnect it.

As Michael says, if you ignore these points carelessly you could be in trouble.

NT

Variac outputs are usually connected direct to the mains, so its live whatever the V_out setting. .

NT

Variac outputs are usually connected direct to the mains, so its live whatever the V_out setting. Look up 'autotransformer' .

NT

thats the one to use. Just put a buffer amp in front of your sound card, as there is no input protection.

NT

The metal band around the front of the tube is to hold the mounting brackets, and equalize the stresses on the tube as it heats and cools. What does that have to do with the fragile neck?

You need to study how a CRT works, if you think that a variac is all that you need. Make sure that your will is in order and say good-bye to your family while you still can. You appear to be pigheaded enough to kill yourself. I've been around when a CRT imploded and pulled the electron gun out of the wall on the other side of the room. I've seen the glass and metal shards just miss a co-worker when a customer installed color CRT exploded. He didn't have the tube mounted properly, and had given up on setting the convergence on his bargain $29 rebuilt CRT.

The chances of dying are a lot higher than .1% if you don't study the safety issues first. Do yourself a HUGE favor and read the FAQ for news:sci.electronics.repair which is at:

Such ignorance, i could hear badly designed tv horizontal deflection(15750 Hz, well below 20 kHz common boundary value) until my 40's, and could hear

I see someone isnt thinking tonight. The a.f. timebase is not a sine wave, it contains harmonics of several times the fundamental, ie the scan coils can run at way above audio if need be. Doesnt imply the scanning circuitry will, we wont be using that. Comprendi?

NT

Not only fixed higher audio frequency, but that frequency is utterly locked into the total energy budget for the entire television. Energy goes into the horizontal scan only during the RIGHT HALF of the sweep. It resonantly inverts and recycles for the LEFT HALF. Anything left over sometimes becomes the B boost for vertical deflection and possibly sound. Plus, of course, the high voltage for the CRT.

A true horizontal linear class A magnetic deflection on a larger tv would take at least two or three kilowatts of line power. This energy recycling was and remains positively brilliant.

Again, the "convert your tv into a giant oscilloscope" was a mainstay outright scam of the classified ads of the 1950's. Which was before the low end real oscilloscopes upgraded to triggered sweep and sweep delay.

Not even wrong.

Ha, ha, I remember another great project from the seventies (I think): multiplexed tv, i.e. one tv that loops over several channels with viewers wearing glasses with mechanical shutters synced to the channel scanner. (BTW, it may now even be possible with lcd shutters...)

--DF

Are you saying the scan coils themselves are resonant? Ie on their own without the drive circuitry. If not, what is preventing them being driven with other frequencies?

NT

Recently used a monitor as a scope. Two sets of isolated deflection coils Y deflection is more turns so higher inductance and less current to deflect. X coils lower inductan i.e needs higher current ie about _+1A for deflection. Gives great lissajou from audio etc. More interesting if the signal drives colour as well or generates a fiels to upset the colour alignment . Try driving coils from stereo audio

No, I am saying that the horizontal scan coil does a hell of a lot of other things besides horizontal sweep. The changing of whose frequency TOTALLY TRASHES the ENTIRE operation of the rest of the tv set.

The point I think some people are trying to make here about converting an ordinary TV by just clipping the yoke leads and slapping on an amp won't work.

The reason it won't work is that the coil's inductance is intricately woven into the fundamental deflection circuit. And the horizontal is even worse - it's right in the same circuit with the flyback - that's where the name came from; that big spike when the flying spot flies back to the other side to start another scan line. Cut the yoke, and it won't oscillate, and you won't get any HV.

Now, admittedly, you could build a stand-alone flyback supply for all of the HVs, and just throw away the vertical sweep osc, and then you could drive the coils with current drivers, but yikes!

It'd be easier to get a used real scope at a ham fest for 5-10 bucks. Even a fixer-upper - you'd get as much experience, and you know that when it's fixed, it'll work!

Good Luck! Rich

What part of "fixed frequency design" did you not understand? BTW what got you to thinking that a proper sweep was anything other than a highly asymmetrical triangle wave.

The input horizontal deflection coil sweep signal for a television set consists of a SQUARE WAVE that lasts from center to right of scan.

Don, you should know damn well that a "square wave" voltage into an appropriate inductance plus a damper and a flyback results in a linear ramp in current, which is what the magnetic field deflecting the electron beam to sweep linearly at the design frequency.

The January thread on converting a cheap TV to an oscilloscope, impractical that it was, did recently give me an idea...

What about using a $20 5" portable TV as a display for one of those cable tuner or comparable homebrew spectrum analyzers?

The key idea would be to rotate the display on its side, so frequency is on the Y axis and intensity on the X.

- Sweep rate would be fixed at the vertical scanning frequency of the TV.

- Horizontal scanning frequency would also be normal. Charge a cap from a current source as the beam scans, and use a fast comparator (better a window comparator) running against the log detector output to turn the beam off (or blip it on-off) at the right horizontal position.

- a small fast microprocessor like a PIC could generate the video timings and also put a reticule on the screen. An FPGA could do that and aslo add on screen frequency display and range info.

Think it might work?

Might it be possible to use the TV's tuner/IF strip to make a very crude "toy-grade" swept receiver? These portable TV's are voltage tuned, right?