NotTaR of Television Sets : Turning a TV (or monitor) into an oscill..  
 Copyright © 1994-2007, Samuel M. Goldwasser. All Rights Reserved. Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied: 1. This notice is included in its entirety at the beginning. 2. There is no charge except to cover the costs of copying. I may be contacted via the Sci.Electronics.Repair FAQ (www.repairfaq.org) Email Links Page.

     << Building a Frankenstein T.. |  Index  | Displaying a video signal.. >>

Turning a TV (or monitor) into an oscilloscope?

This question comes up so often and it does sound like a neat project to give a defunct TV a second life. Don't expect to end up with a Tek 465 on the cheap when you are done. However, it could be a fun learning experience.

CAUTION: See the safety recommendations below.

You will be severely limited in the performance of such a scope. TVs and monitors are designed to operate at a very narrow range of horizontal scan rates and the high voltage is usually derived from the horizontal deflection. So, you would need to retain the original deflection system for this purpose at least.

  1. You will need to disconnect the deflection yoke from the horizontal and vertical deflection circuits of the TV or monitor without killing the HV. (also, doing all this without killing yourself as well). Depending on the design, this may be as simple as unplugging the yoke connector. More than likely, you will need to substitute a load for the horizontal deflection coil. A coil from another sacrificial similar TV or monitor would probably suffice.

    Warning: at this point you have a really bright spot in the middle of the screen which will turn to a really black spot if the brightness is not turned way down really really quickly.

  2. For the horizontal, you need a ramped current source. You are driving a non-ideal inductor (the deflection coil) so it has both inductance and resistance. Thus the waveform is a trapezoid - a voltage ramp (for the resistive part) superimposed on a voltage step (for the inductive part). This should not be too difficult. Don't expect to be able to achieve really fast sweep. Even running at normal TV rates is non-trivial.

  3. Similarly, for the vertical you need to drive with a voltage (your signal) controlled current source. However, if you are just screwing around, then the linearity etc. for the vertical may not be that important. In this case, one way is to put a current sensing resistor in series with the deflection coil and use this in a power op amp type of feedback arrangement. (You could do this for (2) as well.

  4. There is a good chance that the original brightness control will work as an intensity adjustment. However, with some TVs and monitors, this depends on receiving a valid video signal. You may need to improvise. If you do want to control the intensity from a signal source, you should be able to tap into the drive signals going to the little board on the neck of the CRT.

  5. Don't expect high bandwidth, uniform response, or any of the other things you take for granted with a decent scope. That takes work. However, as a fun project, this certainly qualifies. Interchanging the functions of the horizontal and vertical deflection yoke (and rotating it 90 degrees) may provide a better match of horizontal and vertical bandwidth to your intended applications or experiments.

  6. With a color TV or monitor, these experiments could be quite interesting and educational but there may be color fringing effects since you are not compensating for certain aspects of dynamic convergence at all.

  7. SAFETY: Once you disconnect the deflection yoke from the TV or monitor's circuits, move the original circuits out of the way and put a barrier between between you and the rest of the TV or monitor. All you will need are connections to the deflection yoke on the CRT (unless you want to do intensity modulation in which case you will need to drive the video output(s) to the CRT cathodes. I would recommend against doing this if your unit is one of those with a totally 'live' chassis as there would be additional safety hazards and circuit complications).

(From: Chris Crochet (ccrochet@premier.net).)

Hehehe... Actually, I've done this one. :)

I've got two old IBM mainframe terminals, painted like charred metal, hooked up to each channel of the 'B' speaker outputs on my stereo. It's strange looking and always an attention getter when I have guests. Not to mention, the long-persistence phosphor they use makes interesting tracers :)

One caveat, at least on these monitors (I don't know what other monitors this might apply to). When you turn them off, the circuitry shuts down in the following order: horizontal drive first, electron gun second, and vertical drive last. Therefore, if there is no vertical deflection, which would be the case if the stereo is quiet, the active electron beam becomes perfectly stationary during the course of shutdown, thus burning a hole in the phosphor. Oops :) I found it more effective to hook the stereo into the HORIZONTAL drive, thus avoiding this problem. Not quite like your average oscilloscope.

Another interesting effect -- if the electron gun is active during vertical blanking interval, it seems to deflect so far that it bounces off the SIDES of the picture tube, and sprays all over the phosphor, making some interesting images.

(From: Lance Edmonds (lanceedmonds@xtra.co.nz).

Some years ago ELEKTOR and Electronics Australia magazines published articles on a design for this. Dick Smith Electronics in both NZ & Australia used to sell the kit.

Max Bandwidth was a startling 10 or 15Khz. Enough for elementary audio servicing.

Those magazines also published designs for delayed sweep & trigger modules as additions to any basic 'scope. Plus, a storage scope design, logic analyzer design, and a Dual trace emulator design.

Enough to keep the average hobbist/experimenter happy for quite a while (g).

(From: Dale H. Cook (dhcook@rev.net).)

Every few months someone will pop up with this question. A TV would not make a very good scope. Bandwidth would be limited and the amount of work needed to build the horizontal and vertical amplifiers, sweep and triggering circuits and so on wouldn't be worth the effort. You'd need even more work to add modern features such as delayed triggering and variable hold-off. Don't even think about multiple channels and the advantages they offer. In a time when I see used Tek 465s offered for $200 it certainly doesn't pay to try to convert a TV. If you are just looking for a challenging electronic project I can think of several that have a far better chance of yielding something useful. Now, if you were starting with an antique set that used an electrostatic CRT you might do a bit better, but a 1937 Dumont will set you back about $3,000.00 or so - a little too much of an investment.

(From: Tony Duell (ard@p850ug1.demon.co.uk).)

I've worked on the vector monitors that were used on some of the 1970's minicomputers. These are essentially X-Y displays (not raster scanned), and would make audio-bandwidth 'scopes if given a timebase. I would guess at a bandwidth of the order of 100kHz.

Some of them (DEC, certainly, maybe Tektronix) were electromagnetically deflected like a TV. However, there are a couple of things to be aware of. Firstly, the output amplifier, which drives the yoke at constant current, is pretty complex. Secondly, the yoke is specially made - the 2 sets of coils are pretty similar (unlike those in a TV), and the inductance is critical.

So, while I'll keep these monitors running, I'd not want to have to covert a TV into one :-).

(From: David Katz (DAVEkATZ@prodigy.net).)

If by chance what you want is an X-Y display for audio, not a (more typical) X-T, it's easy. Just put a resistor in series with each yoke (about 100 ohms, 5 W) and drive them with a stereo amp.

(From: Steve Roberts (osteven@akrobiz.com).)

Your best hope might be to get a older generation heart monitor from a hospital, these have a professional X-Y display module to begin with, and are surprisingly easy to hack, mine was $10 at the local surplus shop. The ultra long persistence phosphor is a pain/blessing depending on what you are doing.

For a description of what one person did, see: Dan's Home-Built O-Scope Page.

(From: Alan (revidyks@rocketmail.com).)

Apparently it's pretty hard to produce a decent scope.

It is, however, pretty easy to use the CRT as something like a scope, which I did recently with the built-in green screen monitor of a thing called a Kapro 2X. It was being thrown away, so I said I'd take it and have a look inside before throwing it away.

I wondered what if it was possible to drive the CRT from a source other than the computer video circuitry, so I did some tests, worked out how and by what voltage the deflectors were driven, (about 1v, 0.3A measured as an AC voltage).

Once I'd worked out that this was about the same as the output from a small stereo amp, I removed the horizontal signal from the CRT and hooked one channel of my stereo across the horizontal deflector , left the vertical deflector hooked up to it's (60Hz?, 30Hz?) signal, and switched it on. The results look pretty good, I get a full-screen moving trace of the sound wave. One other thing that I did was make the beam intensity constant by turning a knob marked 'B-SUB' a bit, this would have flooded the screen with 'white' ordinarily, but was perfect for me as I could now remove the computer motherboard all together.

I also tried connecting the left and right channels across the horizontal and vertical deflectors respectively (first disconnecting them from their normal inputs), which produced some really cool looking lissijous (sp?) figure type things, that change and throb with the music- each CD seemed to have distinctive characteristics. Maybe I'll try two different pieces of music across the axes, could be interesting...

I'd love to try throwing some different signals of different frequencies and shapes across the axes too, especially in combination a with musical one. The 'best' results so far, have been from music with a strong bass, simple beat (cymbals with a bass drum look great), and not too many layers of guitars, vocals, etc. (too many sounds and it's an uninteresting mess...)

If you want more information or have any advice on or experience with this sort of thing, mail me...

If you're thinking of trying any of this, remember (in case you don't know) that TVs/Monitors can be REALLY dangerous even when switched off and unplugged. See the section: SAFETY.

 <<Building a Frankenstein T.. | ToC | Displaying a video signal..>>