A Question Regarding a Television-to-Oscilloscope Conversion

[ParkerT]

Hi!

So, when I disconnect the horizontal deflection coil, my monitor goes dark. I assume that this is because the yoke is somehow crucial to the operation of the high-voltage circuitry.

I'm thinking that I need to substitute some sort of load in place of the coil, and here in lies my question:

What can I plug in there, and how do i make sure it won't blow up my TV?

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P.S. This is my first project ever and I have the barest of minimums of electrical engineering knowledge. 

[Simon]

The essential difference between a scope and TV is that a tv sweeps the beam side to side and moves down the screen. A scope simply runs the beam acros the acreen on a constant horizontal sweep and the voltage of the signal you are viewing control the vertical deflection. You still need the yokes but they need controlling differently.

[ParkerT]

I understand that much. I'm hoping to experiment with using the horiz. deflection as my input, but disconnecting it from the power supply board kills the whole device.

Hence, I need to know if a dummy load might work to complete the circuit without causing a meltdown.

[Simon]

You need to keep the power drive and change the driving signals. Why are you trying the conversion ? surely a cheap digital scope will serve you much better ?

[ParkerT]

I'm hoping this will be a fun learning experience! (read: i'm dead broke)

Could you explain in more detail exactly what you are saying I should do and why?

[Simon]

Well I am no TV expert but I'd say that your best keeping as much circuit as possible. You still need to deflect the beam but drive it differently.

Instead of the fixed horizontal sweep you need a variable one to cope with different signal frequencies. The vertical needs taking off the video input and driving with an amplifier of variable gains so that you can view different voltages. The power of the beaam now needs to be fixed instead of variable which is will have been before for different brightness levels of pixel.

Really you need to read up on how oscilloscopes and TV's work so that you understand the differences and what needs changing.

Remember you are dealing with up to 25000V BE VERY CAREFUL !!!!!! once powered it will stay charged for a ling time after you turn it off.

[ParkerT]

I can definitely admit to being under-educated. :-P
I'm going to hit the books and keep tinkering.
You've been very helpful. Thank you.

[sigxcpu]

Isn't the flyback transformer primary fed by the horizontal sweep in a TV? (as a safety measure to avoid tube burn when the sweep is dead)

[fcb]

The monitor uses magnetic deflection for H & V.

The analog oscilloscope uses electrostatic deflection for H & V.

They are not directly compatible, however you could mount the scan coil from the monitor onto the tube if you got lucky (neck diameters, gun position, deflection sensitivity, geometry), but the amount of f***ing about would be obscene.

Typically the monitor/TV's horizontal is generated by the LOPT.

[amyk]

The HV supply is also generated from the horizontal oscillator.

[rsjsouza]

I recall years ago seeing a simple circuit that transformed a typical TV into a rough oscilloscope - however, it did not tinker with the TV internals but instead modulated the input signal appropriately to display the desired wave.

I know it is possible (my dad and I did experimented with this zillions of years ago), but I can't recall how good and how stable it was. I imagine that it was very impractical, as we ditched the whole thing shortly after we finished building and testing it.

[N2IXK]

Yes, the high voltage (and focus, G2, and many other voltages for other circuitry) are generated from the horizontal sweep in almost every TV set. The horizontal windings of the deflection yoke are an integral part of the horizontal sweep circuitry, and must be present to keep the system working. They could conceivably be replaced with a generic inductor that has the requisite inductance, current handling capacity, insulation rating, and DC resistance.

You would then need to feed your new sweep signal to the existing horizontal yoke winding. You will then discover that the sweep isn't very linear when you move away from a frequency of 15.734 kHz, the frequency that the yoke design has been optimized for. You will have a similar problem with the vertical deflection system, which is designed for 59.94 Hz.

In short, converted TV sets make for lousy oscilloscopes. About all they are good for is driving the yoke windings with stereo audio from an amplifier, to produce neat psychedelic patterns as part of a lightshow or whatever. You will never build anything that resembles a useful measurement device.

CRT oscilloscopes use electrostatic deflection plates inside the CRT, rather than electromagnetic coils around the outside of the neck. This is done to achieve useful bandwidth and a to simplify the design of wideband deflection amplifiers which only have to drive a capacitive load at miniscule power levels rather than inductive coils needing tens of watts of drive.

BTW, if you are playing around with this kind of thing, be aware not only of the HV supply to the CRT, but the so-called "low voltage" (130V or so) supply as well. It can supply several amps of current, and is generally not isolated from the AC mains input. It is actually MUCH more dangerous than the 25 kV CRT anode supply, which is inherently current limited.

You also want to be sure to turn the brightness control WAY down when normal raster sweep is disabled. The beam energy in a CRT is enough to destroy the phosphor coating, melt the internal metal shadow mask/grille, and possibly even crack the glass faceplate and implode the tube when it is all focused to a single line or spot in the center of the screen.

[xygor]

A second yoke is needed for the deflection.  Use the first to keep the HV alive.  Or, just the horizontal coil might be enough to make the HV work if you want to wind one; and then use the original for deflection.

EDIT: What I said is redundant.  My apology to N2IXK, who said it first.

[xygor]

Anyone remember that ad in the back of 70's "Electronics Illustrated", "Popular Electronics", and maybe other magazines:
Turn your TV into a big screen sensitive oscilloscope!  Plans $2 ...

[kaindub]

One can use a tv as a scope, but the horizontal sweep frequency will be very low - 50 or 60 Hz.
TVs use a coil to deflect the beam, whereas a CRO uses plates to do the deflection.
If you recall coils (inductors) resist changes in current. For a tv the voltage generated on the fly back is low because of the low frequency. Double the frequency, ie change the time base and the drive and fly back voltage doubles, and so on it goes.
Robert