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TV / PC as oscilloscope

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Menticol

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Hello!

There is a previous post about building an oscilloscope using a TV set. Nigel said "Buy a scope! - there's no decent, reasonably priced, way to use a TV as a scope."

Maybe that's true for a serious electronics designer or technician, or a hobbyist on a first world country where used oscilloscopes are cheap as dirt. But here in my country, used oscilloscopes are very expensive (earning 600.000 COP is hard as earning 600 USD on the USA!). Such amount of money to measure a couple of signals on noob projects... is definitely a waste!

I have two options, TV based and PC based oscilloscope (AD converter via parallel port). I'll try the TV version first

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I've applied the classical procedure: disconnecting the vertical coil, but I didn't inverted the yoke as the websites say: I took some measurements instead

Maximum Vertical Deflection: + 4,6 V
Minimum Vertical Deflection: - 4,6 V
Coil resistance: 14,7 ohm

I have some questions, if anyone can help:

Question 1) What signal amplifier would be decent for the Job? I've found (and attached here) an schematic that uses a crappy 741 OP AMP. As Audiguru says, it's a lousy old amp!

Now the horizontal coil. I understand that a Sawtooth generator is required.

Maximum left deflection: XX V
Maximum right deflection: XX V (Tonight I'll fill the missing values)
Coil Resistance: 4,65 ohm

Question 2) Can a 555-based circuit do the job?
Question 3) How can the circuit be synchronized* with the vertical signal like the "Trigger" on a real oscilloscope?). I've read something about messing with the TV's V-Sync, but information was incomplete.

* Sorry, I don't know the real term.

Thank you very much for your help!

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EDIT: The schematics are in Spanish. I've attached them just as an example of what I found. First one is the horizontal driver, second one is the vertical amplifier, third one is the ramp generator. The original author is: https://www.yoreparo.com/foros/diseno_electronico/125029_0.html#461862
 

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1. The problem in driving the vertical coil is the inductance of the coil. You need to know that before you can design an amplifier to drive the coil. Even then the obtainable vertical (input signal) frequency response is likely to be very low.

2. You need a sawtooth generator for the horizontal coil drive. Google that.

3. To sync you need a comparator circuit with an adjustable trigger point to trigger the sawtooth generator. You will also need a free run circuit to trigger the sweep when there's no signal. And you will need to blank the trace during the retrace of the sawtooth and when there is no sweep to avoid burning the tube phosphor (if there is no horizontal or vertical sweep the small stationary beam spot will rapidly burn through the tube phosphor and leave a permanent burn spot on the tube face.)

How are you going to generate the high voltage for the CRT anode? That's normally generated by the horizontal sweep flyback circuit.
 
You're better off going with a PC scope. A low frequency scope is fairly inexpensive. If you are good with software you can build your own using parallel port on PC and an ADC chip.

TV deflection yoke coils are driven at a linear current slope (square wave voltage drive, V= L * delta I/delta t), with fast retrace on horizonal sweep. The inductance is designed to optimize a fixed scan rate of TV signal. The retrace creates pretty high negative voltage driver. A variable scan rate design requires a square wave amplitude proportional to scan rate. Faster scan rates can result in pretty high voltages. On top of all that it requires a positive and negative going supply relative to coil. The square wave tops have a slight slope to compensate for wire resistance of yoke coil to make scan linear.

To get the vertical deflection to accurately represent an input voltage is quite complicated involving voltage to current drive conversion. Again the resistance of the yoke wire will screw up linearity if not compensated.

I cannot remember any real oscilliscope that did not use electrostatic deflection CRT's simply because coil deflection for a scope is too complicated.
 
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Thank you by your fast answer Crutschow! I tried to solve all the points, but I'm still lost on some points

1. The problem in driving the vertical coil is the inductance of the coil. You need to know that before you can design an amplifier to drive the coil. Even then the obtainable vertical (input signal) frequency response is likely to be very low.

That's sad, I guess that's the reason behind using plates instead of coils on real oscilloscopes. Anyway, I'm not looking for a very high frequency response, just enough for audio or general troubleshooting. Do you think that the second schematic may be adequate for that?

You need a sawtooth generator for the horizontal coil drive. Google that.

Copy. The third schematic describes a ramp generator, but I don't know if a 555 based alternative would be better

To sync you need a comparator circuit with an adjustable trigger point to trigger the sawtooth generator.

You mean changing the sawtooth generator frequency until it matches the frequency of the input signal? It will be a headache for my noob knowledge :( A guy described a way to do that, but I don't know if it would work for my case

You will also need a free run circuit to trigger the sweep when there's no signal. And you will need to blank the trace during the retrace of the sawtooth and when there is no sweep to avoid burning the tube phosphor (if there is no horizontal or vertical sweep the small stationary beam spot will rapidly burn through the tube phosphor and leave a permanent burn spot on the tube face.)

According to what you say, I guess my first problem is finding how to turn on/off the electron gun (messing with TV internals). Second problem is, detecting when the sawtooth voltage is rising (gun turns on) and when voltage is falling (gun turns off). Maybe a square signal generator, coupled with the sawtooth generator?


How are you going to generate the high voltage for the CRT anode? That's normally generated by the horizontal sweep flyback circuit.

You're right: when the horizontal coil is disconnected the CRT dies. The problem is solved placing a dummy coil on place, to close the circuit.
 
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I really appreciate your suggestion RCinFLA. Other guy on the same forum posted this diagram, showing a computer based oscilloscope (see attachments). I wonder what would be the frequency response, using the ADC8020?

Here is the program to read the data

Windows version

DOS version


The credits for the schematics:
https://sites.google.com/site/scopeonpc/scopeonpc4

EDIT:
I'll miss the TV based oscilloscope, it was funnier to build.
 

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That's sad, I guess that's the reason behind using plates instead of coils on real oscilloscopes. Anyway, I'm not looking for a very high frequency response, just enough for audio or general troubleshooting. Do you think that the second schematic may be adequate for that?

When I said "buy a scope" I wasn't joking, trying to modify a TV in this way doesn't give you any kind of frequency response worth having, and certainly no where near that required for audio. You 'may' be able to have a bandwidth of a few hundred hertz?.

For both TV's and PC's you do essentially the same thing - build a storage scope (minus the screen) and build an extra circuit to display the stored waveforms on the screen. Using a PC is easier, because you have the processing power of the scope to play with.
 
You could get a lot more bandwidth out of your TV if you turn it 90° and use the horizontal circuits (much higher bandwidth) to display your signal amplitude. However I wouldn't recommend using a TV at all : think about the lethal voltages, and the performance is going to suck anyway.

If you only need low sample rates, you can simply use your computer soundcard. This really is the cheapest option !... and it has more bandwidth than the TV scope (lol).
 
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