To give proper credit, this Instructable is an improvement on Magnelectrostatic's at https://www.instructables.com/id/How-To-Make-A-CRT-TV-Into-an-Oscilloscope/. I wouldn't have been able to complete my mod without it. Thanks!
This mod is not capable of displaying much outside the human audible range (20-20k Hz).
Also, since TVs are highly variable in design, this Instructable is intended for people who can do their own experimenting. This of course requires experience in electronics. Your TV will probably be different from mine. My instructions may not apply to what you're working with. They're guidelines. I hope they help you modify your available resources by providing ONE particular example.
Step 1: Safety
First, the obvious step: Is it plugged in? Unplug it! Isn't it funny that this is the exact opposite of what a tech normally tells you...
When you remove the shroud, be careful not to rip any wires from the circuit board, and do not touch any of the exposed contacts. Identify large capacitors and take note of their voltage ratings. 50v and above are especially dangerous, and should be discharged with a well insulated screwdriver across the contacts prior to tinkering if possible.
Ok, ok, I generally dislike disclaimers because they are almost never legally sound. But for those of you who don't get the idea from the above paragraphs...
Don't attempt this unless:
you have a solid grasp of high voltage safety practices.
you have a solid grasp of electronic equipment in general.
you have someone around to call 911 or give you CPR.
you have experience working with mains (120VAC) power.
you are not a moron.
I take no responsibility for damage to your health or equipment. All damages incurred are the sole responsibility of the end user.
Step 2: How and Why It Works
By the way, this only works with the old CRT TVs...
Each line on the screen of your TV is created by an electron beam being scanned across the phosphor backing of the glass very quickly. This is possible because electrons are charged particles, and can be manipulated by electric and magnetic fields. TVs use coils of wire at the back of the tube to produce the necessary magnetic field to deflect the beam, and these are what we aim to modify.
Scanning frequency is important. We know that video runs around 30 frames per second. Each frame is composed of two "interlaced" images which means the TV actually scans at 60 frames per second. In NTSC zones (America and a few other countries), there are 525 stacked lines per image. So to produce a single frame, the electron beam is deflected the screen's vertical distance every 1/60 of a second (60Hz), and the horizontal distance every 1/(60x525) of a second (31500Hz). That horizontal frequency is awfully fast. So fast in fact that it has to be driven by a 15,000+ volt flyback transformer. For audio, we want the 60Hz provided by the vertical scanning.
When all is said and done, the TV will still think it's a TV. It will attempt to produce an image on the screen, but you will have forced it to draw that image on a very thin line that is vertically deflected by your input signal. For this reason, it is best to use a very old TV that displays white noise, or one that displays a blue screen, but will not automatically turn off the screen.
Oh, one other thing: The deflection coil of the television is an inductor. When a signal passes though an inductor, several things happen. First, the inductor has impedance, which is kind of like resistance. Impedance is frequency dependent, and in this case it means that high frequencies will not make it though the coil as well as low frequencies will, so your oscilloscope will have a hard time displaying really high frequencies. Second, the signal is integrated in the mathematical sense. This means that square wave input will appear as triangle waves on the screen. It will still sound like a square wave on the speaker though. Triangle waves appear (approximately) as sine waves. Sine waves are basically unaffected. Just keep that in mind...
Step 3: Gut the TV
On my TV, there were two potentiometers in the front. One was a combo power on and volume control, and the other controlled brightness. I removed both of them and installed my own power switch (a big red push button). For the brightness, I set it all the way up while the TV was turned on, then removed the pot and checked the resistances. I then soldered the matching resistors directly into the board where the pot was connected to lock the maximum brightness permanently.
The TV's built in volume control can't be used for this build, unfortunately. It amplifies the signal attached to the television signal, and unless you want to poke around looking for the section of the main board that has the amplifier, remove the volume control. This method should be used for all unneeded adjusters.
Lastly, disconnect the speaker wires at the main board and tuck them away for later.
Step 4: Do I Cut the Blue or the Red?
Unless you've taken a physics course and are familiar with the right hand rule F=qVxB, desolder one of the four wires at random. Turn on the TV. If you see a horizontal line, you have disconnected the vertical deflection coil. If you see a vertical line, you have disconnected the horizontal deflection coil. Use a continuity tester to identify the matching ends of coil. Mark or remember their locations.
You do not need the wires connected to the horizontal deflection coil. Remove them at the main circuit board. Remember, they operate around 30,000Hz and 15,000+ volts. They are not needed at all for the oscilloscope. Short them before you touch. Wrap some electrical tape around the stubs at the board and make sure they wont touch anything inside the TV or it will fry.
Now you have a 60Hz vertical line. Unless you like cocking your head to the side, you want a 60Hz horizontal line. So solder the two remaining wires that are currently attached to the vertical coil, onto the horizontal coil.
Now solder two wires onto the vertical deflection coil. This is the oscilloscope input, where you will connect the amplifier circuit.
Step 5: Horizontal Frequency Locking
Connect a signal source to the vertical deflection coil. You don't need an amplifier for this part, your MP3 player or computer headphone output is enough to test your oscilloscope. Find a way to generate a stable tone so you will have a single frequency on the screen. Remove the main board as much as possible from the case without disconnecting anything. Place everything on a non-conducting surface. Identify any trimpots on the board.
Now turn the TV on. Take an insulated screwdriver and *very carefully* reach into the mess of high voltage wires without touching anything and figure out what each trimpot does. Someone watching the screen for changes for you helps a lot.
One of them is going to adjust scanning frequency and you'll be able to lock onto the displayed tone by turning it. You need to replace it. Desolder it and solder three wires into the holes where it used to be, keeping track of the middle one. Measure the resistance of the trimpot and find a nice hand turnable one with similar resistance at Radio Shack or otherwise. My TV's trimmer measured 60Kohm, but I put a 50Kohm in there since I had one lying around, and it works fine. Attach the new pot to the wires, making sure the middle wire goes in the middle, then make sure it all works. Mount the new pot in the TV's case for easy user access.
The horizontal frequency adjust will not allow you to resolve extremely high frequencies any better, it will only allow you to stop the wave form from scrolling across the screen maniacally.
You may also wish to adjust the ring tabs found around the skinny part of the picture tube now. They are usually black or dark grey, and adjust the position of the image on the screen.
Step 6: Signal Amplification
Here is the amplifier circuit I designed for this project. I combined several ideas from other sites, but modified it enough to say it's mostly my work. I made heavy use of Paul Falstad's circuit simulator http://falstad.com/circuit/. It is an Op-Amp augmented audio push-pull amplifier.
The first Op-Amp controls the gain of the input signal by the formula R2/R1 where R1 is the resistance selected by the rotary switch and R2 is the 1Mohm pot. It is theoretically capable of amplifying the input by 1 million times (assuming 1ohm minimum on the rotary switch, which is the jumper wire). It is also capable of significant attenuation.
The second Op-Amp ensures that the transistors receive enough voltage during pos-neg trade off to initiate opening their gates. They require around 0.7v to open, so there is a 1.4v gap between when one transistor closes to when the other opens. This produces a lot of distortion, for which the Op-Amp compensates.
When the circuit is complete, calibrate it. You want the power regulator to supply around 30v so the Op-Amps get their required +/-15v, but its output needs to be a few volts below the voltage across the 1000uF cap to take advantage of it's regulatory powers. Adjust trimmer 1 to do this. An alternative to the transformer and power regulator is a wall-wort. You know, those hideous DC adapters for your cell phone, modem, etc that take up way too much room on the power strip. Just find one that outputs 25-30v.
Connect the circuit output to the vertical deflection coil. Play some music through the circuit and over-amplify it with S1 and S2 so the op amps begin to clip the signal. Adjust Trimmer 2 until the tops of the clips just touch the top and bottom of the screen. This will prevent the transistors from being able to fry the deflection coil.
Now connect the output to the TVs built in speaker. Is it too loud? Add a high wattage resistor. Is it too soft? Add a high wattage resistor to the deflection coil and recalibrate. I was pretty sure I would have to do this, but it turned out that my coil and speaker were well matched. A 10ohm 1W resistor should do the job if needed. You may want to put a switch in line with the speaker as well so you can turn off any annoying signals you may want to view.
1ea TL082 High Slew-rate Double Op Amp
1ea TIP-41 NPN (Can also use TIP-31)
1ea TIP-42 PNP (Can also use TIP-32)
1ea LM317 Power Regulator
1ea 5+ Pole Rotary Switch
1ea 1Mohm Potentiometer
2ea 10k Trimmer
4ea 1A Diode
1ea ~30v Transformer
1ea 1000uF 50v Electrolytic Cap
2ea 470uF 16v Electrolytic Cap
1ea 10ohm Resistor
Step 7: Put It All Together
Connect the transformer to the circuit. Connect S1 and S2 appropriately. Run the two input wires through a hole in the TV case. Connect the circuit output to the speaker and deflection coil. And make sure to remove as much wire length as possible in all connections, especially signal in, to reduce stray inductance in your circuit. Find user friendly places to mount S1 and S2.
Close up the back and take it for a test drive!
Step 8: Have Fun!
The degree of signal amplification is a great feature as well, if you're not using it for anything really precise. The 60Hz noise that the circuit amplifies can be a little ridiculous. This is caused by the input wire's stray inductance, not by the circuit, and could only be reduced by shielded, grounded wire, of which I have none. I have a spool of wire I connected to the input to play with it's large inductance on high amplification. I can detect power sources from many feet away by aiming the coil in the direction of transformers. I can also detect the processor chirping away inside my laptop. I can even use the coil as an inductive microphone by placing it near a speaker playing music. The speaker's coil produces a magnetic field that the detection coil picks up and amplifies and plays on the oscilloscope! The attenuation the circuit can produce is also impressive. I have in fact connected the input to my 120VAC household line and registered a 60Hz wave and the oscilloscope still lives.