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.
There are a few Instructables and otherwise internet based instructions on how to modify a television set into an audio visualizer or other simple oscilloscope-like device. This Instructable will show you how to create an actual lab oscilloscope worthy of a poor, amateur electronics enthusiast. The final product has optional audio output, variable input voltage from millivolts to hundreds of volts, as well as manual horizontal frequency locking. The total cost for this project was around $20.
Step 1: Safety
This project involves working in close proximity to the television's exposed flyback transformer and high voltage capacitors, which are both potentially lethal devices if you do not take proper safety precautions.
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.