I am working on turning an old CRT TV into an oscilloscope for music visualization and perhaps some basic circuit analysis.
Currently I am working on plotting the x-axis with an Arduino and the Signal (y-axis) is coming from an audio amplifier.
The problem is the Arduino can’t supply enough voltage so my x axis is only ~2 inches wide (see photo)
I am using PWM and a virtual ground to supply +- 2.5V, but I’m only measuring 300mv across the deflection coil.
Can anyone recommend any simple circuits for amplifying this signal?
You will destroy the Arduino by trying to drive a coil directly from an Arduino output. The output current for a pin is 20 mA max (recommended), 40 mA absolute max. And then there are inductive spikes to contend with.
The magnetic deflection used in a TV will not work for a signal much above audio frequency, which is why oscilloscopes use electrostatic deflection. Look around for a used scope.
I’m not trying to measure outside of audio frequency, I was thinking there might be a simple small amplifier I could make so that I could use a 9V external power supply and modulate with Arduino PWM
Consider looking at the CRT oscilloscope projects that people have published on line.
So you are using the original horizontal output transformer to generate the EHT voltage?
I foresee you are going to get some serious screen burn! 
You will also require some equally serious amplifiers with current control feedback to drive the deflection - and it will even then, not be linear. 
No the horizontal deflector goes to the fly back transformer to achieve ~16kHz, so it requires extra high voltage. The vertical operates around 60hz normally.
I cut both, and am trying to control the vertical with relatively low frequency signal (e.g. music)
And the horizontal with an adjustable frequency. I am able to adjust this with the Arduino and get really nice waves but they are only ~1 inch wide, so I think I just need a slightly larger current than the 40mA the Arduino supplies. It doesn’t have to be especially high quality either, I’m just trying to get it working now and can tinker around later to fine tune it.
What is your horizontal frequency? The fly back circuit is a tuned circuit, so can't tolerate much variation. You would do better to leave it alone, sample it and synchronize to it.
Also, unless you reduce the brightness, you will get screen burn on the phosphor. Oh, oops, someone already said that...
The fly back circuit is 16kHz in order to span the whole width of the tv for each pass of the 60Hz vertical deflector. (I believe)
I don’t need to do this as I’m not trying to create a full color picture, just one line across the center of the screen. So I removed the whole fly back circuit as it would only show waveforms in the kHz range.
So I am supplying audio input to the vertical deflector, and an Arduino sine or triangle wave to the horizontal with a variable frequency for fine tuning. This works great with the current setup, but only for a ~inch wide section of the tv. So I just need to amplify the sine/triangle PWM waves coming from the Arduino so that the horizontal spans the width of the tv while still being adjustable frequency
I did desolder an old LM386 chip from an old radio yesterday, so I will probably attempt to build a simple amp with that, similar to this circuit:
www.arduino.cc/en/tutorial/simpleaudioplayer
Hopefully with a virtual ground so that I can use the +/- 2.5V sine wave output from the arduino
If you guys have any tips or input on the feasibility of this that’d be great 
