An arbitrary waveform can be generated by cutting it out of cardboard and holding it against a scope display.
I wish I could credit this idea to the right person, but sadly I saw it in a magazine too long ago for me to remember the inventor’s name, or the magazine.
A photodiode pointed at the screen and connected to the Y deflection input with the correct polarity, combined with the right gain setting on the scope, makes the spot follow the edge of the cardboard.
The connection to the Y input is also the output of the generator, with amplitude set by the Y gain (assuming there is enough gain after the photodiode).
Repetition rate depends on the timebase setting.
When I tried it out of curiosity, also years ago, it worked well – even without making the room completely dark.
You do get an aberration during fly-back (or its digital equivalent).
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