In old CRT televisions, the deflection yoke is the device that controls the electron beam flowing towards the screen. It’s run by an internal circuit board, but also has external pins that control the horizontal and vertical axes. Electro-music user dnny exploited this design feature by connecting audio inputs to these pins.
yoke1
Internal view of the CRT with exposed deflection yoke pins.

Please note that hacking an old television like this can be very dangerous. If you don’t know what you’re doing, experimenting can result in fire, explosion, injury, or death. That being said, this successful mod on an old portable TV results in some impressive visualizations when a synth is run through it. The post on electro-music gives detailed instructions on how to do this safely.

[via Palm Sounds]