Denial

In the cage with the VIC chip, I've figured out that the white pot adjusts the composite video signal, but what does the red one do?

Also, other than the s-video mod (which I am not yet prepared to do) is there any way to get rid of the jail bars?

That "red pot" is actually a capacitor trimmer, and it's there to fine-tune the master oscillator clock.

Unless your VIC-20's video output is missing colour, there's no need to touch it.

As for the "jail bars" ( ), you could try an exchange of the voltage stabilizing capacitors near to VIC, check the Denial-Wiki for "Removing vertical patterns & colour phase problems". That mod tackles an issue somewhat unrelated to (the several variants of) the S-Video mod, but likewise isn't easily done unless you're comfortable with soldering on the mainboard.

hm, that "plug'n'play " mod by mrr19121970 looks interesting