The machines that I've upgraded all give a better picture.
There's a couple of things to do:
Replace one capacitor by a bigger one, replace some resistors by 75 Ohm and one from 10 Ohm to 0,22 and add a capacitor there. Also remove a 100pF capacitor at a base of a transistor.
Most descriptions are for the 800XL and 130XL. But the video circuit is basically identical.
RE: 1200XL Video Upgrade. 