A/V mod hasn't really been explored much yet, I managed to make a working mod for a PAL first generation board, you can pick the composite signal or at least the color and luma signals, amplified the luma part and then hooked the color signal on after that. When amplifying them both I lost the color. Optimal is probably to have two good amplifier circuits that you can adjust both the luma and color. I was going to try and hook them to S-video but never got around to it. Picture was crystal clear with no odd RF interference. I think I wrote about it here, check my posts. There's schematics in the wiki where you can make your own solution, we only have schematic for the original VES and the German SABA VideoPlay 2 which is close to the Channel F II:
http://classicdev.org/VES/Schematics
I only have one NTSC machine that I bought NOS so I'm not that keen on tearing the foam off the bottom (or punch holes) to get to the screws - at least not as long as it is working.
For the Channel F II it could very well be easier... in any case if you start at the antenna out signal and work your way backwards there is a composite signal somewhere before it's put on a carrier wave. Just need to find the right spot and then probably amplify it.















