I got the results I wanted by first isolating the luma / chroma / audio on the board. The biggest improvement over the original guide came when I removed R31 (6.8k) at the top right of the area covered by RF shielding. That isolated the amplified chroma signal. Then I removed all 7 resistors at the top right to isolate the luma from the audio and modulation circuit. The FMS6400 improved the picture a LOT and added composite video.
I figured out how to use PCB design software then I made some prototypes and now I'm at version 1.2 after fixing some issues with the composite video.
I posted the schematic I use on my v1.2 boards and the corrected solder points.
I am also working on a more complex s-video circuit that would connect directly to the GTIA on the 5200 and Atari 8 bit computers. This should give the best possible picture quality from these systems. I have the PCBs on order and will begin testing soon.
Edited by low_budget, Sun Jan 29, 2012 8:11 PM.