I recorded a set of videos recently documenting some troubleshooting of an ATARI 400 system, and the conversion from RF to composite video. There was so little info out there about this mod that I thought even my crummy video would help. 

The embedded video above is the 4th and final part that shows the composite conversion. Here are links to the other parts and some docs and forum posts I found along the way... 

Part 1: https://youtu.be/mojxsA4EAFQ
Part 2: https://youtu.be/9mvjiq9HEIk
Part 3: https://youtu.be/qBcRygpFbtI

Atari 400/800 Service Manual:
http://www.jsobola.atari8.info/dereat...

S-Video to Composite info:
https://www.epanorama.net/circuits/sv...

Atari 400 video mod info:
https://atariage.com/forums/topic/178...
https://atariage.com/forums/topic/209...