[old_tv_nut]

Neat project!

I wonder how linear the modulation is - will be interesting to see if you can avoid 920 khz audio/chroma beats when the 4.5 MHz is added. This was the bane of early transistor modulators for VCRs and was the reason the 4.5 MHz injection was kept near the minimum level acceptable to receivers.

Another thing you may want to try is replacing the 220 ohm and switched bypass for 0.7V / 1.4 V video level with a pot, to get an adjustable modulation level.