I modified a Channel Master CM7000 CECB so it can give a B&W modulated TV channel output (without chroma).
Idea is this box uses a special analog filter/mixer video chip, and here I just turn off the chroma input. This chip also provides group delay predistortion (like real NTSC TV stations did back in the day). The switch I used for this used to select which RF channel to put the output RF on, 3 or 4. Now I use it to enable or disable the chroma subcarrier. I hardwired the channel selection to channel 4.