I've modified my Channel Master to produce TV signals without the chroma subcarrier, to feed my early 1950's B&W TV sets. Back then, they displayed the full resolution of the 4.2MHz luma signal, as NTSC color had not come out yet. Later B&W sets usually low pass filtered the luma to get rid of the color subcarrier.
There is a chip in this box that sums the S video signals together to produce the composite video fed to its modulator. Here I insert a switch to pass or stop the chroma from getting to the sum circuit.