This is a way to modify a digital TV set top box converter to produce a modulated B&W RF analog TV signal to feed to the antenna terminals of your vintage B&W TV sets. Especially the pre-color days of TV (before 1953), which had wide bandwidth video amps wide enough that you'd see chroma checkerboard pattern (at 3.58Mhz, which comes out to 227.5 cycles per horiz line) crawling thru the picture of color TV broadcasts. Digital television broadcast the luma (Y) separate from the chroma (U and V), and most converter boxes downscale HD to 525i and create a composite NTSC luma and chroma to modulate onto channel 3 or 4. Here, in the CM7000 I found where you can intercept the chroma before it's merged with the luma, before it hits the modulator.
Looking at the picture, look for the cap arrowed. Near the STV6433 chip (this chip filters and merges luma and chroma to create the composite video output (yellow RCA jack, and also a special composite signal designed to be modulated (group delay pre-distortion and a notch to make room for the sound IF carrier at 4.5MHz) onto a TV channel, which is why I wanted to use this box's intermal TV modulator instead of an external one). What we are going to do is intercept the chroma before this chip, with a switch that will either pass it to this chip, or switch away from the chroma source and switch to ground. I placed this switch before the coupling cap, as the STV6433 has an internal video clamp that charges this cap during the horizontal interval (between scan lines) to give the internal amps inside the chips proper DC biasing. Switching away from the chroma source to ground still gives this charging circuit something to work against.
To do the mod: remove but keep the cap arrowed in the picture. Now stand it upright and solder it to the solder pad it was soldered to closest to the STV6433 chip (see drawing at the bottom of the picture. On the now top end of this cap solder a wire that will go to the center pole of a switch. Another wire soldered to the now unoccupied solder pad goes to one side of this switch, and the other side of the switch goes to ground. Label the switch "color" and "B&W" accordingly.