Modifying RF channel of RS modulator

[old_tv_nut]

The local Radio Shack is closing in a few days. Their cheap NTSC modulators were marked down 70%, so I bought a couple to play with. I wanted to try changing the RF channel so I could put at least two on a cable to feed vintage sets.

The unit opens by finding screws under three feet, and removing the F-connector nuts, followed by a sort of Houdini escape routine to wiggle the board out of the enclosure.

The RF oscillator coils are under an easily removable shield cover. They are accessible through the hole in the upper left, but are covered in what seems to be beeswax for mechanical stability. They are tuned by knifing them (spreading a winding or two). I removed the the cover and scraped away enough beeswax that I could do some extreme knifing of the channel 4 coil, which you can see in the photos.

I connected the modulator to a TV with a synthesized tuner, and knifed the coil first to channel 5 and then continued to channel 6, which is about as far as it will go without replacing the coil.

The bottom shield is soldered in place with a big glob of solder, so it would be difficult to replace the coil. Besides, I am leery of doing any component replacement because the bottom is covered in thin traces and surface-mount components.

i don't know if the frequency is exact, only that the AFC can pull in when changing to channel 6 from above or below, so it's probably within 100 kHz or so.