Atari 800XL RF cable (and I/O cable availability?)

I think that's unlikely. The modulator has a very low RF output (after all, it is designed to connect to an input that typically gets microvolt signals from an aerial) so there is not enough RF power around to do damage no matter where it ends up. Certainly running a modulator into an open circuit or short circuit does no harm (These are big no-nos for radio transmitters)). For that matter, I am not convinced that the aerial input of a TV is going to be tuned to the modulator output frequency. Particularly not if the TV is tuned to a different channel :-). And I have never seen warnings about making sure the TV is correctly tuned before powering up. How come? The elementary books on aerials suggest that a good transmitting aerial is a good receiving aerial and vice versa. -tony