What should be the first step to resolve non-fiber optic cable TV interference caused by your amateur radio transmission?
Why Cable TV is a sealed, shielded system: signals are supposed to stay inside the coax. If your transmitter gets into it, the cable's shielding has been compromised somewhere, and loose, corroded or poorly installed F connectors are the most common leak point. Tightening or replacing the connectors restores the shield and usually cures the problem at its source, so that is the first thing to check before adding any filters.
Watch out A high-pass filter on the TV input is a classic cure for HF energy entering an over-the-air TV receiver, but it does nothing about RF leaking into a cable system through bad connectors; a low-pass filter belongs on the transmitter output, and a preamplifier would amplify the interference along with the signal.
Cable trouble? Check the cable first. Tighten the connectors before you reach for a filter.