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I reviewed the schematic. There is not horizontal retrace blanking only vertical blanking. This means that a negative going pulse is accompanying the video to cause the line.
I see that there is a common Horizontal sweep- video amplifier 100uF electrolytic across the +260v B+ line. I would put a scope probe across it to ground and see if the if a negative pulse dip is be coupled back to the video amp plate circuit.
It could be the pulse is picked up by the antenna. If you switch the rf modulator to another channel, does the line go away?
Look at the sync pulses of the set top box. If you try another set top box, does that remove or subdue the line?
As to experiment to remove it, I might try wrapping a few turns of one end of an insulated wire around one of wires leading to the horizontal deflection coil leads and connecting the other to the CRT grid. This would effectively couple the retrace pulse to the grid to cutoff the tube. This may take a bit of experimentation and polarity is important so you would have to try each lead.
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