Horizontal Lines on display when cable feed is connected

[thetransporter]

here is the diagram

cable provider and satellite feed go into diplexer to use one cable

satellite receiver > to a 20 inch sony commercial crt (used a thick commercial rca-bnc cable at the tv end)

cable receiver > to a 20 inch sony commercial crt (used a thick commercial rca-bnc cable at the tv end)

both receivers & CRT display are 3 prong (so more grounding at that end)

when there is no cable provider feed it the lines go away
everything is heavily grounded. same results with or without drop amplifier

do i need some sort of notch filter to filter out those lines/noise?

[A790]

It almost sounds like interference.

[revelations]

- try switching ports on the duplexer to isolate a potential bad port
- try a different cable box if the problem started happening recently

[thetransporter]

^ it happens even with out cable box connected just when the cable provider comes into the diplexer


i tried commercial diplexers as well.

this not important issue at all just curious why that happens.

thank you all