The 2 SS sets usually will have 75 ohm coax direct to the tuners.
The Zenith has 300 ohm twin lead then a balun to bring it to 75 ohms
at the tuner. So you can have a few feet of twin lead to pick up
trash. A few other ideas... One of the other sets has AC leakage,
the matching transformer is bad. The RF amp, 6HA5 has H-K leakage,
if you try one get the short version.
Try the set with a generator & everything else removed.
73 Zeno
LFOD !