[drh4683]

Stiff tuner is a problem in early production concentric tuners. The 1968 models used an all plastic VHF channel drum, which has a tapered plastic shaft as part of the mold in the center. The UHF drum is also plastic and fits into the VHF drum. The plastic basically deforms, either the UHF drum shrinks, or the VHF drum shaft expands causing the two to lock up. Lubrication will not cure it. You will need to take it apart and use emrey paper to sand the tapered shaft on the VHF tuner. You are only removing a few thousandths of an inch of plastic, just enough to make the drums able to turn freely again. You want no stress on the dial, it should spin free before you put it back together. You will also notice that you will need to add an additional washer to gain clearance between the VHF and UHF drum as the spring tension on the top of the UHF tuner caused enough stress in the center of the VHF drum which ends up deforming the VHF drum. This then caused the tuner to no longer contact the thrust washer in the center and the outside edge of the UHF drum would then come in contact with the VHF drum, causing additional drag (and wear).
Its certainly not the best tuner design around, Zenith could have done much better. I've taken many of them apart, so I am very critical of the design. However, By 1969, Zenith must have realized the design problem (considering an improvement was made only 1 year later, most likely caused by heat from the tubes causing deformation to the plastic components) as they incorporated a brass sleeve insert for inside the UHF drum. This solved the stuck tuner drum problem, but the spring tension on top still caused the VHF drum to felx enough to still contact the OD of the UHF drum. That alone still caused enough drag to make the tuner difficult to operate.
It might sound more complex than it really is, you will see it all when you take it apart.