How long did Zenith use PTOs in table radios?

[AdamAnt316]

Originally Posted by Jeffhs

What is the advantage of using a PTO as opposed to a tuning capacitor? Since so many Zenith radios had them, they must have been far superior to the ordinary two- or three-gang tuning caps used in other makes of FM radios, even Zenith's own (from the X series on).

Not sure if it's the entire reason, but inductive tuning allows for a more 'linear' tuning scale. If you look at the dial scale of an average AM radio which employs a variable capacitor for tuning, you'll see that there's lots of space between markings at the beginning of the dial (55 | 60 | 70), but as it gets towards the top end of the dial, markings starts getting squashed into a smaller amount of space (100|120|140|170). With inductive/PTO tuning, you generally see a much more evenly-spaced set of numbers on the scale. The R-390 series of military radios used a Collins-designed PTO to allow for about the closest thing to digital tuning that a mechanical tuning system could allow.
-Adam