How long did Zenith use PTOs in table radios?

[Jeffhs]

How long did Zenith use PTOs (permeability tuned oscillators) in their radios? I have three such radios with just this type of tuning system (no variable capacitor for FM, but there is a small variable cap for AM). I read here in VK that the company was using PTOs as late as the K-731 series in the late '50s until about 1965.

[dieseljeep]

Originally Posted by Jeffhs

How long did Zenith use PTOs (permeability tuned oscillators) in their radios? I have three such radios with just this type of tuning system (no variable capacitor for FM, but there is a small variable cap for AM). I read here in VK that the company was using PTOs as late as the K-731 series in the late '50s until about 1965.

IIRC, from the very beginning of Zenith AM-FM table radios, at least the tube sets. Post-war to the "X" models.

[Jeffhs]

Originally Posted by dieseljeep

IIRC, from the very beginning of Zenith AM-FM table radios, at least the tube sets. Post-war to the "X" models.

Thanks for the prompt reply.

As I mentioned in my post, I have three Zenith radios with PTOs: C-845, MJ-1035 and K-731. Where is the PTO in the K-731? My other Zeniths have it in plain sight on the chassis, while it seems to be hidden in the K-731. Maybe it is under the chassis?

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).

I'm somewhat surprised that Zenith broke with tradition starting with the "X" series. The only thing I can think of is that, by the time the "X" series was introduced, Zenith was trying to make their radios of lesser quality (read cheaper) than the earlier ones.

I don't know offhand when the "X" series came along (I'm guessing late sixties), but whenever it was, that must have been the start of Zenith's cost-cutting era. It is rather difficult for me to believe that a radio (and later TV) manufacturer whose name had been synonymous with quality for generations would break such a tradition by using a cheaper method of tuning (not to mention other production shortcuts, such as PC boards, in its "Royal" series of transistor portable radios from the Royal 500, et al. in the '50s through the R-series of the early eighties), but times do change, and I guess the '80s was the beginning of the cost-cutting era for Zenith. My R-70 11-transistor AM-FM portable, for example, uses a plastic variable capacitor for AM and FM tuning (however, I'm darned if I know how those little things work, since they do not seem to have metal plates), but the radio has two IF stages for AM and four IFs for FM and still works very well, so this one must have been one of the last small "good" portables made by Zenith of Korea; the R-70, and probably other Zenith radios in the R-series, have a heat-stamped notice on their back covers stating the sets were built to the standards of the Zenith Radio Corporation by the company's subsidiary in that country at the time. In the early eighties, I owned a Zenith four-mode integrated stereo system which was also manufactured by Zenith of Korea, so the manufacture of that company's radios and stereo gear in that country must have still been going on, and continued until the company's complete demise.

The quality of Zenith radios, TVs and so on has, unfortunately, gone downhill since--until the company, along with its famous lightning-bolt logo and illuminated "Z" crest, the latter used as a pilot light on many of the company's table radios of the '50s, disappeared in the late '80s, never to be seen or heard from again. A darn shame, as Zenith was for decades my all-time favorite brand of home-entertainment gear. I still have several antique Zenith radios here, all made in the '50s to the '60s (and my R-70, which was, IMO, the last really good AM/FM portable to bear the Zenith name), which I intend to hold on to because, as I have said many times, they don't make them like that anymore.

[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

[Jeffhs]

I found an under-chassis photo of a K-731 radio (in an eBay listing). The PTO is, in fact, under the chassis, as I had suspected. I also remembered (just now) that, in a thread I saw a while ago dealing with these radios (in a newsgroup, not in VK), there was a warning against moving or otherwise disturbing the lead dress or component placement in a small box under the chassis; if any of this is disturbed, the radio can be thrown far out of alignment, and it will take quite a while to restore it. That "small box" is probably, even likely, the PTO.

The warning makes sense and should be heeded, since the PTO controls the FM tuning. It is possible to throw the alignment so far off simply by moving a component inside the PTO slightly (as in inches or fractions thereof) that the radio's tuning range can and in many cases will be reduced at one end of the dial and expanded somewhat beyond 108 MHz at the other, or worse; the only way to restore the correct tuning range, of course, will be to realign the radio's front end.

I don't know how easy or how difficult the alignment procedure is for the K-731, and I darn well don't want to find out. My set, even after 52+ years, works and sounds great as it is, so I am keeping my hands the heck off the chassis unless I have to change a tube or perform some other service or maintenance procedure. In the meantime I will just enjoy the radio as it is. I see no reason to do anything to it at this point. Why mess with it when it is working so well? This radio pulls in lots of stations on both AM and FM, even in my near-fringe area for Cleveland FM, and sounds wonderful as well.

I don't know how many of these radios (besides mine) are still in use, but the ones which are still in active service are positive proof of the quality of the older Zenith AM-FM sets. Too darn bad the company went completely out of business by the 1990s. The world lost what was one of the best radio and TV manufacturers then. We will never again (unfortunately) see the likes of the original Zenith Radio Corporation of Chicago.

[jr_tech]

In the FM pto vs tuning cap debate, I don't believe that there was a clear winner... tuning caps were indeed used in many very high quality tube type FM tuners. For example here is a description of the tuning cap used in the Marantz 10b, considered by many to finest tube type FM tuner ever designed:

The tuning capacitor was a Hammarlund 8 gang, silver plated, push pull design. It balances out circulating currents and minimizes cross coupling between stages. It also cuts down on common mode feedback. It is calibrated at ten different points. It is also designed to make the radio dial linear. It is the most expensive part in the tuner and you couldn't afford it today.

https://forum.audiogon.com/discussio...und-so-amazing
jr