[Jeffhs]

The K731 is a very fine example of Zenith craftsmanship. I have one myself, also an ebay score a couple years ago, which is the second in my collection of vintage/antique Zenith radios.

The K731 was introduced in the late 1950s (1959 was the first year it was offered for sale in this country, I think, although the tube layout chart on the bottom of the cabinet states the radio was patented in Canada as far back as 1946). It was offered to the public in two different cabinet styles; the Early American walnut cabinet and a later style, offered in the mid-'60s, same type of wood only with short legs (this style has been likened to a small consolette TV). Yours is probably the late '60s model, judging from the fact that it has the short legs. However, your set also has the CD (Conelrad emergency broadcast) icons at 640 and 1240 on the AM dial scale, which could place it in the early '60s (Conelrad was in use in this country until 1963).

The FM sensitivity of the K731, using the line-cord antenna, is adequate for local to possibly far suburban reception areas, but I wouldn't count on good reception in fringe areas without a better antenna (a T-dipole such as Sandy mentioned would work for everything but fringe reception). I live in a small town some 35 miles from Cleveland and 40+ miles from the city's FM stations; my 731 gets every major station in town very well (including a couple low-power NPR stations, one of which is a translator for a station 50-odd miles from here; the radio also gets the latter's main signal extremely well) using just the line-cord antenna.

You can find a schematic for your K731 at techpreservation.dyndns.org, a site which has schematics for dozens of vintage and antique radios, as well as from Sams and other sources. The only thing to watch out for with the techpreservation.dyndns.org site is that you will need a special viewer for the diagrams, which are stored there in .djvu format. Not to worry, though; the viewer is available as a free download right at the site. I wouldn't be too concerned about the small print on the schematics (as they appear on your computer monitor) either. They will print out normal size on your printer (at least they did when I downloaded my '731 schematic a few weeks ago).

The sound quality of the K731 is excellent, thanks to its two-way speaker system and heavy walnut cabinet. The radio has a 5x7 oval speaker for lows and midrange, and a smallish (I would guess three inch) electrostatic tweeter for the highs. (The electrostat tweeter is the small green or blue plastic box next to the main speaker; it has a cable with a small connector on it which plugs onto a nearby terminal strip.) The electrostatic tweeter is driven directly from the plate of the 35C5 audio output tube. IMO, this is one of the best-sounding table radios I have ever heard, second only to the Zenith C845 from 1960 (which I also have in my collection).

I was not aware that the '731's knobs are captivated to the cabinet. Because this radio has one side of its chassis connected to the AC line, such an arrangement is necessary to protect the user from being shocked if the radio happens to have been plugged in such that the hot side of the line is connected to the chassis. Since the K731 did not have a polarized line cord plug (one prong wider than the other, so that the plug could be inserted in the outlet only one way), there was exactly a 50-50 chance that the user would plug the cord into the socket such that the chassis was hot (or grounded through the power line). The polarized plug removes the guesswork and offers nearly 100-percent insurance against shock hazard, as it can only be inserted in the outlet such that the chassis of the device with which it is used is grounded. In addition to installing a line fuse in this receiver (as was mentioned in another post), I would also install a polarized line cord for 100 percent safety.

The dial pointer on your K-731 does not necessarily have to be replaced, as you mentioned. The pointer is just a bit crooked, that's all. At best, all that's needed is to straighten it on the dial cord, but this could be something of a job as the entire chassis will have to be pulled to get at the dial drive. The dial pointer on my K731 is also slightly off kilter, but since the pointer still moves without binding, I haven't done anything with it yet.

Enjoy your "new" K-731. As I said, this radio was one of Zenith's best table-model sets in the '50s and early '60s, second only to the C-845; they don't make them like that anymore. I've been a Zenith fan for years, which is why I started my Zenith radio collection some four years ago with a 1951 model H511. In the '70s I also had several Zenith TV sets, the best of which, IMO, was a 23" b&w console from 1963 I rescued from a curb in my hometown in the late 1960s--the tubes were missing, but once I retubed it, the set worked amazingly well. I also found a 1969 Zenith Space Command "300" 19" b&w portable that worked exceedingly well [except for the remote control; the hand unit was missing] right off the bat as soon as I got it home and for an entire year after that, until the horizontal output tube went gassy and threw the keyed AGC system way out of gear. I also had a Zenith 17" portable, 1959 vintage, I got at a rummage sale; it worked for quite a while, no problems and a great picture, until the horizontal sync went bad (I think the AFC circuit went out to lunch and never came back) a few years later. I had a 1949 Zenith bakelite AM/FM receiver 25 or so years ago which I bought at a thrift store. The only problem it had, believe it or not, was an open fusible resistor under the chassis. I replaced the part, plugged in the cord, threw the switch, and the radio immediately began to play, no hum, no distortion, just lots of stations on AM and FM all across the dials. The fact that there was no hum surprised me, as this receiver was some 31 years old when I got it; normally, in a radio of that age, the filter caps are either starting to or already have dried out, or are shorted. A shorted filter cap will cause the radio to blow a fuse as soon as it is plugged in (no need to turn on the switch), as the filters are in a position to short the power line to ground if the caps themselves are shorted. The same thing could happen if the rectifier tube or selenium rectifier shorts, as these components will also short the power line directly to ground if they are themselves shorted. Another thing to watch with seleniums is that, if they short, they can and often do release a gas that smells like rotten eggs. If you have a radio, TV or any other type of electronic gear with seleniums and you smell this, pull the plug immediately and replace the seleniums at once (or ASAP) with silicon diodes (use a series resistor with silicons to drop the output voltage to that of the original selemium stacks).

The gas released by shorted seleniums can be toxic as well. I would replace selenium rectifiers with silicons as a matter of course when restoring any type of antique or vintage gear, as seleniums become unreliable after 30 or forty years (actually, by the time they are four decades old or more these things have outlived their usefulness and should be replaced anyhow, even if they still work). If you use a radio or TV with old seleniums (before you get around to replacing them), don't leave the unit plugged in when not in use, as seleniums can also present a shock and/or fire hazard if they short. Be careful around the terminal strip to which the lead from the electrostatic tweeter of your K731 connects as well, as the terminal carries the entire plate voltage (B+) of the 35C5.