Working on a Zenith K731

[compucat]

This K731 was a flea market find. This is the first time I have had one of these although I have often read about how good they are. The cabinet is not in bad shape and the set works. There is a slight hum so the electrolytics will need changing. I will probably get rid of the selenium rectifier too. The biggest problem I have is the plastic front bezel is broken at one of the mounting studs so I am thinking about making a new bezel out of wood and staining it to match the cabinet. I know I could easily get another set to rob the bezel from but I don't like plastic cosmetic parts on radios for this very reason. The often get cracked or broken. The sound quality is amazing. It has deep rich tone and plenty of volume. The electrostatic tweeter even works too. I know how to fix the crooked pointer problem on these sets. The pointer attaches to the dial cord at the top and bottom. Straighten the pointer then put a drop of nail polish at both points where the pointer attaches to the string. This set has a very complex dial cord arrangement. I hope it never breaks. I has all of it's original tubes. I think i am the first one to take the back off of this set since it was made. I believe this set to have been made in 1965. Maybe I should sit it on top of my 1965 Zenith roundie color tv. Here are some pics of this great radio.

[compucat]

Here is a closeup of the complex dial cord arrangement.

[Reece]

There is a whole series of Zeniths similar to this from the late '50's through the '60's. I have a couple and they are great performers. If this has lots of disc caps under the chassis you'll probably only need to replace the filters and any paper ones. I always change seleniums even if they work but some others like to keep them. I just don't want to go back in later if I can help it.

[Old1625]

Those are great-sounding radios, and the circuitry is time-tested. By that point Zenith had been using such design in slight variation for many years. Performance is admirable as a rule. I have a mid-'40s Zenith that has roughly the same circuit design.

The style suggests to me some German influence in the day.

Such dial drive scheme can be kind of a pain to restring. The stepping obviously is to lengthen the dial pointer travel length. If you have a Sams on the radio then they will probably give dial restringing details and drawings. Otherwise, be it me, I'd take a few pics from different angles of this drive system with the tuning at one extreme and/or the other. Then count the number of turns of the tuning knob to go from one extreme to the other. Then file such info away for future use should you have to do a restring.

Otherwise Reece gives good advice. If replacing the selenium rectifier is in your plans then substitute a 2.5A 1000PIV diode and a 22 ohm 5W series dropping resistor in its place.

[Jeffhs]

My K-731 is an eBay score from about five years ago. It sounds excellent, the cabinet is in good shape and the sound is typical of any Zenith radio from the '40s through the sixties (mine was made in 1963, probably just after the end of the Conelrad era, as it has no CD icons on the AM dial).

To date I have only had to replace one tube, but then again I don't use the radio that much (not much interesting to listen to on AM except perhaps CFZM "Zoomer Radio" 740 in Toronto; FM isn't much better except for the small stations below 92 MHz).

The circuit design of the K-731 is interesting in that it uses the 12AU6 limiter tube as a third IF amplifier, which explains this set's excellent selectivity. There is no RF amp on AM, but with the (in effect) three-stage IF strip it isn't necessary--this set will pull in stations for hundreds of miles around on both bands, especially at night on AM and when the FM band opens up for DX in spring, summer and early fall. I live in northeastern Ohio, 33 miles east of Cleveland (a far-suburban or even near-fringe area for some AM and FM stations), and my K731 pulls in every station in the city, using only the built-in Wavemagnet antenna, just as well as if I were listening to it in the Cleveland suburb where I grew up.

I also like the looks of the cabinet; my 731 is in the Early American walnut cabinet (no legs). I think the cabinet also contributes to the excellent sound of this radio (the size of the cabinet has an effect on bass response), as does the 5x7 oval speaker and the 3" (+/- an inch or two) electrostatic tweeter. I'm not sure many people would even notice if it were dead; many of these radios were used either for background music or by teenagers on the loudest rock and roll station in town. One way to test the tweeter is to temporarily disconnect the main speaker, but leave the tweeter connected in the circuit. If you hear very shrill sound (almost no bass), the tweeter is working.

I would caution anyone attempting to repair a K-731 (or any other radio or TV, for that matter) to always disconnect the power cord (good practice when working on any type of electrical or electronic device) and discharge the filter cap before any work is begun under the chassis, as these capacitors can hold charges literally for days. In the case of the '731, however, I would add a third caution -- do not touch the terminals to which the electrostatic tweeter is connected while the radio is on. The reason is that these terminals carry the full plate voltage of the 35C5 audio output tube, and therefore are a shock hazard to the uninitiated or uninformed. The K-731 being a transformerless AC/DC radio means there is another potential shock hazard, because the chassis is connected directly to one side of the AC line; depending entirely on which way the AC plug is inserted in the wall socket, the chassis will either carry the full line voltage or will be grounded. It is for this reason that the K731, and most other AC/DC radios (with the notable exception of the Arvin 540T metal-cabinet AA5, et al.) were built with plenty of insulation between the chassis bolts and the chassis itself, plastic knobs and so on. However, if this insulation material breaks down, the chassis bolts will be charged with the line voltage, again creating a shock hazard. The external antenna connection on many older AC/DC AA5s, usually mounted on the back cover, can also be a shock hazard if it is touched, intentionally or otherwise, while the user is in contact with a good ground. There is a blocking capacitor in series with the external antenna input that is supposed to minimize the shock hazard, but in almost all AA5s today these capacitors are defective. Replace the capacitor before attempting to use the external antenna input, or as a matter of routine when servicing a receiver with such a terminal, which will often be a Fahnestock clip. As well, the blocking cap in series with the phonograph input on AA5s and higher-end radios so equipped should be replaced as well before anything is connected to it, otherwise it is an accident waiting to happen.

When repairing an AA5 or any transformerless radio, never take it for granted that the blocking cap is good; replace it as a matter of course. I don't want to read in this forum of any VK member being killed or severely injured because he or she got shocked by line voltage or B+, and the voltage zapped across the heart, or the person was thrown across the room and injured badly. A good rule of thumb is to keep one hand in your pocket while working on a transformerless radio or TV, or any electronic device in which the chassis is directly connected to the line. Better yet, unplug the power cord; this will ensure 200 percent safety, as long as the filter capacitors are discharged to ground before any underchassis work is begun.

[Old1625]

Good advice, Jeffhs. AK-ers that like to delve into old radios are likely acquainted with the shock hazards, but it is still a good idea to be reminded now and then. I've in the past been bitten by AC/DC sets on a lot of occasions--many being of circumstances the most warped mind couldn't imagine. (Owitch! How'd dat happen?!) One cannot be too careful.

A one-to-one ratio isolation transformer is a good thing to have around when working on these chasses. This rids one of one hazard, and leaves just the electrolytics and the tweeter to address.

Isolation transformers of adequate KVA rating should be obtainable through most electronic parts jobbers.