The Wonderful Zenith K731

[Paula]

The Wonderful Zenith K731

The first picture below is of a Zenith model K731 radio that I acquired several months ago. It is an AM/FM receiver with seven tubes, a luxurious walnut veneer cabinet (with solid walnut trim), a large speaker, and a seperate electrostatic tweeter.

With the K731, Zenith had reached the pinnacle of performance for a mass-produced, American-made, tube-type table radio. The handwriting was already on the wall for tube audio, and the economics of high-volume, injection-molded plastic cabinetry had all-but-rendered the solid wood cabinet financially impractical for mass-produced radios.

I've always loved this particular radio, as it is the exact same model that my dad gave to my mom for Christmas around 1965. These radios are still fairly common and easy to find. The one I found was in very good shape, requiring only minor work to make it perfect. Besides a good cleaning, inside and out, I made some repairs to the grille cloth, revived the finish by giving it "The Formby's Treatment", replaced all of the paper capacitors, replaced the selenium rectifier with a silicone rectifier, and added a 1/2 amp in-line fuse.

The second picture below shows the end result of the electrical restoration. Notice that the original filter cap was left intact, and that the new electrolytics, as well as the silicone rectifier were securely attached to solder-lug strips. The nifty powder-blue, 2-watt resistor replaces the original "dropping resistor" to compensate for the lower voltage drop of the new rectifier.

The Zenith now looks and works like new, with unbelievable tuning sensitivity and selectivity on both AM and FM. I listen to this radio for hours on end, and it never wanders off the station. With the large speaker, separate tweeter, and hefty wooden cabinet, the sound is rich and full.

The K731 came in two cabinet styles: the more modern style with legs, like mine, and an Early-American style, shown in the third picture. I think that I actually prefer the looks of the Early-American style, but it does not have the same sentimental value as mine.

Paula

[Celt]

very nice! zenith were definately a cut above the rest.

[Jeffhs]

Paula,

Both your Zenith 731s look great. I too have the Early American model, which works very well for having been made over 40 years ago (mine was introduced in the early '60s). I like the looks of the cabinet and the sound (this is the first table radio I have ever seen in 30-some years of electronics experimenting that has an electrostatic tweeter), which is why I bid on it on ebay last year.

My set worked as soon as I got it here--didn't have to do a thing with it or to it (not even changing the filters), just plugged it in and turned it on. I like the defeatable AFC because there is at least one FM station in Cleveland which is drowned out by another station just 200 kHz away in the next town east of me--I live between two cities (the Cleveland station is on 104.9 MHz; the one in the next town east of here is at 104.7). The 104.9 station is inaudible on my bookshelf stereo and almost every other FM radio I own because most of my sets, including my stereo, have full-time AFC, but the Zenith will bring in the station just fine if I put the band switch on FM. The amazing thing is that I get every station in Cleveland loud and clear on the '731 just using the line cord antenna (I live in a small town some 45 miles from most of the Cleveland FM stations) whereas I must use an amplified indoor antenna on my stereo to get the same results, but I chalk that up to the set's 19T8 FM RF amplifier.

I almost hate to think how many stations this thing will bring in when the FM band opens up to Detroit, Toledo, Ohio and southwestern Ontario, Canada this summer! Those are the areas I usually get out-of-town FM from in the spring and summer, although I have had reception under very unusual conditions from as far away as Florida (I once heard an FM station from West Palm Beach on 107.9 when one of the Cleveland stations was knocked off the air during a thunderstorm some 35 years ago).

I don't know how far you are from Indianapolis (you say you live in northeastern Indiana which would probably spot you a lot closer to Fort Wayne, though if you can get AM radio from Cincinnati, maybe you are closer to Indy at that), but if you're in the metropolitan area you probably will be able to hear the sound from that city's WFBM-TV on Channel 6. Most FM radios will tune in the sound carrier from local channel 6 stations because the FM sound carrier frequency is 4.5 MHz above the lower edge of the channel (channel 6 is 82-88 MHz). This puts the sound carrier for channel 6 at roughly 86.5 MHz. Most FM radios will tune slightly above and below the upper and lower limits of the band so that you'll be able to hear the audio carriers of any channel 6 TV station within range.

[Paula]

Hi Jeff,

Actually, I live in southeastern Indiana, roughly midway between Indianapolis and Cincinnati (OH). Yes, I have picked up Channel 6 on various FM radios before. It kind of shocked me the first time I heard it!

I'm in the process of restoring a Zenith 7H820 AM/FM receiver from around 1949. This radio has the "famous" Dialspeaker (patent No. 2272660) -- a combination tuning dial and speaker in a single unit -- that Zenith incorporated into many of their radios of the era. More notable is the fact that a few of the early versions of this particular radio were equipped with a second FM setting that covered the 42-50 mHz frequency band that was originally allocated to FM, but subsequently reallocated to to TV. Presumably, one could pick up numerous TV audio signals with this radio. One of these came up on eBay not long ago (unrestored), but when the bidding topped $85, I said "No thanks."

In my previous post, I didn't mean to imply that I owned the Early American style K731 -- that picture is from an ad on eBay. It was an exceptionally nice radio, but with an opening bid of $50, I think that buyers got scared away. I believe it went unsold, tho' I now kind of wish I'd grabbed it.

Below is a picture of the inside of that radio, showing the impressive speaker "array". I think it compares favorably with the Grundig 3068 pictured below it. The Grundig goes the Zenith one better by flanking the front speakers with side drivers, for improved dispersion.

Paula

[Jeffhs]

Originally posted by Paula
Hi Jeff,

Actually, I live in southeastern Indiana, roughly midway between Indianapolis and Cincinnati (OH). Yes, I have picked up Channel 6 on various FM radios before. It kind of shocked me the first time I heard it!

I'm in the process of restoring a Zenith 7H820 AM/FM receiver from around 1949. This radio has the "famous" Dialspeaker (patent No. 2272660) -- a combination tuning dial and speaker in a single unit -- that Zenith incorporated into many of their radios of the era. More notable is the fact that a few of the early versions of this particular radio were equipped with a second FM setting that covered the 42-50 mHz frequency band that was originally allocated to FM, but subsequently reallocated to to TV. Presumably, one could pick up numerous TV audio signals with this radio. One of these came up on eBay not long ago (unrestored), but when the bidding topped $85, I said "No thanks."

In my previous post, I didn't mean to imply that I owned the Early American style K731 -- that picture is from an ad on eBay. It was an exceptionally nice radio, but with an opening bid of $50, I think that buyers got scared away. I believe it went unsold, tho' I now kind of wish I'd grabbed it.

Below is a picture of the inside of that radio, showing the impressive speaker "array". I think it compares favorably with the Grundig 3068 pictured below it. The Grundig goes the Zenith one better by flanking the front speakers with side drivers, for improved dispersion.

Paula

I'm not sure, but I think the Grundig set may have been designed for stereo, hence the two speakers at either side of the cabinet. I had a Grundig model 2168 in the mid-70s which had only one speaker--the main one mounted above the tuning dial.

I didn't realize you had grabbed the photo of the Early American version of the 731 from ebay. It's basically identical to the one you have, except for the cabinet, of course (and the fact that your set may have been made a few years later than mine; I read in an item description on ebay for one of these sets that the cabinet with legs was used with later models of the 731, namely, the sets made from 1963 until the end of the production run). If this is true, then the Early American version of this radio may be the earlier model, manufactured in the late '50s. I tend to think my own 731, with the Early American cabinet, however, must have been made in 1963 or later, since the AM tuning dial does not have the Civil Defense icons at 640 and 1240 KHz.

An interesting thing about the K731 series is that the same basic chassis (7K06 and 7K07) was used in several different cabinets under different model numbers. Zenith's model K725B, for example, which used the 7K06 chassis, was in a plastic cabinet and did have the CD icons on the AM dial, so it must have been made before '63 (the year the Civil Defense, also known as Conelrad, emergency broadcast network was abolished and replaced by EBS, the Emergency Broadcast System). The publication date on the Sams I have for my set (it shows the K725B on the first page), however, is 1963. Did the K731 (chassis 7K/7M07) come out before or after that year, with the same Photofact covering every model in the series?

BTW, the 42-50 MHz band found on early postwar Zenith AM/FM radios was later allocated to TV as channel 1 in the late '40s. When channel 1 was abolished in 1950 or so, its frequencies were again reallocated, this time to amateur or "ham" radio as the six-meter band (which itself was later realigned and today is 50-54 MHz, the top end of which is the beginning of the frequency allocation for TV channel 2, 54-60 MHz.)

The standard FM broadcast band we know today at 88-108 MHz was allocated in the '50s. The very first FM broadcast station in Cleveland, Ohio, went on the air in the '40s, I believe, as WBOE-FM and was operated by the Cleveland Board of Education. It operated in the old 40-MHz FM band at the time, 43 MHz, I think. The station is now known as WCPN-FM on 90.3 MHz and is an affiliate of the National Public Radio network, or NPR. The station also has ties to Cleveland's PBS television station, WVIZ on channel 25 (and three translators serving outlying areas of northeastern Ohio) through an organization known as Ideastream.

BTW (2): The small white dot near 100 MHz on many Zenith radios of 1950s-'60s vintage (including the 731 and its offshoots, among others too numerous to mention here) was originally used to mark the frequency, 99.5 MHz, of Zenith's owned FM station, WENR-FM in Chicago.

The marking is obsolete today, as Zenith sold the station many years ago. The station now at 99.5 in Chicago, IIRC, is country WUSN-FM, "US-99.5".

[Paula]

Hi Jeff,

My, you are a font of information! As far as I can recall, the legged K731 that my mom had was purchased in '64 or '65. I have a very vague recollection that the Early American style was also available when she got hers. But I could be wrong about that.

Thanks for your very informative post!

Paula

Sunday, 6 April 2008

Hi Paula,

I have a radio that looks exactly like your wonderful Zenith K731 and has the same model number. I got it as a birthday present when I was in college around 1966 or 1967. It still works and I still use it but the sound sometimes fades in and out.

Someone else with the same radio described a similar problem with fading in and out and suggested it may need cleaning. Since mine has not been cleaned or serviced since I got it 40 years ago, perhaps I should have it serviced.

It was fun to see your radio and I hope it is working well for you.

Sincerely.
Leah

[anden]

The whole generation of 7 & 8 tube Zeniths from the mid 50's through about 1967 are indeed, use Paula's word, 'wonderful', very ballanced and musical in their audio. Sad to say it was the end of the famous Zenith tube radio with point to point wiring, but this method did continue for a few more years in TV's....good habbits sometimes die hard!

I just found a 1955 Y832 for $10.50 and was rather amazed at the excellent sound quality coming from the 8 inch speaker, plus the two 2 5/8 inch square electrostatics.

To get better sound, one would have to move up to a KLH model eight from 1960-64

[ChiefRider]

My dad was a big Zenith fan- He bought a new T-O 1000 in '59, a Royal 50 in '61, and the Colonial style 731 around '63. When it was brand new, I remember he would listen to opera performances on the 731. I assumed posession of it when I entered high school, and had it on all the time. In fact, I remember listening to Howard Stern when he was an unknown local DJ in Hartford, CT. I still have this radio, but haven't used it in years. I figure the selenium rectifier should be replaced, and probably capacitors, too. I haven't done any electronics work in years, so I haven't dug into it. Seeing, this, I'd sure like to bring it back to daily use!

Chief

[Jeffhs]

Originally Posted by ChiefRider

My dad was a big Zenith fan- He bought a new T-O 1000 in '59, a Royal 50 in '61, and the Colonial style 731 around '63. When it was brand new, I remember he would listen to opera performances on the 731. I assumed posession of it when I entered high school, and had it on all the time. In fact, I remember listening to Howard Stern when he was an unknown local DJ in Hartford, CT. I still have this radio, but haven't used it in years. I figure the selenium rectifier should be replaced, and probably capacitors, too. I haven't done any electronics work in years, so I haven't dug into it. Seeing, this, I'd sure like to bring it back to daily use!

Chief

Any antique or vintage radio should be recapped as a matter of routine, as capacitors deteriorate over time. The three-section electrolytic in the power supply in particular probably should be replaced if the radio still has the original, even if the radio exhibits no hum or other problems. These electrolytics can short without warning; when they do short, the radio will blow the house fuses as soon as it is plugged in (doesn't have to be on) because the filters are in a position to short the power line directly to ground when they go bad (short). Dried-out electrolytics will produce a loud 60-Hz hum in the speaker; the radio may still work until the capacitor eventually shorts, but the sound will be terribly distorted.

As to the selenium rectifier, this can and should be replaced with a silicon diode as soon as possible. Seleniums will give off a gas that smells of rotten eggs when they short, so you will know when it goes! (This gas is toxic as well.) The job is not difficult and shouldn't take more than a few minutes; however, a dropping resistor must be used in series with the diode so that the output voltage will match that of the original selenium stack.

[greenmark59]

I love this site. I just bought a K731 Early American style at a thrift store for $15 and I love the sound this radio puts out. I first checked it out before buying it (it works GREAT!) and I must say it is a great piece of history - especially after reading all the comments in this thread. I think it's going to be a keeper.

[wa2ise]

In my B730R (very similar to the K731), I redid the rectifier with a bridge.

I replaced the selenium rectifier in one AM/FM set with a bridge rectifier. Full wave rectification, less power supply hum (120Hz vs 60Hz). The heater string runs directly off the powerline (via power switch). The mid point of the heater string will look to have minimal AC waveform in reference to the B- line of the bridge rectifier. But will have a DC bias of about half the B+ voltage. This is fine, cathodes like their heaters to have a positive bias. One end of the audio output tube (for example a 50C5) (#8 in the diagram) heater is probably already connected to one of the AC lines. Disconnect the heater line of the other audio output heater pin. This line, now disconnected (feeding to tube #7), now will be connected to the other AC line. See diagram. And at the old ground end of the heater string, usually the AM or FM detector/ audio driver tube (#1) (19T8 for example) disconnect from ground and connect to the now loose end of the audio output tube heater. This should minimize hum pickup from the heater line. A small cap of around 0.1uF 400V or more tied close to the heater string midpoint should help reduce the AC waveform some more by holding a bias charge between the heater string and the B- ground of the radio during the time the bridge rectifier isn't conducting. The bridge diodes only conduct at the peaks of the AC powerline waveform.



Pay special attention to the AC line lead dress around the volume control power switch for hum pickup. Hot chassis radios usually switched the local ground feed line to avoid hum pickup from 120VAC to local ground wires. This could ruin the advantage of full wave rectification (less hum, 120Hz vs 60Hz) if not taken care of. Look for trouble by listening to silent passages in radio programming with the volume control at half setting (hum pickup can be more severe at this position). Shielding may be needed.

[myrgatroyd]

Hi, Paula,

it´s interesting to follow the history of radios in america (seen from here). Nice things have you got!

You mentioned a Grundig, there was short discussion about stereo. Perhaps you are happy to know you were right:

The most stereo-tubes in europe used two big speaker, placed in the front. (Exceptions can happen).
You are right about the dispersion. This is called "3d", used to fill the room with sound. The tweeters or midrange speakers were mounted on the side of the radio. Used from ca. 1955/56. Amp stages: Often singled ended EL84, some push-pull EL84, expensive models were bi-amped.
First stereo gear was built in 59/60, only amp stereo, no possibility to stereo reception.
Not the number of tubes tells if a radio is stereo or not. Count output transformers. By the way: Stereo was a new gag this days, so the word stereo was placed in prominent places on the radio.

[Exotica King]

The moment I heard music coming from my just acquired K731, I was wowed! The sound coming from such a old, small radio was phenominal. No cleaning, no servicing, just plugged it in and tuned in a classical music station.

They knew how to build 'em back then. My swap meet find only needs a little oiling of the cabinet and a slight dusting inside. BTW, I found the picture of the electronic clean-up you did very interesting. While the radio works great as it is, I would like to replace the old caps like you did. I'm pretty handy with electonics but have never serviced an old tube radio. Would you be able to give me details of the components you replaced? I wouls really like to replace the old, possibly dangerous components so I can start enjoying my radio.

[DaveWM]

my favorite zenith table top is the 8 tube version of that chassis. think its the H835 or the Y835 or something like that. there are several cabinet styles, but what makes it great is the use of a triode for the AFC on FM, it really locks on to a station (seems better than the variactor diode used on the 7 tube chassis).