Zenith K731, My first post

This is my first post so I look forward to hearing back from you guys.

I am expecting to receive in the mail a Zenith K731 that I won on Ebay. The owner says the AM band works great but the FM band does not work. I am hoping the problem is only one or two tubes need replacing and then radio will then purr like a kitten (figuratively speaking ). We will see.

This is my first “vintage” radio and I look forward to having it. I have 3 decent solid state SW radios that I listen to often, but have long liked “older” things (cars, tools, books, firearms, coins) so I thought “why not radios?”

What do you guys suggest I do as far as maintenance to this radio when it arrives? Of course, I want the FM working so I’ll have to figure out how to test/buy tubes. I read where the first step is just to make sure all the pins on the tubes are clean. And then, if a tube stays cold during operation, obviously it needs replacing. If they are all hot, and still no FM, well, more diagnosing is needed on my part.

I am a Mechanical Engineer by education, so I have no fear of taking mechanisms apart. This has gotten me into trouble once or twice when it was harder to get something back together than I expected. HaHa!

Usually, when I get a new vintage item, I take it apart, clean it, and then reassemble it. Is this the same for radios? Anything that is definitely a “No-No” with radios such as these?

I thought that I would remove the radio chassis (if possible), clean the dust and stuff out of it, try to clean the switches, knobs, finish, etc.

Where do I find a more detailed schematic for this radio, rather than the one on the bottom of the wooden frame? Seems like this would be needed if more than tubes need replacing.

I remember the great old radio my Grandparents used to listen to (I wish I had that one) with fond memories and hope this one, which cost me $32 shipped, gives me a lot of enjoyment too.

Take care and I look forward to hearing from you.

Brad

[Chad Hauris]

Brad, it is probably a weak 12AT7 or 12DT8 tube in the FM tuner that is causing the problem. This radio has the tube filaments in a series circuit so if one actually burns out none of the tubes will light.

It is possible though for a tube to light up but for it to be worn out and have poor emission. I have seen this on several of these sets with weak FM.

Be careful as the chassis is connected right to the power line. Do not try to pull the knobs off as they have safety guards so people cannot touch the live control shafts. You will need to remove the chassis bolts, slightly pull on the knobs to loosen them, and pull the chassis away from the knobs. Make sure the chassis bolts go into the plastic insulators only and do not contact the live chassis.

Some maintenance issues for possible replacement may be the electrolytic capacitor...usually on this late model of radio the cap is still in pretty good condition but may begin to degrade, causing hum.
Also old radios with paper capacitors should have those replaced but in this Zenith they are probably ceramic and mylar and are likely OK. Another part that can degrade is the selenium rectifier but I think this set has a silicon diode.

[Blooze]

Hey Brad, I hope you don't mind me jumping in on your thread, but I just picked up a K731 tonight from a guy and this is also my first radio restore. Plugged it in and everything seems to work, but definitely some hum to it. I was told these had the selenium rectifiers in them as well as paper caps, so I'll need to check further. Took the back off and took a quick look just to see if all the tubes are lit.

One thing I noticed was that the dial indicator bar was crooked and it was about an inch off on the dial for any station. It looks like the indicator had a piece on the back of it that kept it on the tuning wire, but has been broken off. Does anyone know how I need to go about fixing and/or adjusting it?

It looked like there was just the one electrolytic cap--big ole silver job. Is this correct?

And last but not least, a schematic would be nice. Where to get one?

Shane

Chad - Thanks for the initial pointers and tips. Once the radio gets here I'll gets here I 'll post some pictures and many more questions.

Shane - Keep asking questions, the more you ask the more I learn too. Did you get the modern walnut case or the "Americana" case, which is more traditional, I believe. Do these radios have AM antenna leads on back, or is it built in. I think they do have FM leads though, correct?

Brad

[Blooze]

I have the traditional walnut on 4 little legs. The outside casing and grill cloth are in excellent shape--almost new looking.

I'm going to need a new power cord too. This one is cracked in places. Going to have to try and find one? And the indicator bar definitely needs replaced like I said.

Mine has what looks like a loop antenna on the inside of the back panel for the AM I'm assuming. Then it has two connections for a FM antenna on the outside. I've seen pictures of the backs of a couple of these and they were different than mine, but I'm thinking maybe it's because mine has that loop antenna on the inside. One person had the power cord clamp attached to one of the two connectors, but I don't know if this is correct procedure for this radio or not.

I'll pop the back maybe over my lunch hour today and take some preliminary shots for documantation.

Shane

Chad - The "live" chassis design certainly wouldn't fly in today’s litigious society, would it?

What do you recommend I use for a FM antenna? A simple 300 ohm FM Antenna that Radio Shack carried would probably work OK.

[Sandy G]

If that's the translucent plastic "T" antenna, Yeah, that oughta work fine. I try to mount mine close to a window & hide 'em kinda behind the drapes. If you are out in the country, don't expect this little guy to do too well on FM- but if you are in, or close to, a town, you should be OK.

[Blooze]

On the safety issue side, would it be prudent to put , say, a 1/2 A fuse in line with the AC coming in?

Also, what is a good way to put a new power cord on a setup like the K731 has? Just bring a new cord in through a grommett and solder directly? Kind of eliminates the safety factor they have built into this amp, though (as it unplugs the AC if you remove the back).

Here are a few pics. The one of the dial indicator came out lousy, but you can see how it points off to the left (the right from the front).

Shane

[Tom Bavis]

This radio can be found in Sams Photofact folder 653-12. I'd be glad to make copies... but I don't have it. There are a number of online sources for a copy (maybe $5), and many libraries have them - your local branch can probably get it for the cost of copies.

Zenith was using mostly film and ceramic caps by 1963, but there may be a few striped "black beauty" caps - change 'em! The can is a multi-section cap - you can disconnect it and replace with separate caps under chassis. The selenium rectifier is the square finned item to the left in the rear picture - good to change it too. The line cord clamp is used to couple to it as an FM antenna - can't compare to an external one. New line cords are available from Antique Electronic supply, probably from MCM and others too. Look for "cheater" cords.

[Chad Hauris]

Yes, a 1/2 amp fuse is a good idea. You can hook up an in-line fuse holder into one of the power line terminals.
The clip on the power cord acts as a capacitor to couple FM signals from the power line. It should be connected to the "F" terminal.

The set has safety features such as the interlock cord, insulated bushings for the chassis bolts and safety guards on the knobs so it's pretty safe when these features are in place. I would make sure it always has its back firmly attached when operating it.

[Fisherdude]

The K-731 uses the AC cord as its antenna. You can see the white and green wires attached to the metal coupling around the cord.

[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.

[Fisherdude]

And another fine example of a post that results in click & drag with the mouse, paste into Word, "File--Save As", and more incredible info added to the archives.

thanks, Jeff!

I love this place!

I agree with Fisherdude's staement 100%!