Zenith Case Study

Case Study: Zenith Console

Model 10S464, Chassis #1005, c. 1940

I purchased this Zenith console on eBay for $102. Shipping a console is very expensive and rather impractical, so I made sure this item's location was convenient for me to pick up personally.

In our living room. The radio is 42 inches high.

Description of Model

This is an AC, 10-tube model, including the tuning eye. To view a schematic, click here: Zenith_schematic.PDF (requires Adobe Reader).

The set has three bands: standard AM broadcast, medium wave (the obsolete police band located just above the standard broadcast band), and shortwave (6-18 MHz). The tuning dial is the round, black, aviation-type favored by Zenith.

A tuned RF stage is ahead of the converter tube. This is a sign of a high-quality design.

The audio power amplifier section uses two 6V6 tubes in a push-pull circuit, providing plenty of power to drive the 10-inch electrodynamic speaker. With a push-pull circuit, a 6J5 inverter is used to change the phase of one of the two power tubes.

The rectifier section uses two 6X5 full-wave rectifier tubes. Why use two 6X5s when a single 5Y4 could provide about the same output? Possibly Zenith got a bargain on a load of 6X5s, or perhaps it was an example of "tube padding"--increasing the tube count just to make the set seem more desirable to consumers. Indeed, this same model was manufactured in an 11-tube version, with Chassis 1003 instead of Chassis 1005, which employed two separate tubes for the detector and first audio amp; my version sensibly uses a single 6Q7 twin-diode triode to serve both functions.

There are six tone buttons--not pushbuttons, but "pull-out" buttons, like stops on an organ. (These tone buttons, which are labeled low-bass, bass, alto, treble, voice, and normal, can be used in any combination.) There are six station-preset buttons, shaped like the tone buttons, but the presets push in rather than pull out.

The tone buttons are on the left, the station presets on the right. Note the green tuning eye, which is illuminated. The volume control, tuning knob, and band switch are on a common shaft.

This model has some nice showroom features. The chassis is mounted on springs, which you can sense as you adjust the tuning knob or volume control. Also, a heavy flywheel is mounted on the tuning shaft, giving the tuning knob a luxurious, heavy-duty feel. When the tuning knob is spun, the momentum of the flywheel keeps it spinning slightly. The "wavemagnet" internal antenna can be rotated for peak reception. The chassis is colored in a gold hammertone finish The chassis is remarkably deep, 5 inches, conveying an image of mucho stuff inside.

The finish and proportions of the wood cabinet are pleasing to me. Contrasting wood veneers, including a burl pattern on the front, give the cabinet a rich look. The decorative horizontal treatment over the speaker grill avoids the "juke-box" look of some consoles or the spindly effect of lowboy models. The knobs--volume/on-off, tuning, and band-selector--are mounted on a single shaft, reducing clutter. The escutcheons for the tuner presets and the tone buttons are pot-metal, possibly browned. The bezel for the tuning dial is plastic, but matches the two metal escutcheons. The dialcover lens is glass, not plastic.

In 1940, television was much in the public's mind, but still years away from serious commercialization. This radio has a futuristic, conspicuously labeled TV-RADIO switch on the rear of the chassis, with jacks to plug in a television. Of course, this is just a glorified phono jack as used for playing a turntable through a radio; it appears that marketing often superceded engineering even in 1940.

Original Condition

The wooden cabinet wasn't perfect, but was in reasonably good shape, with all the original knobs and pushbuttons. The decals for the bandswitch positions were original and in nice condition. The tuner presets had station call-letter stickers on them, but the only one that is completely readable is WNAX; that 75-year-old broadcast station is located in Yankton, South Dakota--which fits, since I picked up the radio in Minneapolis. The speaker was intact, with no rips. The grill cloth was original and in good condition. The chassis was quite dusty. Several of the tubes were Zenith brand, and probably original. The power cord looked relatively new, but it had been spliced onto a stub of the original cord, which was worn.

The previous owner, a collectibles dealer, had disclosed that he had tried the radio on 120 V, so there wasn't much reason to bring it up on my variac. The tubes lighted and and weak static could be heard. The tuning knob freewheeled and didn't move the tuning capacitor or tuning needle. The filament in the 6U5 tuning eye was good, but the green, circular indicator was dark.

Removing the chassis, I quickly spotted two obvious issues. The tuning belt was broken and laying underneath the chassis, which explained why the tuning knob freewheeled. More interestingly, one of the two rectifier tube sockets was taped over with cellophane tape. Upon further inspection, I discovered that somebody had rewired the rectifier section to employ a single 5Y4 rather than two 6X5 tubes. I'm not a slave to authenticity, but this mod had to be undone.

In the cabinet, underneath the chassis, I found a small cache of large, striped sunflower seeds. It's not unusual for mice to move into an old radio, and often they will cause severe damage by chewing on wiring and other components. But there was no visible damage, just the sunflower seeds.

All the capacitors seemed original, including the dual-section 20/20-µF filter cap. Some of the wiring was frayed, including a couple of speaker wires that carry the lethal DC plate supply voltage through the speaker field coil for it to serve as a power supply choke.

Electronic Restoration

My first step was to assess what replacement parts to order immediately. I had a good assortment of capacitors, so the only thing I definitely needed was a pair of 6X5 rectifiers ($5.90 each for new-old stock--NOS) to replace the non-conforming 5Y4. Even though the two 6V6 power output tubes seemed to be OK, I decided to splurge and order a pair of these, too, since power tubes see almost as heavy stress as rectifiers. (As I write this, I realize that power tubes in a push-pull configuration do not continuously handle full-power output like single-ended triodes, so they see less stress.) Anyway, 6V6s are currently manufactured in Europe and China and sell for about $10 each; no reason to buy new-old stock for several dollars more, so I ordered a pair of Electro-Harmonix 6V6s (made in Russia).

Rear view, after electronic restoration. The brown, rectangular box left of the speaker is the rotatable "wavemagnet" antenna.

To run the radio outside the cabinet, I had to remove the speaker and antenna so that their plug-in leads would reach their sockets on the chassis. I was careful to place the speaker on my bench in such a way that it would be protected from accidental damage. I gave the chassis a rough dusting using canned air.

Next, I replaced the power cord and plug, using a polarized plug to keep the chassis from being energized and to have the power switch on the hot leg of the 120-V line. I noticed that somebody had installed a fuse, but it would soon be blown, to be replaced temporarily with a jumper.

My priority was to rewire the power supply section to its original state so that it would use two 6X5s, as designed, rather than a single 5Y4 rectifier. But first, I replaced the filter cap, which was physically in the way of the circuit mods that I needed to undo. I got carried away, and replaced the two-section 20/20-µF filter with two 40-µF, 450-V caps. When I turned on the radio (with the 5Y4 rectifier still in place), the fuse blew, presumably because of higher inrush caused by the larger filter caps. After jumpering the fuse, I tried it again, and this time the 5Y4 blew. I therefore replaced the filter caps with smaller ones. For the cap ahead of the speaker field (d.b.a. filter choke), I went back to the original 20-µF size, but for the cap downstream of the speaker coil I used a 33-µF. Plugging in a spare 5Y4, everything held with the smaller caps.

Restoring the butchered rectifier wiring was fairly straightforward--just follow the schematic. The 5Y4 basing scheme is different than a 6X5, so several wires were now on the wrong socket pins and there were a couple of other hard-to-understand wiring changes that had me scratching my head. Simplifying the process, the unused 6X5 socket had been abandoned in place, so its wiring served as a model for restoring the socket that had been rewired for the 5Y4.

Why had the 6X5 rectifiers been replaced? It turns out that Zenith had many warranty claims arising from the 6X5s. These tubes, which have indirectly heated cathodes, would sometimes short the high-voltage cathode to the heater circuit, blowing out the transformer, which may have been a little undersized to begin with. Zenith's "fix" was to replace the transformer with one having a 5-V winding, and convert the set to use a single 5Y4. The later vintage 6X5s, with an X-shaped plate configuration, are less prone to the problem.

I installed a CL-90 current limiter in series with the AC power switch. This is a thermistor which has higher resistance when cold, but then much lower voltage when warmed up. This reduces the current surge when the receiver is turned on, and reduces stress on the tubes, including the 6X5 rectifiers. Also, a current limiter permits using a smaller fuse in the AC line, which provides better protection against short circuits.

Before plugging in the new 6X5 rectifier tubes, I checked the voltages on their sockets. All seemed in order. After plugging in the 6X5s, everything seemed to be about where I expected. I could turn the tuning capacitor by hand, and receive one or two strong, local broadcast stations, but overall sensitivity and performance were poor.

Now, following my usual practice, I recapped the entire set with 600-V "Orange Drop" capacitors. After each cap was replaced, I turned on the radio to make sure I hadn't introduced some new problem. During this process, I exhausted my supply of 0.047-µF caps (which I was using to replace nominal 0.05-µF caps that are ubiquitous in this radio). To avoid delays, I paralleled two 0.022-µF caps for these remaining applications; it looks strange, but so what?

After the recapping, I was finished under the chassis except for another dusting with canned air and squirting Radio Shack contact cleaner into the volume control and into the various parts of the multi-gang bandswitch. Redirecting my attention to the top of the chassis, I removed all the tubes and cleaned the chassis with a rag moistened with Radio Shack electronic cleaner (this differs from contact cleaner: contact cleaner includes a lubricant, which would be a dust catcher if used for cleaning the chassis). The electronic cleaner's aerosol can includes a nozzle brush which was very useful for cleaning the leaves of the tuning capacitor. Cotton swabs, moistened with electronic cleaner, were helpful for reaching nooks and crannies.

Next, with the tubes still removed, I replaced the broken tuning belt that connected the tuning shaft to a transfer pulley. Zenith was famous for using belts instead of dial cord. However, replacement belts or modern O-rings that may be substituted for them, come in various non-interchangeable sizes, and installing a replica belt would have required disassembling the tuning assembly to get the belt over the end and around the shaft. I used a simpler approach: generic belting available from Radio Daze http://www.radiodaze.com/ This rubber belting can be cut to length, and then the two beveled ends joined with super glue. On the first try, the belt was too loose, but it was simple to cut it apart and shorten it.

One of the two dial lamps was burned out. Replacement was simple: I keep plenty of #44 bayonet-based lamps on hand. I cleaned the tubes with Windex and reinstalled them and the metal tube shields. I replaced the 6U5 tuning eye tube and installed an in-line fuse holder with a 2-A slow-blow fuse.

Now, I had a radio that was electronically restored, but just didn't work very well. After aligning the two IF transformers (actually, just adjusting the primary and secondary adjustment screws for maximum volume), everything worked better, but the set had a tendency to squeal when the tuning was toward the upper end of the frequency range.

Troubleshooting the squeal was time consuming. Eventually, I determined that the squeal was originating in the R.F. section, the circuit with the 1232/7G7 tube. Evidently, the value of some resistor or other component must have drifted, causing too much R.F. gain or resonance. Rather than rip into this part of the radio, I came up with a rather inelegant fix: I just bypassed the R.F. section, and jumpered the broadcast-band antenna signal directly to the 6A8 converter. Many radios didn't have R.F. sections, and this one worked great, without squeal, with the R.F. tube bypassed. Interestingly, if you study the schematic for this set, you will see that the R.F. section is used only for the broadcast band, not for the police and shortwave bands. I cut the pins off a dud locktal tube, and plugged it in the R.F. socket for cosmetic purposes. Also, I put a tag on the radio, explaining what I had done--for the benefit of any future owner.

The only chores left were to give the cabinet a treatment of Kramer's Antique Improver and reinstall the chassis, speaker, and antenna.

After intense spousal negotiations, this beauty now sits in our living room.

Chassis, after electronic restoration. The two 6X5 rectifier tubes are visible on the left side; one has a brown base.

What's the History of this Particular Set?

It's interesting to speculate where this old radio set has been for the last 65 years.

Even though it is a 1940 model, it's likely that it was manufactured and initially purchased in 1939. Radio models, like cars, were customarily introduced in the prior calendar year.

I conclude that this radio was owned by someone in the Upper Midwest (Minnesota, Nebraska, Iowa, or the Dakotas)--based on my purchasing it in Minnesota and the preset call-letter sticker for station WNAX in Yankton, South Dakota, located in the southeast corner of the state, but within radio range of portions of Nebraska, Iowa, and Minnesota.

The website for WNAX has a history of the station. Their old programming focused on agriculture and on advocacy of populist politics. Before jumping to the conclusion that the radio was owned by a Midwestern populist farmer, consider that the WNAX label is the only one of the six station preset stickers that is completely legible. The others are more worn, suggesting that WNAX was not the most-listened-to station.

Another potential clue is the cache of large, striped sunflower seeds that were under the chassis, presumably carried there by mice while the radio was stored in a garage, shed, or barn. Sunflowers grow wild and are cultivated in flower gardens throughout the U.S., although I think ornamental sunflower seeds tend to be smaller than the ones I discovered inside the radio. In parts of the Upper Midwest, however, fields of tall sunflowers are grown as a cash crop. Could this point to the owner being a farmer? Of course, large sunflower seeds are commonly used in city and suburban bird feeders.

Sometimes there will be an old repair tag or penciled notations that provide interesting information. This set had none.