Zenith Royal 500 AM Radio Restoration - July, 2019

Intro and Disassembly

This radio belonged to a family member who was not aware of the sorry state it was in. When I found it and revealed the advanced battery corrosion, he agreed to let me have it, and if I happened to get it working again, I could keep it. I initially thought it would just need a quick clean-up of the battery compartment with some vinegar and a toothbrush, but I soon realized it would need a full restoration, inside and out.

The Duracell batteries, with expiration date 17 years prior, had caused quite a bit of damage. On the inside of the battery compartment door was a badly damaged transistor layout label. Crucially, the chassis number was still legible. This number is important for finding the correct schematic and other info about this radio because Zenith produced several variants, all nearly identical externally, and bearing the same “Royal 500” model name.

In 1956, just two years after the Regency TR1 debuted as the first commercial transistor radio, Zenith entered the market with their Royal 500, affectionately nicknamed “Owl’s Eyes”. They produced several variants, each with a different circuit especially taylored to specific batches of transistors. Since transistors were relatively new and in short supply, Zenith wanted to ensure they could have a manufactureable product regardless of transistor source. They designed different circuits to accommodate all NPN transistors from Sylvania, all PNP transistors from Raytheon, or a mix of NPN & PNP transistors from Texas Instruments. The radio shown here, with chassis number 7ZT40Z1 uses the mix of TI transistors.

After removing the batteries, and opening it further, I could see most of the internals were in good shape, and that this radio would be salvageable. I’m not 100% certain, but it looks like the tuning capacitor is dated September 23, 1957. The serial number, 247044, appears in the hole stamped through the battery installation label.

Another major problem was one of the transistors had become dislodged from its socket, and settled next to the tuning capacitor. This particular transistor is one half of a matched pair to drive the audio output transformer. I plugged it back in and found it to be quite loose. Later on, after removing all seven transistors, I slightly bent their leads to ensure good springy grip in their sockets. These transistors were manufactured by TI, but labeled with Zenith part numbers. Package style is TO-22.

Thanks to the handy disassembly instructions inside the case, I had no problem removing the chassis to reveal the other side. The speaker was in good shape, albeit a bit dusty. Around the battery holder, there was some blue-colored corrosion on the electrolytic capacitor leads and a couple other wires. The circuit board was in good shape; no corroded traces.

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Restoration Process

The first major fix I did was to re-wax the AM antenna. The 62-year-old wax had yellowed and was cracking and flaking off. I threw together a ghetto rig to heat up my wax melt pot on a stove-top heating element, complete with a vintage Weston analog ammeter. Once melted, I dipped the antenna into the wax, stirred it around a bit, and let the old yellow wax just dissolve into the pot. After a couple of quick dips, the antenna had a nice new coating of paraffin, good for another 62 years.

In the mean time, while waiting for wax to melt, I cleaned up all the corroded parts in a bath of 50% vinegar and 50% water, including the transistor layout label on the battery cover. A few small areas of the label, which before were still legible, had completely turned to mush after the bath. But it’s much better now that the label is chemically stable. The chassis number remained intact.

Naturally, such an old radio would need to be recapped. On my DMM, one of the 50 µF, 6 V electrolytic caps was about 40,000% out of tolerance! While rooting around my capacitor drawers for potential replacements, I found a nearly identically rated cap, at 47 µF, 6.3 V, but probably only about 10 years old. I probably salvaged it off of something I took apart in the past few years. A half-century of innovation can be seen when comparing the size difference between these two caps. I ultimately used tantalum capacitors to replace this and other aluminum electrolytics, but with voltage ratings considerably higher those originally installed. I can imagine the Zenith engineers who designed this radio went along with a 6 V cap on a 6 V rail because there’d be no space or budget for anything larger. In the service manual for the 7ZT40Z1 chassis, the 50 µF cap (of which there are two in this radio) cost $1.50.

I removed all the transistors from their sockets, and sprayed contact cleaner on the sockets, in the volume-pot/power-switch, and on the ear-phone output jack. I put some fresh grease on the pot’s shaft, and the tuning cap’s bearings.

One peculiar thing I noticed was how this yellow 0.1 µF cap seemed to be too large and out of place. It might have contributed to that one transistor falling out of socket. On the bottom of the circuit board, it was soldered adjacent to the leads of another component which had been clipped off. In searching online, I found pictures of other radios’ internals either with this plastic film/foil cap, or with a smaller cap that appeared to be ceramic. The service manual lists this part as a 0.1 µF, 30 V ceramic disc (costing $0.95, in case you wanted to know).

Functionally, it is a snubber cap, in parallel with the primary coil of the audio output transformer. Ceramic caps are generally not recommended for such an application. It’s possible that Zenith manufactured a certain batch of radios with a ceramic snubber cap, then realized there was a problem. They would have had to rework the radios with a larger film/foil cap instead. The rework on this particular specimen was definitely done by Zenith, not by a second-hand service person, because the film/foil cap has the same yellow fiber sleeving as other parts on the board.

Although this cap would still be perfectly functional, I replaced it with a much smaller modern film/foil cap (“104J” in the photo). Now there is plenty of room for that transistor. In the photo, you can also see a red-colored tantalum cap that I installed next to the pot, replacing a 3 µF, 12 V aluminum electrolytic.

I tidied up some of the messier solder joints on the circuit board, and began putting it all back together. The two yellow blocks dangling off the bottom of the circuit board are 47 µF, 25 V tantalum caps to replace the 50 µF ones.

Once all back together, I touched up the front “ZENITH” with gold-colored marker.

The knobs needed some work too. The volume knob was cracked in the middle, as if someone had pushed it too hard on the shaft. Some superglue fixed that with no problem.

The tuning dial has a vernier movement, originally with a clear plastic disc surrounding the knob, and a painted white mark pointing to the frequency. However, with age, the clear plastic disc had started to yellow, and the dial numbers were less visible behind the plastic. This would only get worse in the future, so I decided to snip off all of the disc’s outer perimeter, save for a small area containing the white mark. Overall, I think it appears much better this way because there is greater symmetry between the two knobs.

Functionally, this radio works perfectly. There was no need to perform an alignment. It picks up many more stations outdoors than indoors, probably because my house is older than this radio, and has aluminum-backed insulation. I often sit in a back yard lawn chair with this radio in my lap while the dogs do their business. I scan the dial, searching for any decent programming. Naturally, most AM stations are talk and/or sports, but I prefer to search for music or any other eclectic program I can find. Just a couple days ago, I happened to find a station playing a radio drama from 1948.

This page last updated: 2019/08/09