I bought a bunch of Zenith Royal 500 and similar AM transistor radios from an eBay seller. These were made in the late ’50s to early ’60s. They all use air variable capacitors like the 5 tube sets. They all use four AAA cells for power. The cases on some say unbreakable nylon, but there are chips and cracks. They are relatively easy to work on. I got fifteen for about $15 apiece.
The seller did not put enough packing around the radios, but they survived the shipping.
So far I have downloaded several schematic diagrams for them. I have an old Zenith TransOceanic Royal 1000, but I had never seen these smaller Zenith radios until now. They have an RF amplifier stage, oscillator, mixer, IF, driver and push-pull output stages.
I opened most of them up. The first one I worked on was the one with the oval speaker in the upper left corner. Most of the problems seem to be mechanical. The battery holders have corrosion from leaky batteries. And the volume controls get worn out from use.
I couldn’t power up the radio because the negative battery contact was mostly eaten away. The other contacts were in much better shape.
I disassembled the radio, unscrewed and unsoldered the battery holder and drilled out the rivet holding the negative contact. I cut out a similarly shaped contact out of a can lid, using a pair of scissors. I used a 2-56 small screw and nut to hold the replacement contact in the battery holder, and reconnected the battery holder to the circuit board. Using a power supply, I powered up the radio and found that the volume control was defective – it was dead until turned nearly fully on, and then the radio was playing full blast. So I unsoldered and removed the volume control, and tried to clean it. But that didn’t help. I opened it up and found the carbon element was falling apart. It had to be replaced.
I spent hours looking for a replacement potentiometer. I quickly found a guy on eBay who machined replacement pots, but he sells them for $26 apiece, which is nearly twice as much as I paid for each radio. Someone suggested trying Parts Express, but they didn’t have anything close.
I tried Mouser’s search engine to see if I could find something close. They have all those filtering choices so I limited my choices, and it came back with nothing found. I had to turn off one filter after another, and finally it gave me a few poor choices. So essentially Mouser doesn’t stock anything like it.
I tried DigiKey. They had lots of filtering items to choose from, but the most important, the risistance, wasn’t anywhere to be found. That was really dumb!
I thought I might be able to use a 10k slider pot, so I looked in my box of potentiometers and I found some 10k miniature pots with the 1/8 inch diameter shaft. The threaded shaft was extra long for mounting in a very thick panel. So I decided to try one to see if it would work. I had to saw off and file the D shaft to get it to fit into the knob. After a few times of adjustments to get it to fit, I soldered the wires on and powered it up. It worked, but there is no on/off switch. It was a linear taper, so to “delinearize” it, I put a resistor between the wiper and common. I tried some values and found that about 680 ohms seemed to work good at making the volume spread out over its rotation. I will explain this in my blog in the near future.
I tried to tune in some stations but I heard very little. I found one strong one on about 820 on the dial. I listened for awhile and heard 640 KFI, but it was way off. I think someone had played with the adjustments. I used a tiny screwdriver to adjust the RF osc coil to get this 640 station back to 640 on the dial. That helped, but it still needs the other adjustments adjusted. It needs to have a realignment like they would do at the factory.
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