From FB group Electronics Hobbyists, regarding the Zenith Royal 500 radios.
Clyde Hendrickson
There are two 50 uF, 10V electrolytic capacitors under the battery holder. Replace them with 47 uF, 10 or 16 volts. There is also a smaller capacitor next to the other, it might be a 3 uF, or a 16 uF, 10V. Use a 3.3 uF or 22 uF. All three of the replacement capacitors can be soldered to the copper side of the PC board.
The fourth capacitor, 3 uF, goes from the center pin of the volume pot to the PC board. This is the one that causes the volume to be very low or zero. There is enough room to solder the replacement capacitor (3.3 uF) between the center pin and one resistor.