I'm currently restoring a Zenith Radio, of which there are many things wrong it. One being that the Rotary switch used for the Tone control has been a faulty item since coming from the factory and obviously got passed quality control. This is the part in question. The bass boost capacitor C13 is only shorted out in 3 positions according to the schematic, however I discovered that due to one of the rotors on the switch not having the right "cut outs" on it, it meant that C13 was shorted out all the time, except one position Here's the switch, with the lower rotor which permanently shorted out C13. I have a couple of other Zeniths with the same tone control, and they are fine and work well. In the mean time, I purchased a cheap rotary switch from Jaycar to get the circuit working and have purchased a better replacement off ebay (it could take months to arrive?)
The lesson learned is, don't assume your radio ever worked correctly in the first place, this one would have never sounded that great, now has a great sounding bass as designed.