1965 Zenith Screen Shots

[DavGoodlin]

The farmers in midwest seemed to care more about TV back then. Most of the rural folk in PA-MD were indifferent to TV in general. Most had late 50's and newer BW sets lasting well into the 70s, mostly GE, Silvertone, RCA and Philco. Most farmers I knew were not afraid to attempt repairs first and most succeeded. A tube tester was in most auto parts stores, so that was an invitation to try. I guess when you get a few raps off your electric fence, you're not afraid of HV.

The sound problem on that set (27KC20?) could be an open "buzz" control, a wirewound pot in the cathode circuit of 6BN6. I had that happen on a 20X1C38 chassis (6Z10), which is the same design IIRC, just 4 years newer.

My TAB Zenith service book also says to check the solder connections on the sound IF coil terminals after lifting off the can.
Your set does not appear to have the bass and treble controls, but the plug to them could cause your symptom.
You could inject an audio signal (from a VCR or DT box) into the tone or volume controls to eliminate the output stage.