1965 Zenith Screen Shots

[jstout66]

This set, I need to actually fix. Great picture, BAD audio.. and its got me stumped. You have to turn the volume all the way up to hear anything. It’s got dual speakers, so the sound should be fantastic. The sound isn’t distorted, just extremely low output.
This set has an interesting back-story. I got it off of Craigslist a few years ago. Supposedly it was a one owner set. (owned by farmers) they passed quite a few years ago, but the kids farmed the land and left the house untouched. I was told that when they were in the house, they would watch it once in awhile. This set had a new flyback installed during its life AND an off-brand rebuild 21FJ installed in 1989!
I did have this set listed for sale in the classifieds, but have decided to keep it. In this area, I haven’t seen a tube set in years, let alone a roundie. And I figured I’d probably regret it if I let it go.
Anyway.. here are some screen shots I took.

[sanjarali]

Many TVs that are happy to look at in now times!!!

The two sounds are upper caste, yes! What a good man.

[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.