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I'm not familiar with your Zenith's audio curcuitry, and don't have a schematic, but, I know some old tube sets can lose their SOUND as a result of sweep circuit failure.
This seems counter-intuitive, but many sets used a quadrature detector tube for detection of the audio signal. A Quadrature tube (also known as a gated beam discriminator) required a low B+ voltage with virtually no reactance. Getting the B+ from the usual B+ supply is out of the question (the source of the usual B+ is a huge inductor (transfomer secondary) and a huge capacitor (AC ripple filter). So, the plate of a gated beam discriminator as attached to a big resistor to the quite high voltage of the "B+ boost" in the sweep circuit.
No sweep circuit, no B+ boost, no B+ boost, no B+ on the detector, and the only sound you hear is the noise in the audio amplifier.
Of course, if your Zenith uses a ratio detector of Foster-Sealey detector, all of this is no help to you :-)
good luck with your Zenith, Rob
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