A few suggestions:
Turn to channel 6 and see if you can receive a local FM radio station at around 88.3 Mhz or so. The split sound IF should allow you to do that. Be aware that Fm radio stations use higher frequency deviation than TV sound carriers use, so the FM radio station should come in louder.
With the VCR output, try loosly coupling the signal into the antenna tterminals. Do it so the picture is snowy, see if that makes a difference.
With no signal on the antenna at all, you should get a fair amount of FM hiss, like on an FM radio between stations.
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When I run my signal generator and inductively couple the output to the TV's antenna terminals (just place the coax nearby), I can tune in very crisp horizontal bars, as shown. Tuning the signal generator to a slightly higher frequency results in fuzzy bars, and a very clear, loud tone from the speaker. If I turn the volume control up, the tone is loud to the point of risking damage to the speaker (and my eardrums). I'm convinced the audio amp circuits are golden.
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Sounds! like the FM detector is acting more like an AM detector. I'm guessing that your bar generator is just an amplitude modulated carrier at a frequency near channel 3 or such*. Video is AM modulated onto the picture carrier. An FM detector would just get quiet (hiss going away) when you get the carrier frequency dead center of the FM detector.
An AM detector will not get much signal out of an FM signal. but some if you "slope detect", ie, tune it off center. It would sound lousy though. Not that your TV set was built with an AM detector, but maybe that small electroytic on the ratio detector went bad, shorted or completely open. Try replacing it with any modern electroytic of about the same capacitence (you could just salvage one from a modern circuit board, I've done that and it works fine). Or some other part close to one of the 6AL5 diodes is bad. Or a dirty tube socket.
Also check that the 6AL5 heater is getting about 4 or more volts. Check R48, a heater voltage dropping resistor (used to reduce "contact potential" on the diode cathodes. Maybe that resistor went bad.
*And check to see that your generator can produce a clean audio tone on an FM radio set to 88MHz. That would tell you if it's producing true FM signals. If the FM radio seems to get audio slightly mistuned high and low, but quiet on center, then the generator is producing AM.