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Sounds like the audio section is OK. If you get white noise on FM but no hum, power supply and filters should be OK. Certainly it needs the paper caps replaced soon but first you could do some testing.
I'm wondering if the converter is working. You could do the test with another operating radio. Put another operating radio set to 1455 next to your Zenith and tune your Zenith through 1000 Kc. AM with its volume down. Should hear a squeal or rushing on the other radio. If not, suspect the 12AT7. If that's bad neither AM nor FM will work. You may get a little "bleed through" of a strong station on AM, as you seem to be getting. Try tapping the tube while set is on, may have an intermittent connection. If no squeal stop here and try another 12AT7.
Just for the heck of it, check the audio section. Set the phono switch to phono, turn up the volume, put one hand in your back pocket and with your other hand touch your finger to a screwdriver shaft and touch the screwdriver to the center of the phono jack. Should get a loud hum. Switch back to radio.
If audio's OK, look at the schematic and check the voltages around the IF, converter, AFC, and RF tubes. You tried new IF tubes?
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Reece
Perfection is hard to reach with a screwdriver.
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