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You might want to start by looking at the input to the 6SN7 driving the tube when on FM. Note the 2.2 Meg resistor #17 taking voltage off the discriminator output. Also the 2.2 Meg coming off the disc. tube going to the AGC line and with a .05 cap to ground. The small 'lytic in the disc. circuit is almost certainly needing replacement as well.
Where i"m going with this is you may have insufficient or unbalanced output from the discriminator to give the right input to the eye-tube circuit. Measure the voltage on that first resistor I mentioned, and you might find the discriminator is way out of whack and producing a large voltage which will knock the eye tube off. Think of the eye-tube being a center-tune meter - the expected voltage should be zero if tuned in properly, IMHO.
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