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You have to be careful here. there are many hidden things that may be going on here. Increasing the gain of the IF amplifier to max with the pot may induce the IF amplifier to oscillate. I would reduce the gain and increase the RF generator to +10 or +20 if necessary to force a signal through.
What you must confirm is that when you see a deflection on the meter, always confirm you cange vary the level properly eg a smal increase in input yields an increase in the output and vice versa. If you overload an amplifier, sometimes you will find the change of input yield no output change. Or increasing the input may reduce the output. Always make sure increasing the input will increase the output.
You have to understand and get a feel what the circuit is doing. Instead of the VTVM, you could use an oscilloscope on pin 1 of the 12AU7 to see the 1kHz sine wave amplitude. I sometimes like to check things visually to confirm there is actually a sine wave there and I am not just measuring noise. Additionally if the amplifier is overloading, you will see the sine wave clipped. And if the amplifier is oscillating, you will see that too.
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