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I am pleased to learn that the problem is solved.
Replacing R12 with a diode could produce too much base current in Q8 which can damage this transistor. Try putting R12 back in the circuit and putting the diode (1N4004 or equal) in place of R33 and R34 with the banded end toward C18/L4. This has almost the same effect as what you did and increases the maximum modulation from 90% to 100%. This introduces some audio distortion but you probably won’t hear it.