As a general thing, first find the spike diode. Should be easy to spot, just follow traces from the audio tranformer pins until one of them leads to the anode of a black epoxy 1-Amp rectifier diode. On the diode's other end is the cathode, the end with the band on it. The trace it feeds into is our modulated B+ that will power and modulate the driver and final transistors. Downstream from the diode you'll find a small-value electrolytic capacitor, almost always under 10uf with the positive end connected to the modulated B+ line, and the negative side connected to a diode. Remove the solder from one side of this capacitor to break the circuit and see if it affects the modulation limit. Good chance it will.
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