Hello Don,
By checking that the audio amplifier is working you have already started back tracking the circuit.
Have you got a RF probe or oscilloscope etc. for RF voltage measurements ? If not make a simple
RF probe with a germanium / schottkey ?diode. First thing is to check that the final IRF 510 is not
oscillating by removing the mike, pressing PTT and in the same condition i.e. mike removed check that there is
no carrier ( RF voltage ) present after the 2 diode Balance Modulator / De-modulator. My experience is there is
heavy carrier leakage. ( I am facing the same problem of removing the carrier leakage here ). You can check the
carrier leakage before the crystal filter and even after the Ring diode mixer as there will be RF voltage present
in athe all of the above stages without any audio signal from mike. Go stage by stage measuring forward or backward.
Hope this will help you.
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Satish Chandorkar