M600: No transmit or receive after warm-up.
PLL unlock. Replace unstable C228 trimmer capacitor with a ceramic equivalent and adjust the lock voltage for 2 volts instead of 2.5 volts as stated in the manual
M600: Won't turn off.
Replace bad Q1 transistor on the relay unit.
M600: Distorted TX audio in SSB. RF feedback.
Add a ground jumper at J10 connector, pin 4 on the main unit.
M600: Won't operate the tuner.
S1 and/or S2 switches on the logic are not set in the proper position. Q52 transistor may be bad. Set switches correctly. Check Q52 transistor for failure.