RW / David, would appreciate you bringing your powers to bear on how the balanced modulator circuit works. Audio in is to 'Signal In' (Pin 5), Carrier in is to 'Car In' (Pin 7), balanced mod out is to OUT 2 (Pin 3) and on to Q520. Looking across to the right, there is a signal BFO_FRQ coming in from the right and I think that is a PWM signal coming from the processor.
That PWM waveform is converted to its DC voltage equivalent by filter / averager U510D and the DC voltage is then used to shift the frequency of the 10.6975MHz 'BFO' oscillator by applying it to the varicap D507.
In SSB TX mode I think the purpose of this will be to shift the carrier frequency slightly up or down so that only one chosen sideband (the upper or the lower) slides through the SSB IF filter.
Q505 / Q516 both look as though they are designed to intentionally offset the carrier null adjustment set by W502, so that the modulator does produce a carrier output in CW mode for example.
Datasheet for the balanced modulator IC:
https://www.mouser.com/datasheet/2/2...94_E-62933.pdf
I've just noticed it is also a demodulator, or at least can be used for that purpose.
Edit: (cross posted with above) ...David is obviously a mind reader....
The issue at the moment is that crackle doesn't seem to have his DSB signal on Q520 collector, hence asking him now to look for the audio input going to SIG IN.