Nick,
It sounds like your LSB crystal has drifted way off, the trimmer capacitor needs cleaning or someone tweaked it sans knowledge.
You could do the following without test gear.
1. With an antenna attached, tune to 40 SSB. Turn volume up and switch back and forth between USB and LSB. When optimal, you will heard no difference in the noise content, a balances mix of highs and lows in the noise.
.
2. Test tweak
Go 40 or 75 LSB and tune to a signal.
Now whilst listening critically, adjust TC501 trimmer cap on the last card (AF BOARD).
Does this test make it any better to copy a SSB signal? If it does nothing or very little, let me know.
You should be able to adjust from all lows to all highs (big changes)
?
Cheers,
Randy
