The tip on adjusting alsamixer did the trick on the 3B+, 4B and even the zero, HOWEVER, the received text via the Signalink USB is garbled on the zero w. I cannot imagine the zero shouldn't work with the Signalink USB. I am using a Icom 737 without rig control and using port audio as the Signalink as both tx and rx. I think it would be a hoot if I can use the zero, has anyone tried the zero for PSK31?

Older thread, but I am now using the Pi-zero with PSK-31. The trick is using a more simplified program. LinPSK works like a charm and offers several modes including PSK31 and RTTY. The latest version 1.3.8 was just updated by the author Volker Schroer after some beta testing by me...fixed some issues with logging and time zones. Very nice program that I am using with a tablet connected to the Pi via WiFi.

=Vic=