[Elecraft] K3 Sub RX Not Working

[H D Barr]

My K3 (non-S) is equipped with the second rx, which I rarely use.  After 
infrequent use (got a K4) the sub rx seems to have stopped functioning; 
no signal or noise  can be heard from it. When the "Sub" button is 
tapped, "No Sub" appears in the display. The main rx and tx functions 
seem normal.  Before I open up the K3 to see if any cables have come 
loose, I am searching for more obvious (or easier to check) possible 
reasons, including operator error.  Any ideas are welcome.

Thanks and 73.

Dave, K2YG