I didn't try "trace_tuner=1". Would that be more informative than the "stalk_debug=1" command which I did use?

I had also tried connecting with hyperterm with the following commands:

stty -echo raw 19200 < /dev/ttyS0
od -txC -v -w4 /dev/ttyS0

however the results were similar to "stalk_debug=1".

David of SONICblue's Technical Support believes that it is, "a dry (solder) joint at best or a faulty opamp at worst". Hopefully a trip to England will do the trick for my tuner/RM-X4S problem. I'll let you know how it turns out. ...Mark