It would have to be something internal, exclusive to channel 6. It sounds like the meters arent registering anything, even on a tape that has signal on track 6.
Seriously doubt external cabling is the culprit.
So, when a known to be functional sound card is plugged into channel 6 it ceases to work?
And when # 6 goes into another slot it works??
I do have the manual, but I guess my question is how do I go about looking for the culprit, how do I check to see where the signal fails? can I use a multimeter and just follow the path to see where the signal stops?