My first guesses would be a dirty head followed by a poor contact for the DIN plug to the amp.
If
the weak channel is the right one then it could be the belt needs
replacing. I had this problem with my BC8500 - thinner tapes (C90 and
longer) showed the problem less. My theory was that the thicker tapes
(C60) didn't pull dead flat over the head for want of decent tension
from the belt.
Steve