It would seem the amplifier is OK due to the problem going away when using the CD. Old equipment can commonly have bad connections at the selector knobs from dirty contacts. A lot of times a litlle contact cleaner in this area and working the knobs will fix it. Also did you eliminate the turntable having the problem. Since the turntable is having a problem with only one output. You could switch the right and left connections and see if the symptom is the same or moves the other side. Just some thoughts.
