According to the service manual, this is an indication of either "AMP overload" or "abnormal DC voltage in the power supply". I would expect that the problem is in the AMP section as this is the most common failure. Since this unit uses discrete transistors as outputs, I expect to find at least one that is shorted which would cause the symptom you describe. If you are comfortable working on these, finding and changing the outputs should be relatively easy. Always change them in pairs. This is because when one fails it places extreme stress on the other one in the pair.
Hope this helps,
Dan