This Kenwood KA-550D had multiple problems, some quite challenging from a mechanical point of view.
On channel was very quiet. That was due to the volume control potentiometer being damaged.
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The potentiometer was a little unusual in that it had an additional fourth connection that was a centre tap to the pots carbon track. The centre tap has a resistor to ground that’s used to make a linear potentiometer act like a logarithmic one.
The big problem with the volume pot used in this amp is that while it was a dual pot (as expected), each pot was individually adjustable as the volume control shaft supported two separate knobs, an inner and an outer one.
Try finding a new one of those!
The simple way to repair it was to fit a standard dual pot with a single knob to adjust the volume.
Note that D profiled potentiometer shafts like this amp uses don’t all have the flat bit in the same place relative to its rotational position. If the volume knob has a marker on it it might not line up with the front panel numbers, so the whole pot needs to be fitted in a rotated position.
This amp is about 30 years old and the contacts in the input selector slider switch were dirty causing the sound to drop out or go very distorted so it needed cleaning.
The output transistor bias was correct as per the service manual.
Those problems sorted, the amp was repaired.