Woody wrote:
It sounds to me like a bias fault - one of the output transistors is
getting hot and a sensor - usually a transistor fitted into the
heatsink - is sensing the rise in temp and trying to switch the output
transistor off. Net result is a seriously distorted output.

It's possible one of the devices in the output stage has lost contact
with the heatsink (Loose securing bolts/ clip). I've had that happen
with similar symptoms before now. It may even be the heat sensing device
in the bias setting chain being defectove or loose, which is usually
either a transistor or diode secured to the heatsink with a spring clip.

At least with the other channel apparently working correctly, there's a
comparison avaliable without using the schematic.

--
Tciao for Now!

John.