i found this schematic on the web, the components location numbers match on a 95%, i was able to confirm that the pre amp for both channels are working fine, and it looks like the driver circuit is the problem, however, except for the output transistors, i exchanged all driver circuit components from the good channel to the bad one and vice versa, and the problem remains on the bad channel. i also noticed that the bad channel have a square wave at its output, instead of audio signal. the output filter composed of c190 and r296 is working ok