[morzh]

OK, so at 1/2 volume you have normal signal driven into the transformer.

At the same time from your previous pics at 1/2 volume your output starts distorting. And both channels are doing the same.
This tells me you might be OK and what you are seeing could be normal, or at least agree with the design.

Definitely look across the power, see if all is recapped, like C19/C20, C23/C24 and such (you probably recapped C29/C30). Also the power supply caps.

Now, does the input signal come from inside the amp or do you supply it from outside? Is it a test tone or something?
The sch says "11.8V across 4 Ohm load", which makes it 34W output.

What I wanted to know was what is your scope's V/div so we know what is the peak (or RMS if you use a meter) amplitude where it starts distorting. It is possible you are exceeding the max power and this would be OK, as the distortion should not occur only before exceeding it.

The gain is about 26 so if you supply 1V of input this would exceed it. You should be roughly under 500mV RMS (700 peak or 1.4V p-p) on your input signal.