I have used an oscilloscope and a signal generator to more tests regarding the audio distortion.
I have followed the signal thru the audio chain after the last mixer when appying a known clean and weak signal on the input so no front end or mixer overload ocurs. Everything looks clean on the oscilloscope until the output of final two transistors. The signal on the output of Q71 looks fine. On the output of Q72/ Q73 the signal is distorted around the zero crossing. https://groups.io/g/BITX20/photo/59179/3?p=Name,,,20,1,0,0
I wonder, is the biasing actually correctly done?
Should it have been like this instead? https://groups.io/g/BITX20/photo/59179/0?p=Name,,,20,1,0,0
Here is the distorted signal out of the amp (blue trace, note different Y axis scale). The red trace is the signal earlier in the audio chain. https://groups.io/g/BITX20/photo/59179/1?p=Name,,,20,1,0,0
Pls post your comments.