Re: no audio
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Confused here. You said you do not have an oscope, so what is it that is making weird lines?
If injecting audio at the mixer output did not produce audio at the regular output then you have
probably isolated the problem to between the mixer and the audio output. Follow the audio
path on the schematic and inject audio (use a 0.1 mfd cap for DC isolation) at each stage
in the signal path from mixer output to audio output. At the point where you start hearing audio
measure DC voltages and look for any that are different from normal values. If voltages near the
fault are all normal then you will probably need to look for an open in the circuit path (cold solder
joint or possibly a bad component).