After I have repaired the power supply of the second unit and check all of the provided voltages it was time to continue with the amplifier. Since there is no sound on both channels it must have something to do with either the supply or controls.
In a first step I checked the voltages and noticed F with +38V was missing. Checking the circuit has shown the fuse resistor 3404 is broken. Unfortunately I do not have such a fuse resistor so I replaced it for the moment with a bunch of resistors to get 4.7Ohm. Of course it need to replaced it with a fuse resistor when I get one. But it will do the job for the moment to continue.
Next I had to understand the undocumented transistor circuits on the FM board close to the input switches. The board looks quite different to the documentation. I checked the output to the preamplifier which is situated on the equaliser board. In the beginning I used an audio signal but of course this does not give me any meaningful measures on the oscilloscope in order to read frequencies and amplitudes. To overcome this issue I ordered the frequency generator which will now help me during this step. I used 1kHz at 1V on the AUX input for my measurements. The input selectors are working fine and the signal gets forwarded to the preamplifier. Then I made a mistake which costs me some time. I have not disconnected the cable between preamplifier and the TDA2030 amplifier. Be ensure to disconnect the cable when measuring the functions of the preamplifier.
After I disconnected the cable I the oscilloscope has shown the output was not working. However I decided to remove the volume potentiometer since its movement feels scratchy. Oh and what a surprise, I noticed that it was already removed before and put in back the wrong way. Now I know for sure that this machine was a donor for someone and it was clear why all boards are broken. Or maybe someone tried to repair the device, who nows…
After I cleaned the potentiometer and put some vaseline on – well I am pretty sure there are better materials out there – I soldered it back the right way to the PCB. And voila – afterwards I got a the sinus signal on both channels at the output of the preamplifier.