Well, the preamplifier is working and not to continue with the TDA2030. Since the channels are separated but both do not have a signal it was quite clear it can only be a control of both channels which is not working. Luckily there is only one circuit that can cause the troubles.
The circuit around 7310-7314 mutes the signal for a short time when switching to the tuner input. It seems there is something wrong. A first visual check shows that 7310 got burned. There must have been a high voltage on F to burn this little fellow. I replaced it right the way and also 7310 and 7314 since I had them all on stock. Afterwards there was still no signal so anything else must be broken. The diode 6311 is right in the line of a possible overvoltage and a measurement with the voltmeter on diode-mode has shown it is broken.
After replacing 6311 the amplifier is finally working. A quick check of the tuner has shown it is fully functional. Finally, I learned a lot when repairing this board.