Well, after studying the service manual for a couple of days and after some measurements I got a first impression of what is broken.
The three keys for the CD-Player Play/Stop/Pause are controlled by the tape deck CPU. As we have seen that no key of the tape deck shows any reaction I have a suspicion.
To check if I am right I measure the signals from the tape deck CPU. Again, the service manual helps us to verify if the CPU is working. Starting with the output gates as mentioned in the service manual we get… nothing. The tape deck CPU is not working. VDD as well as Reset is fine and the oscillator is working at crispy 5.3MHz. Btw, the oscillator also drives the CD-CPU 7501, hence they are more or less synchronised. The transistor 7521 prepares the signal for 7501. The CPU also communicates with the serial display driver MM5452N for the tape deck lcd screen. Since all options are visible on the screen there comes no clock signal from the CPU.
I also conducted all the other tests from the service manual to just verify my suspicion. Finally, the tape deck CPU is broken and needs to be replaced. Now we have a serious problem. Even if it is possible to get the 68xx based Motorola CPU MC14605G2 we do not have the program stored in its eprom.
However, today there are other possibilities to overcome this situation. The solution for me is to replace it with an 8 bit Arduino. I do have all the hardware and knowledge by hand to replace the CPU.
In a first step I replaced the CPU with a 40-pin socket to be able to connect cables. An Arduino Mega is just laying around and I am using it for some first tests. A quick win would be to get the CD-Player keys working. This allows me to check the overall CD functions.