It was time to fix the microcontroller board. Since the display sometimes showed the correct segments it was clear that the serial bus of the CPU is working and the CPU itself might be ok. There can always be a toasted input/output but I have never seen such a failure.
It was obvious that the CPU sends a command to the tape mechanism and is waiting for its execution. To replace the belt was already a first step to get the capstan wheels back working. The only difficulty is to understand what the CPU is expecting at the start-up procedure and what is it trying to do. Unfortunately, the service manual does not point anything about this topic. It describes quite well how to measure the functionality of the CPU, but if the initialisation routine already hangs you will not get the functions of the default application as described in the service manual.
This means there must be a default state of the tape mechanism which cannot be switched to. This mode can be easily figured out when you have a working unit:
- Turn the head to side A
- Move the slide servo down
- Switch off record-mode
If one or more of the above steps fail the CPU will never react and your sound machine becomes useless since you even cannot handle the CD player anymore.
I have never seen a CD555 for sale which does not have a hanging CPU. The failure is quite command but easy to solve if you know what you have to do. If ever see a non working Sound Machine for sale do not hesitate to get it, you can fix it. It requires a better quality of the firmware with appropriate error handling which we are going to add with the replacement CPU (of course also add more features). The Philips SER1 tape mechanism was only used in a few devices and it is hard to get spare parts. Wondering if Philips replaced it because they had a lot of such problems, what other reason could it be to replace it so fast? However, I think the mechanism is great and much better than later ones.
Have a lock at the video to follow the process…