Small update:

According to the service manual there are some tests to run on the microcontroller, by shortcircuiting 4TP1 twice briefly.

After the first short, all leds should go on, which it does.

After the second short, the leds should blink and then you can choose the test by pressing a specific key on the MCP.

This second short doesn't change anything, the leds do not blink and the only thing I can do is restart the unit.

So I'm afraid the microcontroller is toasted. the controller is a Siemens SAB 8032B-P in this particular unit.

I'll give it a try, and ordered a new one on Ebay and try to replace it. Hopefully..........

Are you sure, you need to short 4TP1 twice?

Martin

According to the service manual:

Microprocessor test

Press  RADIO

Short-circuit (twice briefley) 4TP1

Press TAPE

Display: 2=processor OK   3=Error in processor

I just downloaded the BM5500 service manual and yes, that's what it says.

However, that manual claims that a uC8031 is installed, not the 8032. Are you sure you're having a BM5500?
In any case, I'd try the test mode procedure from the BM6500. This one does use the 8032 and the test modes are somewhat different.

I found another post where a user was successful in applying the BM6500 procedure on a BM4500 unit. If the BM5500 is a successor of BM4500 the new/different test mode procedure might work on your unit too.

Honoustly I don't know for sure if the microprocessor board is really from a 5500, I have seen other 5500's where indeed the processor was a 8031 instead of a 8032. This is a  unit which was "serviced" by several people so maybe the brain is from a 6500.

I never serviced a 6500 so I can't tell you the differences, but as metioned, the unit is somewhat different from other 5500's I've seen.
The service manual of the 6500 is downloaded so i'll try the testprocedure of the 6500 and share the results

To be honest, I've never serviced either of them. My answers are based on theory and the service manuals only.

The 8032 is fully backward compatible to 8031, but it has more RAM. So it is thinkable that hybrid versions exist, I guess.
I'd say there's one way to check which test mode procedure to follow. The 5500 manual says that the ROM 4IC4 is an 8x16kbit ROM on the 8031 processor, the 6500 manual shows a 8x64kbit ROM on the 8032. A bigger ROM suggest a more elaborate firmware and more functions. So, if you have the bigger ROM try the test modes as per BM6500. If you have the small ROM, it's more likely that the simpler BM5500 test mode is coded.

Okay, I did try the test procedure from the 6500 series. And yes, this works! 
Relevant tests were succesful. So now I can confirm this microprocessor board is in good shape. Jay!

Also I did manage to get the tuner to go to FM, with <GO TO>  and <TURN> instead of <TURN>
It doesn't receive any radio stations but now I am able to tune in on a frequency which it did not do before.
So now I have to find out why there is no FM reception.

Thanks for your great support!

If there is a BM6500 processorboard placed inside the BM5500, then you get problems with the connection board. Wrong sources are choosen.
But you can say that a processor works well when you get a standby led lit. Obviously the startup routine has correctly been done. 

The problems you mention are not the case, so then it should be some sort of V2 processor board in the 5500 I guess?
Just like what manfy already suggested.

But I think the troubles with the processor board are now sorted, I have to concentrate now on the tuner board for the reception troubles.