I do have a certain knowledge in pinball system board repair. So I
guess this is more or less the same.
I have a dvm and a test probe.

All other 'sources' like CD and AM radio sound very good. only fm
sound output 'disappeared'. the sound stopped while connecting the
second pair of speakers (in parallel) to the red and black speaker
crimps.

Thanks for any help

Do you have any experience in troubleshooting/repair of home electronics?
Do you have test equipment? You'll need a print, DVM and possibly a cheap
scope to fix this one.