On the Beocord 8000 through 9000 series of cassette decks one of the first things to check is the tape transport mechanism. Servicing will always require new belts of course but there are quite a few other things to do regarding the mechanics.
The first thing I saw when I opened up this Beocord 9000 was a loose part that had broken off the transport. The inset picture shows the little leaf spring. There are three of these springs that hold down the tape head transport as it slides forward to engage with a cassette. Without these springs the tape head transport can teeter up and down which would not be good for the sound. The red line shows where the broken spring should go. There was also a 2.5mm ball-bearing that is part of the spring assembly. That ball-bearing is gone but I have plenty of spares.