Clearly a belt fault.
There are two screws under the black grill at the rear.
Make sure you get the correct ones or you will dismount the lid spring.
That's not the 4 large head screws but the two smaller.
And watch out when you lift up the right hand section, there are some cables and plugs
that prevents the section from being lifted more than a few centimeters.
Grab a belt kit, printed instructions with photos and text can be included, it will
take you through it all in detail.

Martin 

Thank you Martin for your reply. Y have extracterd the two screws and unplugged the cables so i can get up all the part. But now, i see all the tae deck but i suppose that i need to unscrew something more to see the belts of the tape. What do i have to do? how does it looks? Do you have any picture of it? Forgive me, but i'm very passionate with this beocenter and i don't wanna break anything.

You need to remove three screws holding the tape drive section.
You may also need to lift up the turntable section to undo the connectors and free the cables, they
are sometimes too short to allow for flipping the tapedrive upside down.

Martin

I finally took to the down part of the tape. What i think was the belt that was rounding a white wheel, now is gone, and i think that the rest of the belt has become now something like grass, that stained me extremely. Needed many soup and water to clean me hands. If i buy a kit of belts for the tape, where could i buy it? How much would it cost? How long would took to arrive to my hom (i live in Spain)?

Thanks a lot.

I can sell you a set of belts.
You have a PM, check your inbox at the extreme top right corner of this page.

Martin