WM-EX606 gear slipping

I recently bought a WM-EX606 listed as not working. I replaced the belt and it played fine, but when fast forwarding there was some occasional clicking. Now both rewind and fast forward do not work when a cassette is inserted (the gears slip). What should I do?

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I had issue like that with FX811, changed belt as it seems it was too loose. Now it works ok

 

thanks for the tip. I have already ordered belts, just waiting for it to arrive...