Medogsfat: I wouldn't like to comment on the cassette cleaning as I too own a beocord 6500 and have used it just the once in the last 10 years and only keep it to make my system look complete to be honest.
I have one too - I can't see how it would be any different from any other cassette deck. Use the same (or preferably new ) cotton bud and alcohol to clean the tape heads, the capstan shafts (the metal axles on either side of the head that rotate once you turn the Beocord on) and the pinch rollers (the rubber wheels that press against the capstans, once you coax the mechanism to engage without a cassette - I think you need to push a sense flipper in the rear of the cassette compartment).
Try to avoid too much pressure on the heads, it's a rather delicate rotating mechanism on these auto-reverse decks. And do not overdo the amount of alcohol on the pinch rollers, the rubber might not like being soaked in it.
And this needs to be done hardly ever, unless you really use the deck regularly.