I have had some success cleaning cassette heads in place with long cotton swabs, which are often sold for 'wound care' in pharmacies. Usually they only have the expensive sterile ones, which are wasted on this. Clean gently with rubbing alcohol, and try to leave no fibers behind. They do make special 'head cleaning solution', but rubbing alcohol seems to do the job just as well in my experience.

It is a challenge to see what you are doing in there.
It is much easier to do a good cleaning job by removing and opening up the headunit, but perhaps not worth the security reset issues in this case.

I have had no luck with those special 'head cleaning cassettes' that that they sell.

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1986 300E 5-Speed 240k mi.