I also greased the Sharp MD-MT831 unit (though I don't recall re-greasing the Sony MZ-R900...).

What little skipping no longer happens on that unit.

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Also on the Sony MZ-R500: a plastic screw end on the front end needed gluing in; not mentioned on the previous post.

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Finally broke my main noise-canceling headphones (right ear section cracked), but I fixed them; they are holding well so far now.