I just acquired a spare 8216A digital cassette drive from eBay. It wasn't advertised as working, but for $30 including 1 used and two new tapes, it was a no-brainer.
The drive works mechanically: that is, tapes spin, it goes throw the NEWM routine, and so forth. Most attempts to read and write from the unit fail with a "medium error" (once or twice I've gotten a good directory listing).
A closeup of the tape head shows the likely problem:
The question is "Is this contamination of some sort, perhaps flaked oxide, that can be removed, or is the head toast?" Medium hard rubbing with a cotton swab has no effect.