Isn't it an optical detector? There is a small glass tube in the vicinity of the write protect detect arm. The arm swings fine, so not broken. Have hit with compressed air and a brush, not alcohol tho. Oy.
Ah, you have a really ancient drive with a reed tube + magnet working as the detector. First check whether the swinging arm still has the magnet in place, next check that the reed tube starts conducting once you introduce the magnet to its vicinity.
