If the drive is dirty, open the metal cover (nothing more), and clean it carefully. In particular clean the 4 LEDs and the area immediately around. Since you already have the drive open check the "Take Up Leader" (used to feed the tape) is not bent or broken. If you have to replace it, the DEC-PART-NUMBER is 74-28268.
DO NOT PUT THE MAGNET NEAR TO THE READ/WRITE HEAD OF THE DRIVE !!!
This procedure is not without risk, and we accept no responsibility for non functional drives!
If you can't operate the drive open, use the bulk eraser ;-))