I have a Pers LaserWriter NTR which prints fine from the manual feed but always thinks the multipurpose tray is empty. The black paper-sensing plastic thing on the bottom of the printer is intact and moves nicely, but even if I tape it up to the top, it still thinks the tray is empty.
Interesting to note, with the tray in place, if I lift the front of the printer to about 45° with the back still on the table, it will start to print - it stops and the paper light comes back on if I set it back down. Is there a mercury switch in there somewhere?