Apple LaserWriter 12/640 PS - Manual (Page 229)

Checking the optional 500-sheet cassette and feeder
m
Remove the paper cassette to see whether the jam is in the paper cassette area. If so,
gently release or pull out the jammed paper.
Check that all paper edges are under the bracket at the front of the cassette.
Papers edges over the bracket are also considered paper jams.
If you have removed the jammed paper, open and close the top cover to reset
the printer.
230 Chapter 11