Larry Pina's book 'The Dead Mac Scrolls' has some useful information on fixing 400k, 800k and 1.44Mb floppy drives and a section on how to re-align the heads without the need of an Oscilloscope. That tends to deal with the later drives rather than the earlier ones, but I suspect the procedure is similar.

Well worth getting a copy if you can find one cheap enough.

ISBN 1566 09016 4