Hi everybody.  Here are my suggestions for in-situ checks on the
alignment of the three important components in a spool box.

Horizontal check: insert a music roll into the chucks at the top of the
spool box but don't pull the paper over the tracker bar.  Rest a small
spirit level on top of the roll, the tracker bar and the take-up spool
in turn and hope for identical readings.

Vertical check: get a bit of plate glass about an inch wide and eleven
inches long.  The dimensions aren't at all critical as long as the
glass fits inside the roll spool flanges.  Also get one of those little
pen-sized laser pointers that lecturers love to wave around, usually to
the intense irritation of their audience.  If you can't get a laser,
use a slide projector, or any convenient source of a reasonably
parallel beam of light.

Temporarily mark in some way centre ('center' to U.S. readers) points on
the tracker bar and the two roll spools.  Set the laser on a stable
surface like a table or a step ladder a few feet in front of the spool
box.  Adjust it until the beam is falling on the centre of the tracker
bar.  Blu-Tack could be useful here.  Rest the strip of glass on the
front of the tracker bar and note where the reflected beam strikes the
opposite wall.  Unless you are an orang-utan this is made much easier
if you have a helper.  If the helper is not your wife, partner or
companion discourage them from using a permanent marker pen on the
wall.

Repeat this process with the two spools.  You will need to raise or
lower the beam a trifle but don't alter the laser's position otherwise.
By noting the relative positions of the three reflected spots you can
quickly see whether or not you have an alignment problem.

This is quite a sensitive test so don't panic if the three reflected
spots don't lie in a vertical line.  The geometry of the situation is
quite easy to work out but my guess is that in a normal-sized room the
reflected spots could be separated by several inches and you would not
have a serious problem.

This sounds complicated but it worked for me.

John Phillips in Hobart, Tasmania.