john laid this down on his screen :
Thanks - the trouble is that everthing is inside the plastic shell. I don't
want to try and remove it without a clue of what I might be up against. Any
idea how the notors know when to stop? There are 4 positions.

My own guess would be....

A small motor driving a screw thread which sets the beam. A
potentiometer which provides feedback of the position of motor to the
controller. The switch provides 4 resistor values and the controller
compares the voltage from these against the feedback voltage, driving
the one way or the other until the two voltages are a close match.
There has to be an acceptable 'close match' (hystrerisis), otherwise
the motor would be constantly hunting to find a none acheivable perfect
match.

Your description of the problem suggests it could be (in most likely
order)...

1. Intermitant remote potentiometer or intermitant wiring between.
2. The switch.
3. The controller.

--

Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk