Nikon Labophot fine focus problem

I have in front of me a Nikon Labophot. Its stage slowly creeps
downward due to gravity...with the fine focus knob slowly turning in
response to the stage creep. Is there an user adjustment that I can
make to correct this problem? Also what causes the focus assembly to
do this...obviously it did not do this before.

I would like to find a copy of the user and accessory documents that
apply to this instrument....can you point me to where one might find
it?

Thanks for whatever help you can offer.
.