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Frank A,

Attached a file with info about the same problem with a Davis Mk15 sextant.

Hope this helps to solve your problem.

regards,
HermanD

File:
[attachment replaced with direct quote of text --FER]

I was messing around with mirror adjustments etc. and discovered something weird: I couldn't get the index arm mark to line up with a whole degree with the micrometer reading anywhere near zero. More fiddling, and I find that the micrometer knob is slipping on its spindle. Eventually the micrometer will no longer adjust elevation at all, the knob is just spinning. 
It turns out, the micrometer knob has slipped off its spindle to where it's just loose. I set the index arm to zero degrees, aligned the micrometer to zero, placed my thumb on the micrometer knob and finger on the back of the index arm, and "squeezed" the knob back onto its spindle.