"Bill" wrote

I've owned this F Photomic since 1969 ...
but when you wind the film, the
mirror flips into the "up" position.
Upon releasing the shutter, the
mirror returns down.

What a great feature -- don't tell marketing.

None of my Nikons has ever done this so it's
all supposition.

I would look at the mirror lock up control and
fiddle with it. It seems the mirror wants
to be in the up position because of the mirror
lock but it has skipped a cam and the mirror
goes down when the shutter is released, like
it normally does -- except that it should
be staying up because the MLU is engaged.
If you are lucky you have a half-engaged MLU
and with some futzing you may be able to get
it fully engaged.