F's viewfinder not locking in ! !

Check the pushbutton & prongs

Check the pushbutton & prongs

Check a couple of things: make sure the focus screen is nestled in all the way, then make sure the release-button on the back isn't hung up. Push it in/out and watch to see of the catch 'em prong thing actually moves.

Oh, and do you have an "F" prism FOR the "F"? I've heard/read that the F & F2 prisms/bodies are mutually compatible but the front nameplate on one or the other gets in the way and has to come off.

Sometimes there's gunk in the button-to-prong link and the button/prong gets stuck in the Open position. Hope this helps. Nikons are sturdy, but avoid the temptation of using the Shemp mallet.
 
Assuming that the screen is no mismatch and nobody made a mess of a repair, nothing. It can be fiddly to get a tightly sitting finder to lock to a worn body or vice versa.
 
I have had similar problems. I have a late F eye level prism (circular eyepiece) which is a very difficult fit on my 2 F bodies. Comparing it with an earlier F eye level prism, there are significant differences.

I also have a DE-1 which doesn't seat properly on any of several F2 bodies I have tried it on.
 
Easy.

There is a piece of foam between the prism assembly and the metal top housing of the finder to make sure it doesn't bump around.

When the finder is refurbished and the foam is too old or was replaced, tightening the screws (they are hidden under the leatherette on the top, in all four corners) that keep the prism assembly into the metal top of the finder can pull the whole prism assembly too far into the metal top housing and the body cannot grab a hold of the finder anymore.

Easy solution: loosen the screws under the leatherette corners a bit, say two strokes. And re-fit. If it does fit now: take off the finder, check for the amount of play (might be half a millimeter only) and loosen the screws completely to take the housing off the prism. You can see the foam sitting on top of the prism. Replace it with a thicker piece that will take up the play you recorded before.
Then re-fit the prism in the finder housing and mount the finder on the camera.

This can be a rather precise job, the 'height' of the prism may have an effect on the focus, I presume. If everything is fitting tightly, focus should be spot on.
 
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