Switching to manual (M90, whatever) won't release it? I had that problem with a little FG years ago, which came down to the computer needing replaced. Those haven't been available for probably 20-25 years now.
I agree with the other posts, cheaper to replace them if you can't fix them yourself. I'm not even sure what Nikon's I have right now; EM, FG's, FE2, F3HP, Nikkormat, F1's, F2's, F100, N90, so on and so forth. There's probably 25-35 different lenses for them too. I love my Nikons (actually, I love all my film cameras). Meter's in the old F's die off, but hey, who needs a meter anyway? There are hand-held meters and you can always just look at your shadow and work from there.