Got a free Nikon F3 from someone who passed away, was curious as to why the light meter was borked
Well this certainly explains it. The ASA/ISO glass potentiometer has a crack in it (left side). Quick check with a multimeter shows it is indeed a problem, even though it doesn't go all the way through
At least this is a popular enough camera that parts are available, and old enough to have a repair manual available online. Nice to see that troubleshooting steps and causes used to be available.