EOS 1N RS Stuck Eyepiece Shutter

The eyepiece shutter on my 1N RS seems to be stuck. I've tried moving it back and forth, but it only seems to move between blocking the lower right corner and halfway. Does anybody know how to fix this? I couldn't find any guides online. Is it something that I could fix myself, or am I better off getting it repaired somewhere?
Thanks,
Eric

Broken or dislodged pawl. It is usually a service job.
Sometimes tapping the eyepiece surround with a small rubber mallet can reset the mechanism, but if not, to the service bench. This glitch has also affected my EOS1N, albeit when it was almost new, and never since (around 1996).