Recently I purchased a Mamiya M645. While its exterior is in great condition, the shutter does not work. It has a peculiar issue where the shutter button only opens the shutter and it uses the battery check button to bring in the second part of the shutter (to stop the exposure). If needed, I can record video of this. Is there any way to fix this without having to buy a new body? The exterior is nearly perfect and it'd be a shame for it to just be a paperweight. Thank you.