I had a problem a while ago with my Bronica SQ-A. I had a film jam that damaged the transport mechanism in my film back. I've repaired quite a few cameras over the years (non professionally) so I thought I would give it a go.
The symptoms were as follows - I could turn the film advance lever as many times as I liked, but the frame counter wouldn't move and the shutter wouldn't lock. Changing the back returned the camera to normal operation. Removing the film from the back I found that turning the film advance crank did not result in the take up spool turning. With a bit of assistance the spool would turn when the advance crank was turned but there was little or no torque being transferred from the crank to the spool.
As it turned out, there's a spring in the drive train (to absorb shock loads I assume) that was no longer transferring power from its input to its output. For details of the strip down and fix, I documented the process in one of my Flickr sets here:
Hopefully this will help other people as I couldn't find any similar information when I took it apart. N.B. Be careful to pay attention to the screws that I have noted have left handed thread!