The symptoms were:
Prior to the jam the film advance was slightly stiffer than usual.
Once jammed, the film advance crank would turn as freely as when there is no film in the camera.
The shutter would never prime nor the film transport lock - you could keep turning the crank as the film would never advance.
Brass swarf in the film chamber(!)
Once the film was removed, holding the film take up spool whilst advancing the film you could feel very little torque being transferred to the spool.
Film backs for this camera are still pretty expensive, so I was keen to try to fix this one rather than just buying a new one. It turned out to be a pretty straight forward fix and didn't require any replacement parts or special tools.
Step 1: Disassembly
A small Phillips screwdriver,
A small to medium flat blade screwdriver,
Two pairs of needle nose pliers or small general pliers and
Remove the film back from the camera and then remove the film insert (see photo) from the back, as this is the only part we will be working on.
Step 2: Disassembly - Leatherette
There is a little rectangular bit of thin metal attached to the underside of the leatherette. In the 2nd photo I have shown where it goes, but there is no need to detach this from the leatherette.
Step 3: Disassembly - Rewind Crank
Step 4: Disassembly - End Plate Removal
In the 2nd photo you can see the mechanisms that are behind the end plate. They include the frame counter, film transport, exposed frame indicator and dark slide indicator.
For this repair, I am only interested in the film transport so will ignore the rest.
Step 5: Disassembly - Takeup Spool Drive Removal
To do so, first remove the plastic piece that runs along the rear edge of the insert. Four screws are holding it in place (2nd photo). Then remove the 2 screws (3rd photo) that are holding the one-way ratchet for the film advance, then remove the ratchet also (4th photo).
This will reveal the gear train for the film transport. Remove the two steel gears (5th photo) and this will provide access to the takeup spool drive unit (6th photo). There are three screws holding the unit in place (7th photo). Remove these screws and retract the unit.
Step 6: Disassembly - Takeup Spool Drive Fix
Once the unit is unscrewed into three parts, I focused my attention on the right hand part (in 3rd photo). This part broke down further into smaller component pieces. It was also covered in brass swarf.
Basically the mechanism is a metal form that holds a strong cylindrical spring, with a geared top cap and a bottom cap that engages the take up spool directly. Under normal use, turning the top will result in the bottom turning also. The spring appears to perform the function of buffering shock load in the gear train.
In my insert, the top could rotate freely and transferred very little torque to the bottom of the device (where the film spool would be). This appeared to be the problem.
The top cap is held in place when the unit is assembled. In it's current disassembled state, it just lifts off. This gives some access to the spring. As the form has been created to hold the spring, it could not slide off. However, I could remove the spring by lifting one edge of it over the lip of the form and then chasing the edge round the form. This effectively unscrewed the spring from the form. Once I had removed the spring, I could clean the swarf from all the parts.
The bottom cap is attached to the form that held the spring. The top cap is free fitting. There is a notch in the bottom cap and also in the top cap that the ends of the spring fit into so that it can transfer torque between the two. Taking a closer look at my spring, I could see that there was a right angle bend in one end of the spring to create a leg that fits into the bottom notch. There was no similar bend in the other end of the spring for the top notch. It appears that there was once a bend in the spring, but it wasn't there now.
I can only assume that the excess load caused by the film jam resulted in the spring being pulled through the notch and caused the bend to be removed in the process. The loose end of the spring then proceeded to rotate inside the housing gouging the softer brass as it went, causing all the brass swarf.
Using two small pliers, one to hold the spring and the other to hold the unbent end, I bent the spring back into what I believe to be its original shape - i.e. a right angle foot on both ends (8th photo).
I then reassembled the drive unit in the reverse of how I disassembled it.
Step 7: Reassembly
Congratulations, your Bronica 120 Film Back should now be in full working order! :D