RB67 Pro Back: Counter Failure From a Loose Gear Plate Stud
Originally posted on Instagram. Notes from the repair bench, kept here as a searchable archive. View the original on Instagram
This Mamiya RB67 Pro back came into the shop with complaint of the counter not working, so the film would just wind through without stopping. When I removed the top cover, the parts shown in the second photo came spilling out of the top plate mechanism. What had happened was that the stud for the gear plate had come loose and so all these parts then became disassociated with the top mechanism plate. Straightforward to put everything back in place but the spring that fits on the bushing was nowhere to be seen. No real way for it to escape so my thought was that this back had been disassembled at some point and this part was lost. Another clue to previous disassembly was that some of the parts inside were bent as seen in photo four. After installing a new spring and attaching the stud with some very light sealer, all is well. Of course, light seals also needed replacing.
These backs take a lot of force from winding and repairs often require new parts. I also see a lot of loose parts in these backs as the top mechanism plate is fairly thin so the threads on studs don’t extend more than about one millimeter. Or the studs are staked in place.
I do install most of the screws with a very light touch of thread sealer. I often find threads with a very heavy application of sealer and it makes non-destructive removal difficult at times. These 1.4-2mm threads don’t need much to hold them, certainly not high strength Loctite!
