I acquired a free Sigma 24-70 f/2.8 lens that had a few issues–namely, autofocus was broken and the zoom was incredibly hard to turn. After using the lens in full manual for a while, I determined that I would attempt to repair it.
I first did a quick disassembly of the mount, exposing the autofocus motor and the top of the lens components. It was clear that the metal bracket on the AF/MF switch was broken, one of the metal arms that push down the gear was dangling from the bracket. I removed the bracket and mixed up some J-B weld to glue the pieces back together.
After some googling, I determined that the lens must have already been disassembled for repair (made apparent by scuff marks on screws) as the spring that engages AF was on the wrong side of the gear. This resulted in the bracket pushing down on the gear rather than pulling it up against the spring’s force, causing the bracket to break.
After gluing up the bracket, I continued disassembly to see if I could do anything about the sticky zoom. Taking the remainder of the lens apart was tough, as Sigma lenses have soldered-on ribbon cables instead of nice detachable ribbon cables like all Canon lenses have. In addition, you must remove a single setscrew to take off the zoom ring which is hidden under the zoom ring’s rubber grip.
When removing the bottom of the lens, I pulled a bit too hard and ripped the aperture drive motor ribbon cable–I didn’t even notice I had broken it until I looked inside the disassembled lens body. Be incredibly careful when pulling apart separate pieces–these ribbon cables are very fragile! I left the aperture cable broken for the time being, as it will take a fairly long amount of time to solder back up. Note: be very careful when removing focus/zoom rings from your lens, zoom/focus encoder brushes are also very fragile.
When I reached the zoom portion, I found several small plastic bushings which were apparently crushed. The previous owner of the lens must have dropped it, crushing the bushings and skewing the alignment of the zoom. Several screws for these bushings were loose, and tightening them did make the zoom a bit easier to use. However, I could not do any more to fix the stuck zoom issue.
I also noticed that the encoder brushes for the focus assembly were bent and broken. I attempted to bend these brushes back in, but I was largely unsuccessful. I couldn’t find any replacement brushes online that matched the brushes in this lens, so I left them as-is. Interestingly enough, AF worked fine without the encoder brushes.
After waiting for the J-B weld to cure, I reassembled the lens with the spring in the right location and tested it out. AF worked properly, and the zoom was a bit easier to use. Other than the aperture being stuck at F/2.8, this was a very usable lens.
I have since repaired the broken aperture cable, and I’ll throw together a post on that procedure in the near future.