The replacement aperture flex cable for my Sigma 17-35mm lens arrived. It was time for some desoldering and even more dismantling than last time. (Part 1.)
The new flex cable (photo from ebay)
After desoldering a few flex cables and unscrewing one screw the PCB could be lifted revealing the whole AF assembly and the aperture electronics.
On the left side you can see the snapped aperture cable
Then I proceeded to remove all the flex cables from the PCB, removed the AF unit and the zoom ring with the DoF scale ring. The aperture electronics became much more accessible.
At this point I dropped a tiny screw in the lens which jammed the zoom, I had to shake it out.
I removed the broken flex cable, cleared off the glue residue and starting plotting the placement of the replacement. It took a bit of origami-ing to fit the new cable, the glue on it stuck everywhere but finally I managed to jam it in the correct position, resoldered the connectors to the aperture motor and screwed the cable on the metal prong that holds it in place when zooming.
Broken flex cable removed
New cable attached
I hoped that I have done everything properly and started reassembling the lens. Reinstalled the zoom ring, AF unit and fixed the non-moving part of the aperture cable.
The aperture cable is on the left side
While doing this I examined the zoom's electronics. It uses 5 connections on the encoder brush for 5 bits of information on the position of the zoom. The focal length info is used for correct flash usage and also to maintain a constant aperture along the zoom range. When you set the aperture to f3.5 the aperture is completely open at 35mm but has to stop down at 17mm.
Zoom encoder brush
I then reinstalled the PCB, resoldered everything, double checked every soldering point and completed the reassembly by screwing on the lens mount. I first tried the lens on my old Canon EOS 500 film camera just to be safe. It didn't cause any errors but the AF was rattling, it only moved one small step to one direction then one to the other. I put the Sigma on the 550D next to check the AF and the aperture using Magic Lantern. The ML can give focal length info, it has focus pulling and focus peaking.
To my satisfaction the aperture works like a charm.
Sadly the AF has a problem on every camera body. Even unmounted from the camera when I turn the focus ring while in AF (I shouldn't but I had to) I can turn a few millimeters then it gets stuck again and again. I don't see any physical issue with the AF motor or gear. MF works fine and MF focus confirmation is accurate too. Magic Lantern always says "Soft limit reached".
The AF/MF unit
Now I have a Sigma 17-35mm f2.8-4.0 EX HSM lens with working aperture and bad AF. It is way better than before because its intended use is for landscape shooting where I use f8 or f16 and manual focus is not an issue.
i know autofokus problem
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