Sigma 17-35mm aperture flex cable repair, part 1.

A few weeks ago I purchased a Sigma 17-35mm f2.8-4.0 EX HSM wide angle zoom lens for Canon EF on ebay. It arrived a few days later. Nice packaging, seemed like it was in good condition, the seller said the lens worked correctly. On the first try I noticed that while zooming there was a grinding noise coming from the lens barrel. I immediately thought it was some cable that was in the way of the zoom. It was still usable... well almost. AF worked, zoom worked but the images were blurry, most likely there is some decentering. I wanted to try smaller apertures, this lens is for landscape shooting with a 5D so it should be used at f8 or f16. Then came the real problem: the aperture did not close. It didn't work on the 550D neither on the 5D. I even tried it on a Canon EOS 500 film camera but it did not work on it either. That was the point where I was almost certain that the aperture cable was broken. It was a cable that caused the noise while zooming.

I had to gather some strenght to start disassembling the lens. It is an electronic lens after all so it is not as simple as a Helios-44M for example.

The Sigma 17-35mm before disassembly

First the rear plastic dust guard had to come off, it is held on by 3 screws.

The mount of the lens. The black ring is the dust protector part.

After this the PCB became visible. The soldering was fine, I had to dig deeper. 

Visible PCB

To remove the Canon EF mount from the lens I had to unscrew the electronic contacts from the mount (2 screws) and unscrew the mount from the lens body (another 4 screws). The PCB was almost free, only a brass shim had to be removed. The shim is for setting the correct flange focal distance.

The PCB with the brass shim still on.

The PCB

The PCB can only be removed after desoldering all the flex cables from it apart from the Canon EF contacts. For better access I unscrewed the black plastic ring that has the AF/MF switch. It was a bit tricky because the switch has a metal hinge on the inside that moves a gear and also a microswitch. After a little wiggling the ring was free.

Focus gears under the PCB

After all this I was sure that I had to desolder all the flex cables from the PCB, remove the PCB to access the aperture drive motor and all parts of the aperture flex cable. The replacement flex cable was already on its way. My soldering skills are not good so I postponed the task until the new cable arrived. (Part 2.)