Repairing a Sigma 24-70 EX Lens

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.

Ethan is a computer engineer and open source hardware/software developer from Michigan. He enjoys AVR and linux development, photography, mountain biking, and drinking significant amounts of home-roasted coffee. Find out more at ethanzonca.com.

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27 comments on “Repairing a Sigma 24-70 EX Lens
  1. Massimo says:

    Hello Ethan.
    I have also this zoom and I have to dismount it to fix a problem on the front lens.
    Have you the diagrams or the manual? If yes, please can you send me?
    Thank you in advance.
    Max

  2. Rimvydas says:

    Hello πŸ™‚ I am planning to dismantle this lens too..
    i have problems with AF parts.. it looks like sth is stuck there.. Could you send me any info regarding to dismantling if you have?

    • Hey, unfortunately I do not have a disassembly guide, and I didn’t take any photos of the teardown process. As I remember, there might be a disassembly guide floating around somewhere online. Most sigma lenses are very similar and the teardowns aren’t too difficult, just remember that there is a screw under the rubber focus grip, and watch out for those easily-breakable ribbon cables!

  3. Alberto Zimmermann says:

    Hi there!

    I had one of the screws inside my lens just going around and when not locking the focus it would lock the zoom. Had a quote from a few camera shops, and as it would cost 300 bucks to fix it, i decided to disassemble it myself as i’ve had success with a couple of old pentax primes before. However, didn’t happen this time. it’s a pain in the ass to find the correct way to put the elements back together but at least i’ve found the loose screw and put it back again. Have any of you guys found the disassembly guide? that would help a lot putting the missing parts back together. cheers.

  4. Alberto Zimmermann says:

    Just fixed it… By the last steps i ended up braking the aperture connection, but you can get that on ebay anyway… good luck to everyone!

  5. Donnie says:

    Hey Guys

    I have det same problem with loose front end and stuck focus ring.
    I’ve tried to disassemble it and i can only get to where the zoom ring is removed (picture 5 on this page).
    What will i have to remove next, to get to picture 6?

    My goal is to get it working with manual focus, since i’ll use it for filming more that still.

    Hope somebody can help.

  6. steve says:

    The broken zoom coller fingers are probably the sensors for the cameras 3D flash settings, ie. the lens reports the range of the focus and adjusts the flash accordingly. If these are not functional, the lens is still very useable, might nit even miss that function or know if it is working.

    • Hmm… if I remember correctly, there were actually two sets of gold fingers inside the lens. The exposure on the lens seems to be off at different zoom levels, so I think at least one of the encoder rings must have reported the current focal length of the lens to calculate exposure. I have no idea what the other set does–you’re probably right in that they could be for flash settings.

    • Philippe says:

      Still error 01 on my lens 24-70 2.8 Sigma after sending it 3 times to New York headquarter repair shop
      Grrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr
      Don t know what to do.

      Anyone ???

  7. yani says:

    @phillippe try to check ur apeture knob it must on f32.. still no site for manuasl???

  8. RCW says:

    I too am stuck at picture 5 with the zoom ring off, but I can’t figure out how to get the focus ring off. Help?

  9. Ben says:

    So good!!! God a used one off ebay, said in near mint condition;( Can’t hardly move the zoom ring, hoping God willing ebay will help out. Great info here. Thanks so much to everyone for sharing. That really helps, Ben.

  10. socrates says:

    Do you know how to remove the front element out of the barrel?

  11. Piet Heijn says:

    Had a stuck manual focus ring after dropping the lens.
    I removed the front element and front barrel (just turn the black ring on the front glass element counter-clockwise and it should come off, there are 2 small dents to give you some grip with a small screwdriver. Then remove 3 small screws next to the glass element and the front barrel comes of.)
    I shook the lens a bit in all directions. Some broken small plastic rings came out. After that it just worked fine again…

  12. Ben says:

    So so cool you got that lens working for you!! I had gotten one from an apparent pawn shop and it wasn’t working…thank God the took it back and I was fortunate to get another!! So good.

  13. Vicken Mardiros says:

    hello i’m Vicken,I have an out autofocus in my lens sigma 24-70 f2.8 for Nikon any help to solve this issue??? please thank u in advance.

  14. adi says:

    hi!
    I own this Sigma 24-70mm lens and i have a problem with the aperture. When I set the aperture over f4 the iris remain closed (does not come back, open) but if I remove the lens an reattach to camera the aperture is coming back to f2.8…. so, what should I do? I also disassambly the lens and check the aperture flex and it is ok (not broken)…

  15. Marvin says:

    Hello, I had a little accident with my lens while removing the camera from a cheap school tripod -.-
    Anyway the ribbon cable that connects the board with the mount contacts ripped, is this easily fixed? Or can it get quite expensive since the ribbon cables are not detachable?

    • I actually ripped a ribbon cable while taking this lens apart to repair it (see the 6th photo in the post). If you (or someone you know!) is good at soldering, you can fix the ribbon cable. Use a knife to scrape off the top kapton layer on the cable. Then form solder bridges between the contacts of each cable, or solder small wires between each cable. Good luck!

  16. Toni Hallikas says:

    Same problem here. I cannot tear it apart more than taking zoom ring away. So is there any special tricks to get inner barrels out of the body.. as siin on picture number 6?

  17. Thomas says:

    Thanks for this information.
    I dropped my camera with sigma 10-20 lens attached and it hit ground head first. The autofocus and distance indicator was stuck so I disassembled the lens yesterday.
    There is a plastic plate around the front lens element that I had to pry open with a sharp flat head screwdriver. A more elegant way is probably to stick some double side tape on the plate and try to unscrew it. Underneath the front plate and the rubber focus ring there are some screws. I added a little bit of grease lubricant to the spinning gear and gave it a few turns. I was lucky, the stuck part mysteriously recovered itself through this process. Would recommend to tape/cover the lens element at the beginning to avoid scratches.

  18. Fish... says:

    Hello Ethan
    My lens is having issues focusing from the view finder it goes dim when trying to focus but works Normally via live mode… Do you have an idea what could be the cause ???

  19. Mantas says:

    Where is possible to get free spare arts for this lens? I need first ring where AF button.
    grimas25@gmail.com

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