F-words repairing Canon lens

F like F…. FLEX. The f.cking flex.

I had to repair a 17-55 with aperture problems. The lens started to close the aperture more and more at certain zoom values.

Simple case, isn’t it? You have to change the flex. As on the shelf I had only 1 pcs of this cable (there are not too many such cases), I worked very carefully. Before disassembling the aperture module, I made a small testing assembly: the board, the stabilizer and the aperture module, connected to a camera (an old 40d) wih an extension cable. Be careful: by this lens the aperture will not work without the IS connected!


The aperture was not working, better to say, working only when the cable was bent in certain manner. So, broken cable. (by the way: absolutely no visual signs of broken lines!)

I changed the cable, cleaned all the elements, degreased and re-greased the whole assembly. I always check the coils with the multimeter. (ca. 17 – 19 ohm)

After finishing the WHOLE work, I have done the final check: aperture was not working. F.ck, f.ck, and again f.ck! I have no time at all, and now I have to disassemble it again.

I had an another old aperture unit, which was considered bad and replaced with a complete one. Has the flex too (just not in perfect condition, as it has been disassembled from the lens.

I checked it with the “test bench”. Not working – but in an other way!


The aperture has a small stepper motor, and an optocoupler,  which gives the ZERO (open) position. The basic position is the most open one – and then the small black “flag” interrupts the light.

There can be 2 possible failures:

– No interruption at all (when sensor is dislocated, as the flag can not interrupt the beam). In this case the motor will bring it back to the start, but, as gets no ZERO signal, is buzzing till the electronic timer will stop it.
– ZERO signal all the time: for example no grounding of the interruption signal, because a damaged optocoupler – or a broken wire.   The optocoupler is in fact a npn transistor without a base. In this case the motor will try to open more and more the aperture, as the feedback line gives the information that is at the starting time all the time.

The disassembled old unit had a dislocated coupler. I pushed it into it’s place – and was working perfect.

The repaired lens module was bad with the symptom 2. – I was sure that the coupler didn’t survived the soldering.replace the repaired modules cable to a new one – so I desoldered the cable from the bad one, resoldered on the other (as I had no second cable, as i told)

(By the way: I tried to replace the bad optocoupler with one from an old 18-55 lens. On the photo the left one is from the 18-8, the right one is the original 17-55 assembly. It does not fit 100%, there is ca. 0.3 mm difference, but after working on it with some sandpaper was ok. I am sure from the function they are similar. At the end although I decided for the complete replacement)

I am a convinced atheist, but I must say THANKS GOD there was there the test bench. I checked it – not working.

After a half an hour of desperate measuring I had the explanation: the BRAND NEW cable was defective.

Now I am here, crying on the edge of the table, I will have to spend the whole time again for this work (totally: 3 complete repairs!!!)


When the brand new flex should be checked for continuity that is the end of the world.

I am sad.



Dear friend! When You reached the end of the page, means that you are interested in this subject. Case You are a native speaker, and You would like to help me spending some time, may I ask You to help my work by correcting the text from the point of view of the English grammar? If yes, just email me the corrected text – You would make me very happy.


About canonrepair

Ruzsa János. Amatőr fotós, Canon DSLR váz. Amateur photographer. Canon DSLR user.
This entry was posted in 17-55 F2.8 IS, Canon lens, EF-S lens, test bench. Bookmark the permalink.

8 Responses to F-words repairing Canon lens

  1. Petra says:

    Hi I have a problem with a canon 15-85mm. The ring that sets the distance to an object keeps on turning. The distance is set op olmost infinity. How do I fix this?

    • canonrepair says:

      Hi, if you can still focus with AF, then probably you have a loose USM motor. The USM vcan be disassembled, the roller have to be reworked (unpolished), then will be as the new one.
      Case You have no AF and no MF, could be anythng, a blocked focus ring or loose screws in the inside.

      Anyway, the lens has to be disassembled.

    • canonrepair says:

      I would say send me the lens and I will try to repair it, but I suppose you live in Netherland, so would be not as easy. The problem is, that case somebody has no knowledge about the internal build of the lens, will be destroyed during disassembling.

  2. Filip Lolic says:

    I had the same experience with the broken (new) cables twice, both times on the EF-s 17-85 lens.
    On one cable there was short circuit between two tracks that go to optocoupler, and on other cable one of the tracks was interrupted. Both failures was visible only under microscope.

    After that bad experience, I ALWAYS test every cable with the multimeter for shorts and open tracks.

  3. fiffer says:

    it is extremely rare that a new cable is broken already, what usually happens is they dont cut the end of the cable properly with a very sharp tool, but rather shear it, and the tracks are forced together at the end of the cable, causing short, check it under microscope, but not the flat side but check the thickness of it u will see the metal tracks are touching each other, u can fix that with sharp tweezers and knife

  4. Cardoso says:

    i know some technicians that put the new flex cable on top of the old one. Saves time and some possible problems when desoldering but i think leaves to other problems in the future.

  5. Been there done that, sometimes those are just so tricky and hairs are getting more and more grey while inspecting the problem. Once I replaced aperture flex cable, still not working. Well, there is usually small adjustment possibility, oval shaped screw hole, for optocoupler (which only tells aperture open position for aperture control unit). It just required tiny amount of adjusting and voila, it worked!

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