Canon EF 20-35mm f/3.5-4.5 USM Lens Repair

Discussion in 'Canon EOS' started by piers_hendrie, Jul 24, 2008.

  1. Canon EOS EF Lens Repair: 20-25mm f/3.5-4.5 USM autofocus / manual focus failure
    This is my trusty wide angle lens: It has gone in numerous small dusty caves, damp caves, muddy caves, and I take it hiking in the sun and in the rain alike.
    The Problem
    The problem was failure to autofocus, followed shortly by failure of manual focus. Within 1/2 hour, it was shot. It was stuck at a focus distance of about 1.0ft. High ISO and small apertures got a few more shots out of it that day at Natural Bridge Caverns, TX, but that was pretty much the end of it.

    This lens was about $400 new, had served me well, and I was confident it could be fixed. With an upcoming climbing trip to Crestone Needle (Ellingwood Arete ascent), I didn't want to spend the time or money sending it off to be repaired. I was sure it could be fixed in my kitchen, using my Swiss Army knife and a set of small Wiha screwdrivers.

    While this report is specific to the 20-35mm f/3.5-4.5 USM wide angle lens, the nature of the failure is such that it is likely the cause of other "my lens cannot focus even in manual mode" problems, and thus generally useful.​
    Many will be familiar with the operation of the full time manual mode, which allows both the USM drive and the manual focus to occur at the same time, by virtue of the focus ring rolling on three small wheels sandwiched between the manual focus ring and the USM armature ring. Without sufficient compression force between the drive rings, neither can turn the focus ring's three small wheels, and thus the lens loses ability to focus in both auto and manual modes alike.

    There is an excellent description of the workings of the USM lens here (which I discovered after the repair):
    A guided tour through the inner workings of the EF-28-105 USM
    To disassemble the USM workings of the lens, remove four screws of the mounting plate, two screws for the electrical interconnect, and one screw for the bayonet stop. To remove this black plastic part, you first need to pop four tabs from below. Use Swiss Army screwdriver blade. Extreme care must be taken here so as not to damage or rip off the flexible PCB, which is attached at this time. Then use the Swiss Army knife blade to ease around the now-visible gap on the top-side, and pop out this part as pictured below:
    From here, the control PCB and electrical interconnect are visible:
    Carefully disconnect the four thin ribbon cables (two are friction fit, two are tabbed low-insertion-force type), and remove the PCB, which is held in place by one screw only:
    Now, remove the four black screws holding in place the large plastic collar, taking care not to damage the ribbons -- two of the ribbon cables are secured in place by each fitting onto a plastic pin.
    Life the collar off:
    Now, we can remove the components of the USM drive:
    Numbered in the order they were removed, we can see the various rings:
    1. Retaining ring,
    2. Wavy spring,
    3. Felt-like spacer,
    4. USM motor stator,
    5. USM motor armature.
    Wear component
    The problem component is the felt-like spacer, indicated with the red arrow. Clearly a wear-out-and-send-lens-to-service component, and probably suceptable to moisture, it is very disappointing. It was visibly compressed in some places where in contact with the wavy spring, and no longer provided sufficient spacing and consequently sufficient compression forces to allow the focus ring wheels to operate properly.
    I took some measurements:
    Above, on the left, we see the nominal thickness of the felt-like spaer. On the right, we see the thickness in one of several "thin spots". I found my wife's card stock to be of use, and simply cut three different rings of cardstock, whose total thickness was a little thicker than the felt-like ring was at is thickest spot. My paper rings were not perfect, because I couldn't find the blade to my wife's circle cutter, so I used the Swiss Army Knife scissors...
    This collection of three cardboard spacers now replaces the felt-like spacer in the USM motor focusing unit.

    Reassemble the USM motor components in the correct order, using the card rings as appropriate, and replace the lens components in the reverse order to which they were removed.

    Verify manual focus operation, and attach the lens to a camera and verify auto-focus operation.
    Note that no optical glass components were touched, altered, misadjusted, or otherwise compromised in this procedure. Note also no special service tools were required.
    This whole process took under an hour, and I bet many lenses that suffer the same failure mode have the same felt-like spacer, which seems to be a wear component and needs replacing. You may want to order the correct part from Canon, if you can find it -- the Parts Catalog I found here only lists the "Focusing Unit" as a single part number.

    If you are competent at tinkering, have suitable small tools, a trusty penknife, and especially if you are the "engineer type", do not be disuaded by the thousands of comments on other forums that claim "there is nothing that you can fix in a Canon USM lens", "DO NOT take it apart", "Only Canon Service can work on these lenses", etc. There is fear of the unknown, and hopefully these pictures will remove some of the mystery of at least some parts of this lens.
    The author makes no guarantee that the reader can repair his or her lens, and no warranty (expressed or implied) is provided. Perform this procedure at you own risk. If your lens is under warranty, an Athorized Canon Service Center, through your local camera shop, should be your first choice.

    Might help some folks :)
  3. Great write-up Piers! I'm not having any problems with USM at the moment but I've saved your procedure for when I do. Thank you.
  4. An excellent exposition about how to dismantle the ring type USM lens.

    I have found that it is usually sufficient to rotate the face cam, "retaining ring" as you call it, to increase the clamping force. This ring is usually held in place by three spots of varnish which often come loose.

    Rotating the cam ring clockwise re-establishes the clamping force which allows the auto and manual focus mechanism to generate enough torque to move the focusing group. If you cannot tighten up the assembly enough with the face cam you will need to replace the washer.


  5. The Canon 50mm f/1.4 requires a slightly different approach:

    These "can do" approaches (along with such things as Ken Phillips' repair of the IS unit in his 70-200 f/2.8) make me wonder if something similar might resurrect a 200mm f/1.8 or 80-200 f/2.8 with a failed USM motor, since spares are no longer available.
  6. Piers
    This sounds exactly like what happened to my 20-35 3.5/4.5. One moment it was working then it stopped. I wasn't able to manual or auto focus. In fact I posted something on this last week to see if anyone knew how much repairs might cost and if the lens was discontinued. I probably won't try the self repair you have posted though and will most likely go through the Authorized Canon Service. But yeah, I was quite bummed. I like this lens a lot. I've gotten some great shots with it.
  7. Arthur: in this instance you might do better finding a traditional camera/lens repair shop and printing out this thread for them. Canon would likely refuse the repair if they don't have original spare parts - and they don't usually disassemble sub-assemblies like motors and IS units.
  8. Hi, i wish i had seen this earlier lol, it would've made my repair of 10-22mm much easier.

    It's essentially of the same design, with the USM assembly being identical.

    Though i disagree with your idea that the spacer needed replacement. the spacer doesn't wear out because its in a
    stationary part of the USM assembly.

    If you rotate the "retaining ring" clockwise, it would increase the pressure on the spring and fix the problem.

    thanks for the tutorial!
  9. My 28-135 has a slightly different problem: it can autofocus at 28, but if I zoom in just a little bit, AF struggles and fails.
    I followed this instruction and tried both the cardboard ring as well as the clock-wise turning approaches, none of them helps.
    I wonder what could be the link between zoom and USM failure?
  10. My 28-135 has a slightly different problem: it can autofocus at 28, but if I zoom in just a little bit, AF struggles and fails.
    I followed this instruction and tried both the cardboard ring as well as the clock-wise turning approaches, none of them helps.
    I wonder what could be the link between zoom and USM failure?
  11. I have a 35-135 USM which had the same failure as described. Used the instruction to disassemble the lens. In the last step where you are removing the 1. Retaining ring, 2. Wavy spring, 3. Felt-like spacer, 4. USM motor stator, 5. USM motor armature, I noted that I could readly lift up the Retaining Ring, no turning of the ring required. As it turns out the retaining ring, as it is called, must be pressed down and rotated. You will note that there are three notches on the inside of the ring and when the ring is rotated the notches go behind a lip to secure the retaining ring. After this was done, and the lens was put back together, the lens was working without problem for both automatic and manual focus. Thank you for your help.
  12. Thanx to Your post I have successfully mended my EF-S 17-85 IS USM lens.
    My lens was broken in a different way: It would focus, but the AF performance was very-very poor. Sometimes it just kept swinging the focus back and forth without getting a lock at all, sometimes it eventually managed to get a lock. I was able to focus by hand. First I thought the camera body was to blame, but with a different lens it worked perfectly. When I disassembled my lens I noticed that USM motor was sticking. The reason: the same stupid spacer that had lost it's shape, but in my case the loss of elasticity was asymmetrical so that stator was tilted and the motor got sticky and inaccurate as a result.
  13. Thank you very much for this.
    It enabled me to fix my lens (EF 20-35 3.5-4.5 USM) and have a working camera.
    A Canon repair would have not been economic.
    I had the same symptoms (failure to autofocus, followed shortly by failure of manual focus)
    Instead the cardboard spacers, I rotated the felt washer and 're-sprung' the wavy spring.
    The felt washer ranged between 0.5mm to 0.8mm.

    I had a devil of job removing the black plastic part with the four tabs.
    I removed the single screw holding the PCB and then the ribbon cables followed.
    This allowed me to more easily pop the four tabs.
    (I was considering not removing the black plastic part with the four tabs, but reassembly would have been difficult)
    The reason for difficult removal was Loctite holding them.

    I only had 3 black screws retaining the large plastic collar.
  14. Following reading Mitchell Macha , Jun 14, 2009; 04:48 p.m. response (Thanks)
    I dismantled again and rotated the retaining ring under the lip as suggested.
    This must be the cause and fix, gradual slippage or something and then it dislocates.

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