piers_hendrie Posted July 24, 2008 Share Posted July 24, 2008 <STRONG>Canon EOS EF Lens Repair: 20-25mm f/3.5-4.5 USM autofocus / manual focusfailure</STRONG><BR><P>This is my trusty wide angle lens: It has gone in numerous small dustycaves, damp caves, muddy caves, and I take it hiking in the sun and inthe rain alike.<BR></P><P align="center"><IMG src="http://www.xenopuselectronix.com/USM_Repair/__IMG_9181.jpg" width="400" height="295" border="0"><BR></P><P><STRONG>The Problem</STRONG></P><P align="left">The problem was failure to autofocus, followed shortly by failure of manualfocus. Within 1/2 hour, it was shot. It was stuck at a focus distance ofabout 1.0ft. High ISO and small apertures got a few more shots out of itthat day at Natural Bridge Caverns, TX, but that was pretty much the endof it. <BR><BR>This lens was about $400 new, had served me well, and I was confident itcould be fixed. With an upcoming climbing trip to Crestone Needle (EllingwoodArete ascent), I didn't want to spend the time or money sending it offto be repaired. I was sure it could be fixed in my kitchen, using my SwissArmy knife and a set of small Wiha screwdrivers.<BR><BR>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 thusgenerally useful.<BR></P><P>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 sametime, by virtue of the focus ring rolling on three small wheels sandwichedbetween the manual focus ring and the USM armature ring. Without sufficientcompression force between the drive rings, neither can turn the focus ring'sthree small wheels, and thus the lens loses ability to focus in both autoand manual modes alike.<BR><BR>There is an excellent description of the workings of the USM lens here(which I discovered after the repair): <BR><A href="http://www.photoscene.com/sw/tour/inside.htm">A guided tour through the inner workings of the EF-28-105 USM</A></P><P><STRONG>Procedure</STRONG></P><P>To disassemble the USM workings of the lens, remove four screws of themounting plate, two screws for the electrical interconnect, and one screwfor the bayonet stop. To remove this black plastic part, you first needto pop four tabs from below. Use Swiss Army screwdriver blade. Extremecare 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 toease around the now-visible gap on the top-side, and pop out this partas pictured below:</P><P align="center"><A href="http://www.xenopuselectronix.com/USM_Repair/_IMG_6124.jpg"><IMG src="http://www.xenopuselectronix.com/USM_Repair/__IMG_6124.jpg" width="400" height="267" border="0"></A></P><P>From here, the control PCB and electrical interconnect are visible:</P><P align="center"><A href="http://www.xenopuselectronix.com/USM_Repair/_IMG_9182.jpg"><IMG src="http://www.xenopuselectronix.com/USM_Repair/__IMG_9182.jpg" width="400" height="296" border="0"></A></P><P>Carefully disconnect the four thin ribbon cables (two are friction fit,two are tabbed low-insertion-force type), and remove the PCB, which isheld in place by one screw only:</P><P align="center"><A href="http://www.xenopuselectronix.com/USM_Repair/_IMG_6128.jpg"><IMG src="http://www.xenopuselectronix.com/USM_Repair/__IMG_6128.jpg" width="400" height="301" border="0"></A></P><P>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 securedin place by each fitting onto a plastic pin.</P><P align="center"><A href="http://www.xenopuselectronix.com/USM_Repair/_IMG_9184.jpg"><IMG src="http://www.xenopuselectronix.com/USM_Repair/__IMG_9184.jpg" width="400" height="302" border="0"></A></P><P>Life the collar off:</P><P align="center"><A href="http://www.xenopuselectronix.com/USM_Repair/_IMG_6129.jpg"><IMG src="http://www.xenopuselectronix.com/USM_Repair/__IMG_6129.jpg" width="400" height="288" border="0"></A></P><P>Now, we can remove the components of the USM drive:</P><P align="center"><A href="http://www.xenopuselectronix.com/USM_Repair/_IMG_9185.jpg"><IMG src="http://www.xenopuselectronix.com/USM_Repair/__IMG_9185.jpg" width="400" height="282" border="0"></A></P><P>Numbered in the order they were removed, we can see the various rings:<BR>1. Retaining ring,<BR>2. Wavy spring,<BR>3. Felt-like spacer,<BR>4. USM motor stator,<BR>5. USM motor armature.</P><P align="center"><A href="http://www.xenopuselectronix.com/USM_Repair/_IMG_6132.jpg"><IMG src="http://www.xenopuselectronix.com/USM_Repair/__IMG_6132.jpg" width="600" height="231" border="0"></A></P><P><STRONG>Wear component</STRONG></P><P>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 suceptableto moisture, it is very disappointing. It was visibly compressed in someplaces where in contact with the wavy spring, and no longer provided sufficientspacing and consequently sufficient compression forces to allow the focusring wheels to operate properly.</P><P>I took some measurements:</P><P align="center"><A href="http://www.xenopuselectronix.com/USM_Repair/_IMG_9188.jpg"><IMG src="http://www.xenopuselectronix.com/USM_Repair/__IMG_9188.jpg" width="400" height="267" border="0"></A><A href="http://www.xenopuselectronix.com/USM_Repair/_IMG_9190.jpg"><IMG src="http://www.xenopuselectronix.com/USM_Repair/__IMG_9190.jpg" width="400" height="267" border="0"></A><BR></P><P>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 differentrings of cardstock, whose total thickness was a little thicker than thefelt-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 usedthe Swiss Army Knife scissors...</P><P align="center"><A href="http://www.xenopuselectronix.com/USM_Repair/_IMG_9194.jpg"><IMG src="http://www.xenopuselectronix.com/USM_Repair/__IMG_9194.jpg" width="400" height="267" border="0"></A></P><P>This collection of three cardboard spacers now replaces the felt-like spacerin the USM motor focusing unit.<BR><BR>Reassemble the USM motor components in the correct order, using the cardrings as appropriate, and replace the lens components in the reverse orderto which they were removed. <BR><BR>Verify manual focus operation, and attach the lens to a camera and verifyauto-focus operation.<BR></P><P><STRONG>Notes</STRONG></P><P>Note that no optical glass components were touched, altered, misadjusted,or otherwise compromised in this procedure. Note also no special servicetools were required.</P><P><B><STRONG>Conclusion</STRONG></B></P><P>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 <A href="http://photography-on-the.net/forum/showthread.php?t=356679">here</A> only lists the "Focusing Unit" as a single part number. <BR><BR>If you are competent at tinkering, have suitable small tools, a trustypenknife, and especially if you are the "engineer type", do notbe disuaded by the thousands of comments on other forums that claim "thereis nothing that you can fix in a Canon USM lens", "DO NOT takeit apart", "Only Canon Service can work on these lenses",etc. There is fear of the unknown, and hopefully these pictures will removesome of the mystery of at least some parts of this lens.<BR></P><P><B><STRONG>Disclaimer</STRONG></B></P><P>The author makes no guarantee that the reader can repair his or her lens,and no warranty (expressed or implied) is provided. Perform this procedureat you own risk. If your lens is under warranty, an Athorized Canon ServiceCenter, through your local camera shop, should be your first choice.</P> Link to comment Share on other sites More sharing options...
chrisjb Posted July 25, 2008 Share Posted July 25, 2008 http://www.photo.net/canon-eos-digital-camera-forum/00QFDm Might help some folks :) Link to comment Share on other sites More sharing options...
Jeff Lear Posted July 25, 2008 Share Posted July 25, 2008 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. Link to comment Share on other sites More sharing options...
HenryUK Posted July 25, 2008 Share Posted July 25, 2008 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. Henry Henry Link to comment Share on other sites More sharing options...
mark u Posted July 25, 2008 Share Posted July 25, 2008 The Canon 50mm f/1.4 requires a slightly different approach: http://www.fotomozaic.ro/artikel.php?idstory=225&s=1 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. Link to comment Share on other sites More sharing options...
arthur_reyes1 Posted July 25, 2008 Share Posted July 25, 2008 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. Link to comment Share on other sites More sharing options...
mark u Posted July 25, 2008 Share Posted July 25, 2008 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. Link to comment Share on other sites More sharing options...
kim_zhou1 Posted October 18, 2008 Share Posted October 18, 2008 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! http://photography-on-the.net/forum/showthread.php?p=6505691 Link to comment Share on other sites More sharing options...
thu_nguyen Posted December 16, 2008 Share Posted December 16, 2008 <p>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.<br> I followed this instruction and tried both the cardboard ring as well as the clock-wise turning approaches, none of them helps. </p> <p>I wonder what could be the link between zoom and USM failure?</p> Link to comment Share on other sites More sharing options...
thu_nguyen Posted December 16, 2008 Share Posted December 16, 2008 <p>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.<br> I followed this instruction and tried both the cardboard ring as well as the clock-wise turning approaches, none of them helps. </p> <p>I wonder what could be the link between zoom and USM failure?</p> Link to comment Share on other sites More sharing options...
mitchell_macha Posted June 14, 2009 Share Posted June 14, 2009 <p>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. </p> Link to comment Share on other sites More sharing options...
antti_andreimann Posted July 16, 2009 Share Posted July 16, 2009 <p>Thanx to Your post I have successfully mended my EF-S 17-85 IS USM lens. <br> 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.</p> Link to comment Share on other sites More sharing options...
william_hester Posted July 1, 2012 Share Posted July 1, 2012 <p>Thank you very much for this.<br /> It enabled me to fix my lens (EF 20-35 3.5-4.5 USM) and have a working camera.<br /> A Canon repair would have not been economic.<br /> I had the same symptoms (failure to autofocus, followed shortly by failure of manual focus)</p> <p>Instead the cardboard spacers, I rotated the felt washer and 're-sprung' the wavy spring.<br /> The felt washer ranged between 0.5mm to 0.8mm.<br /> <br /> I had a devil of job removing the black plastic part with the four tabs.<br /> I removed the single screw holding the PCB and then the ribbon cables followed.<br /> This allowed me to more easily pop the four tabs.<br /> (I was considering not removing the black plastic part with the four tabs, but reassembly would have been difficult)<br /> The reason for difficult removal was Loctite holding them.<br /> <br /> I only had 3 black screws retaining the large plastic collar.</p><div></div> Link to comment Share on other sites More sharing options...
william_hester Posted July 1, 2012 Share Posted July 1, 2012 <p>Following reading <a href="../photodb/user?user_id=5201364">Mitchell Macha</a> , Jun 14, 2009; 04:48 p.m. response (Thanks)<br> I dismantled again and rotated the retaining ring under the lip as suggested.<br> This must be the cause and fix, gradual slippage or something and then it dislocates.</p> <p> </p><div></div> Link to comment Share on other sites More sharing options...
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