There is a similar thread to this one started back in '09, but at the risk of being accused of duplication I thought that I would share my repair experience with the DA 16-45. The other thread, which was a fantastic reference: pentaxforums.com/forums/10-pentax-slr-lens-discussion/68137-da-16-45-repair.html
I bought my example of eBay a few months back for a pretty good price from someone in the eastern Australian states. In due course it arrived, and I took it and my Helios 44K-4 ( also reasonably fresh from the courier bubble-wrap ) into some of the older and prettier suburbs in the city to just wander around and takes some snaps and get used to the lenses. The 16-45 was on the camera first and I took a few photos without any issues before some flowers drew out the Helios ( I'm a tragic for flowers+camera, especially if there are bees around
). When I swapped back to the 16-45 later on, I found that it wasn't focusing to infinity. The AF would simply lock up at a little over the 1 metre mark on the focusing ring and refuse to go any further. Manual focusing was no different. I contacted the seller, asking him whether the lens had any repairs done to it in the past or whether it has been disassembled at any point. He said no, he's owned it from new, and no repairs of any kind have been done. Since the lens worked fine when I received it, I couldn't yell unfair ( it could have been damaged during transit anyway ); so following some guidance from the other thread and took it to bits to see what was wrong.
After disassembling it, I found that there was a small screw lodged in the end of the front element barrel. Seriously, it took some effort with the screw driver to release it, so my repeated forcing of the focusing ring had jammed it in pretty tight [ Picture #2 ]. I checked the rest of the lens assembly ( without taking the inner element barrel apart ) for a missing screw, but found no empty threaded holes; so I still don't know where the screw came from, and hopefully I never have to find out! I reassembled the lens with quite a bit of frustration trying to figure out how it all fits back together again, and found that it would now focus to infinity; but now had another problem ( also experienced by the author of aforementioned thread ). When the camera was pointed upwards at an angle any greater than the horizontal plane, the front element barrel would literally move back into the lens by around 3 - 4 millimetres at the 2 metre mark on the focusing ring, and the focus ring would jam. Curing it was as simple as pulling the front barrel forward again, or pointing the camera at the ground; and the AF would happily carry on like nothing happened.
I took it apart again, and after playing around with it for a while I found that it was jamming up where the focusing ring locks into the inner element barrel. The focusing ring mounts onto the inner barrel by way of a sort-of bayonet system, where the two pieces will only slot together at one point which is marked on each by a small notch [ Picture #3 ]. The natural instinct when assembling it is that once the focusing ring is on, you twist it by 180 degrees to get the focusing ring away from the one point where it will fall back through the bayonet tabs. The problem is that when you use the focusing ring to wind the front barrel back on, the focusing ring has done another 120+ degrees or so of rotation on top of the 180, and the two notches nearly line up again! And when the lens is assembled and the focus ring is used either manually or by the AF system, at approximately the 2 metre mark the two notches cross over and the weight of the front barrel tries to disassemble the lens internally ( it makes a lot more sense when you're trying to reassemble the lens, trust me ). It took a few attempts at reassembling the lens before I got everything lining up properly, and it now works well, although mine seems soft at the long end.
Sorry for the long spiel, but I hope this information is of use to someone else at some point.
Similar Threads 
