I want to share with you guys here a small project I have done to repair my 24 70 F2.8 (version I) which has a problem focusing manually. When I rotate the focus ring in manual mode, the ring slip at certain points and cannot achieve focusing. Auto focus is fine. So I ordered a set of JIS screwdriver and opened up the lens to check it out.
I discovered that the silver retaining ring of the USM unit was not engaged thus the pressure required to control the USM system has lost and have to be adjusted. I screwed the retaining ring back in, added a few drop of loctite (blue) to fix it in position. Put everything back and it's working okay now.
Since I have not dismantled the Helicoid and the other glass elements, accuracy of focus is not affected. Do not attempt to DIY like I did. You risk sending it back to camera repair shop and pay $300 or more to make it work again.
I like the cardboard tray to catch loose parts.
I have repaired countless pieces of electronics over the last 40 years, and losing a screw, a nut, a washer, a jumper, a harness, or a widget can drive you nuts. However, I have learned one trick over the years when I lose track of something under those conditions.
Don't get up. Stay seated. Remind yourself of this, "It is within arm's reach." It works 90% of the time.
I just performed this repair and wanted to add a couple things that might be helpful to anyone else looking to do the same.
The silver metal retaining ring that allows manual focus to engage will have some blue adhesive on it already. In my case, it was dried up and not sticky anymore (probably why I was having this issue). Now, to get the silver ring to engage properly, it needs to be tightened just slightly. You'll notice some little indentations around the outside edge of the ring. Carefully use a flat-head screwdriver in one of the indentations to turn the silver retaining ring clockwise while gripping the body of the lens. I moved mine a matter of degrees and it's no longer getting stuck at infinity. After moving mine I wiped a little adhesive where the ring is held in place.
Note you don't need to remove the entire USM assembly to do this, just get past the PCB and the zoom ring assembly and you'll have access to this ring.