Originally posted by pygmalionundone Dave, thanks so much for the pictures. Even after shipping I only paid $35, and so I will only gain about $15 to send the lens back for a refund, which the seller has agreed to do. My sense after looking at your post is that somehow the ring mechanism got detached. Now would it be possible a piece broke in there (the initial resistance with turning the ring that finally gave way), and if so would it be difficult/impossible to repair?
If some part is broken, the only source is another lens or making it yourself, which is not that easy. That's somewhat rare, though. Loose screws are the biggest problem I've seen, and one possible culprit here. But I was curious, so I did some further disassembly on my lens. I removed the three phillips screws going into the lens body, which are exposed when you remove the aperture ring. One of the heads is right under the yellow "m" on the distance scale. Then you can pop off the whole aperture adjustment assembly. On my lens this part was kind of stuck on, but came off without a hammer or violence.
Then I could remove the ring on the right, which is upside-down in this photo.
These parts form the semi-indepent aperture movement I referred to in my first post. I'm not sure if I can describe it with words but here goes, referring to the second photo. The lens aperture has a round pin connected to it which fits in the small fork riveted just inside of the aperture lever, top center of the mechanism on the left. There's a return spring, mostly hidden at the bottom of the mechanism. A stamped steel not-quite round piece has the aperture lever and fork riveted to it, and is attached to the mechanism body with one screw at the bottom that also holds the spring. That allows the aperture lever to move sideways and spring back.
The ring on the right has four attached protrusions to move the lens aperture to the aperture ring setting. On the bottom is the key that fits into the aperture ring. Sticking up is a part which goes through the lens mount and should engage a lever in the camera mount. I'll skip it for brevity, it doesn't matter for our purposes. Then there are two tiny pins on the left and right, one shiny and one dark. These fit inside the stamped steel part on the left (remember this part is upside down). The stamped steel ring has an arc in its shape that engages one of these pins. The spring holds it against the pin. When the aperture ring moves, the pin moves the stamped steel ring and through it the aperture blades, while still allowing the aperture lever to move the blades between that setting and wide open.
Whether anyone can follow that or not, here's a theory about what has happened to the broken lens. The tiny pins were broken off when the lens was set to f1.7. That would prevent setting any other aperture and the lever could move without any actual effect. The broken pics could maybe be replaced with small screws, filed down after installation.