Originally posted by TimB COUNTER-RANT
Simmer down now. I didn't realize there are significant differences in aperture assemblies between various mounts.
Besides, there has been useful info provided so far, especially the mallet technique.
Having said that, my lens is an M35/2.8.
I appreciate the repair service referral, but it'll probably be too expensive to send the lens out.
Plus, I was looking forward to a DIY opportunity here.
Tim, I don't have the 35/2.8 but several (over the years) 35/2 Ms. These are infamous for the sticky aperture blade syndrome. Pentax obviously used a grease which after a couple of years migrated into the aperture mechanism.
In my experience this needs a major operation, as it usually does not work, to just clean the visible part of the aperture blades - which could be done easily enough by unscrewing the rear lens element. You really need to get onto the aperture group itself, which is done via the front of the lens:
- remove name ring, by screwing it out through the filter thread
- remove front barrell assembly (usually fixed by three or four screws
- remove (probably, but here the case with your lens may be different from the lenses I cleaned) front tube from focus thread (you will have fun reasembling this!)
- if you are lucky, you can now reach the aperture assembly, by unscrewing several tiny screws. Alternatively you have to remove those screws from the back of the lens, in which case you first remove the K-mount, then the rear lens element (beware of losing the tiny ball, which provides the click stops of the aperture ring)
When you take out the aperture assembly be careful. Sometimes their housing is screwed together, sometimes it is loose and falls apart, as soon as you try to get it out of the lens. But as you have to take it apart anyway... You need to clean out the interrior of the aperture assembly housing and the blades itself with Isopropanol. Don't use acetone. it is very effective, but sometimes aperture blades and housing parts are made of plastics, which will be damaged by the acetone.
Do not applay any oil or grease at all to the aperture mechanism. It is not necessary and the mechanism will work fine without any lubrication.
Have fun and keep track of all the tiny screws, as these are hard to buy as replacements.
Some remarks: Pentax on many lenses used Locktight for securing the screws. The Locktight they used is often more of the Superglue variety and it might be very difficult to remove the screw, without shearing off the screw head. In very stubborn cases I used a gas soldering iron to heat the glue, until it was soft enough to turn the screws. In one extreme case I had to heat a thread around the complete lens barrell in that way, which is nerve wrecking, if the lens is an expensive one...
Ben