Originally posted by stevebrot but it is not uncommon for users to put the ring at f/22 without depressing the small button allowing it to move completely to the A position.
That would be my first thought.
If the pin mechanism is the same as on the 50mm f/1.7 then the pin itself is spring loaded. The pin assembly is lifted or pushed into the raised position by a slot on the ring and a tab on the pin assembly.
So some of the ways this can fail are:
- the opening for the pin is blocked
- the slot or tab is worn
- the ring is not seated properly so the tab and slot don't align in the "A" position
- the "A" setting button is blocked so "A" mode cannot be set or switched out of*
*this is a common problem with the 50mm f/1.7 as the spring clips for the "A" button often break loose.
If the mechanism is the same as the 50mm f/1.7 then accessing the "A" pin mechanism is rather simple - it involves removing the K-mount bayonet, a ring or two and perhaps the aperture ring. The "A" assembly is 3 or 4 very tiny parts and there is a spring and ball bearing that are easily lost for the aperture ring.
It could just be a matter of adding some grease to the tab and slot on the aperture ring. If it is like the 50mm, then this can be done without removing the aperture ring and risk losing the ball bearing or having to fiddle around with put it back together.
https://www.pentaxforums.com/forums/114-maintenance-repair-articles/88051-pen...ng-repair.html