A year ago I purchased an old and cheap 28mm F2.8 manual focus lens to use it in macro-photography and with an old Minolta; Its diaphragm did not functioning properly, closing slowly and with uneven blades, or even not closing at all.
I dismantled it trying to fix that problem, as you can se in the first steps, but I didn't success because the spring was inside a block.
Months ago, I realized how to repair it... with a rubber band; I did it, and It is still working today.
I hope this may be usefull.
Thanks for your time!
Step 1: Opening the Lens:
You must retire these three silver philips screws. Pull the top and look at the second pic
hey! you can see the little pics if you becames member; it is free, and fast; do it!
Then unscrew that little black screw in the aperture ring, and pull it up while it is in the more closed aperture. Do the same with the third ring and its very little black screw below the first one.
Step 2: The Inner Part:
The lever of the right sets the aperture and you can move it like the aperture ring move it.
You can unscrew the four small black screws (two on the upper left and two down on the right) to let the lens reduce to the inner block as shown in the second pic.
I could retire the glass covering the diaphragm (pushing counterclockwise with a small flat screwdriver in one of the slots of the black mount) and touch it and even oil it (don't do. I must clean it after that. Don't oil a lens. Never.)
None of that works.
Step 3: The Rubber Fix:
I remount part of the lens and fix the rubber with this knot (I can't remember its name, but you all know how to it) to the lever of the aperture ring (I do it in the maximum aperture).
As seen in third and four pics, in the other side of the lens there is a lever, used by the camera to open the diaphragm to the max in order to let you frame and focus properly, with the more possible light. Simply attach the other side of the rubber on it.
Now the diaphragm will be close fast and evenly.
Step 4: And It Is Done!
As you can see, now the blades close evenly, and fast enough to be used.
The lens still work on the Minolta, and of course in the macro attachment.
I hope this may be usefull to you.
Thanks for your time!