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:
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:
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.