Some DIY experience and basic tools needed, 30-60 min
Symptom: badly exposed pictures. Depending on the way the aperture is stucked, you'll see mostly all pictures over- or underexposed, regardless what manual aperture settings you use. Autofocus also doesn't find correct settings.
Step 1: Move the lever back and forth and look through the lens: the aperture on a good lens should open and close and spring back into position
Step 2: Remove the two black screws holding the contact plate in place. Be careful: lift up the contact plate slowly. Underneath the contact plate sits a tiny spring. The spring sits in a hole pressing against the contact plate from below.
Step 3: gently take aout the inner tube. Take care about the contact plate: it is connected with some fine wires to the optical parts and cannot be removed
Step 4: In my case I noticed another problem: the lever would not move with ease and seem to stuck and stop sometimes. Time to remove the metal ring.
Step 5: That retaining ring (arrow) took me a while to sort out: it doesn't belong there, but rather fell out of ther just removed metal ring.
Step 6: three metal rings are on the lens body: do not mix them up. Picture shows the correct order.
Step 7: At the same time try to align the two aperture levers that they are connected
Step 8: put the remaining parts back. Always tighten the screws very loosly at first and continue tighten them in a starway fashion (opposite sides).
Step 9: I was not able to remove the optical part from the housing. Hopefully some ifixit members can jump in here.
Arbeite die Schritte in umgekehrter Reihenfolge ab, um dein Gerät wieder zusammenzubauen.