EF 20-35 1:3.5-4.5 USM aperture problems

[kitor]

Not sure if this should go here or into general chat, since it's not camera and ML related.

Recently I got old EF 20-35 3.5-4.5 USM lens. Pretty good external and internal condition, focuses fast, shoots OK, but...
Shoots OK only wide open. Trying to shoot with lower aperture ends in err01 both on R and 5d3.

After some guessing I noticed that lens reacts randomly to aperture changes - this is best noticeable in movie mode where aperture should be kept at selected step all the time.
General rule is that closing from 3.5 in direction of f/29, step after step - it will close two steps, then open by one, then close by a few, then open a few and so on. When camera says it should be f/29, lens is really at about f/16 maybe. Behavior is inconsistent - most of the time it will change aperture in proper direction, sometimes it won't react at all and sometimes goes into opposite than requested.

I disassembled it twice - checked aperture ribbon - seems ok (both visually and using multimetr), disassembled even aperture itself - i can move it in both directions as freely as stepper motor allows me to do.
With each aperture "step" motor is making noises.

I wonder if the problem is in lens controller, ribbon or motor itself. Will use it wide open (as there are no hopes for parts for ~$150 lens, unless someone sells me dropped one with broken glass), but maybe someone had similar problem with any of Canon lenses?

[kitor]

I fixed it! (well, almost)

Problem was with aperture motor itself. After disassembling it (it's held to aperture assembly by few plastic tabs and 4 solder joints to ribbon), using some wd40 for cleaning it up (just two electromagnets and pivot that should not touch magnets) aperture works OK.
Camera will still drop Err01 when I'll do some shots on f/16-f/20, but that seems to be a bit different problem. Seems that it's not aware of real aperture position since it has only one sensor. And it's done by simple wire spring (not-)touching metal element - contact is broken by pin moving with aperture itself. I noticed it was barely touching when fully open, so I slightly bent it and that made it working more reliably.

My theory that err01 is now caused by this sensor is backed up by behaviour - I can play how long I want in movie mode, it won't fail. It will fail in photo mode always after photo is successfully made with proper exposition - when it opens aperture back and maybe overshot now? After i bent spring, it may lose contact for fraction of second when going back from fully closed...

Anyway, it works even on R high-fps mode when closed at least to F/8 without errors, so it's usable again