Canon nFD 300mm f4 aperture problems

[sasquatch]

I got a Canon nFD 300mm f4 really cheap due to some listed issues with fungus. The worst of it was on the back side of the front element and was easily cleaned, still some on one of the rear elements but my main concern is the aperture seems faulty.

I know nFD lenses unmounted sit halfway open around f8ish.

When I mount this lens on an FD-E adapter with the adapter set to open the aperture opens fully.

When I set the adapter to Lock the aperture returns to the halfway position same as unmounted(f8ish), the aperture ring has no effect on the blades.

I decided to remove the mount to have a look at what is going on and found this.


From what I can tell and I'm a novice when it comes to fd mount lenses the highlighted section should link the aperture ring to the silver ring (labelled A-1) and turn it to open and close the aperture. If I pull up on the silver ring while turning the aperture ring it does engage but as soon as I release it it drops back down and no longer catches. Without the silver ring turning the long aperture transmission arm doesn't catch and move so I can't test to see if the transmission arm is correctly placed to actuate the aperture.

For some reason the silver ring is sitting 1-2mm too low for it to catch the black aperture ring tooth.

Is this normal behaviour, I don't have a clue?


As it stands I can operate the aperture from fully open to around f8ish using the open-lock ring of the adapter so the lens is kinda useable. Its not exact but a workaround for what was a very cheap buy.


To get to the last fungus infected element would require a much more involved disassembly which I have read is not a task to be taken lightly with an internal focus nFD lens and the test shots don't seem to be obviously suffering from the fungus so I might just leave it but I would really like to get the aperture working as it should.

Any ideas? Suggestions on next steps?
TIA