...I don't have the skills to strip this lens down to clean and re-lube the focusing helicoid, but maybe it's possible to do the job 'in situ'- as it is on the 100mm lens. Has anyone done this successfully? On the back of the lens is a dished cover (slotted for the focusing cam) and a helicoid key. I'm thinking of removing them to see if the helicoid is revealed. Any precautions I should take? Thanks