Minolta AF 135 mm f/2.8 lens service report
I bought this superb lens second hand through Ebay. Unfortunately it must have been sitting on the shelf for a long time, because the focus mechanism was everything but smooth and the camera was clearly having trouble with that. I had a 50 mm f/1.7 to compare it to, and I concluded it needed service before it could be put to work. This is a step by step explanation of how I took the lens apart and the incredibly simple solution to the problem. Do this at your own risk!
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1. Slide out the hood, and remove the three screws it exposes. The hood together with the front cover comes off.
4. The mount has a focus position switch attached with 2 wires. Remove the two screws holding the switch.
5. Remove the focus spindle. This is the part where the camera's screw drive pin locks onto. It might fall to the floor if you don't remove it first.
7. Remove the four tiny black screws. On my copy these were sealed with some red stuff and wouldn't budge. Put your screwdriver in the screw's head and tap it firmly with the lid of another screwdriver to break the seals. Don't end up destroying the heads, like I did.
8. Lift the lens housing from its interior. Notice the matching slots on both the housing and the interior. These must be positioned correctly when you put it back together.
9. Remove the two screws holding the focus scale. Take notice of the position marker and the thin plastic spacer ring that comes off.
10. After measuring the gap between the stationary part and the moving helicoid part, remove the two oval-like shaped focus track adjusters. When putting it back together, the track adjusters must, of course, be in the same position to achieve infinity focus and for the focus scale to match up.
11. Don't remove the three guides that hold the focus lens group, as it may be hard to get things back into place. Instead, just lift the helicoid mechanism together with the focus lens group, as far as it'll go. Clean the exposed bearing surfaces, inside and out, with rubbing alcohol or cleaning benzine and a q-tip. On my copy these tracks were visibly dirty, probably from grease residue.
Sorry about the average photography; the shots were taken quickly with varying light and AWB and JPG quality. I'll do better next time. Promise.
If you can't properly reach all areas that need cleaning, you might have to remove the focus lens group and completely remove the focus mechanism. I haven't done this, can't help you there. To prevent this from happening again I used a very small amount of lubricant (sewing machine oil) to make the focus run extra smoothly. I willingly took the risk of oiling up the iris blades if the oil should ever start to creep.
Update: the oil hasn't crept into the iris blades after two years of operation, and probably a few thousand shots, so I think it's safe to say you can use sewing machine oil on the focus mechanism.
After making sure the focus mechanism is turning freely again, reassemble the lens from step 11 back to 1, continuously checking focus smoothness.