Minolta Rokkor 58mm f1.2

Minolta Rokkor 58mm F1.2
Problem: Stuck Aperture

Got this lovely lens by Minolta made between 1969 to 1973. This is the 2nd version of this lens which is all metal and really feels like a tank. More info can be found here: rokkorfiles

The glass was in very nice condition and the focusing was quite smooth, but the aperture was stuck wide open. Rotating the aperture ring or moving the "Auto" lever on the back had no effect.

  Fig 1 - The Rokkor PG f=58mm 1:1.2

Fig 2 - The aperture stuck open

9 out of 10 of the times (not really a based statistics) is that degraded oil or grease from the focus helicoil drifted along the years to the aperture blades and/or the aperture opening levers and springs making them stick. So we need to get to the aperture mechanism.

First remove the bayonet. (Fig 3) Note there are 8 screws, 4 short ones and 4 long ones. You really need to remove only the long ones. Removing all of the screws will make your life a bit more difficult (as was mine).

Fig 3 - Unscrewing the back mount ring

If you did remove all the screws, by accident, or on purpose to clean the "Auto" lever ring, you will find a set of 5 small (very small) bearing balls and a set of 5 spacer segments. Take care so not to loos them on the ground, on the table or even in the lens it self. (Fig 4)

Fig 4 - Note the small bearing balls (3 out of 5 shown) and the spacer "rods" between the black ring and silver mount ring.

Ones the whole back mount assembly (mount, "Auto" ring and base ring) are removed, you can remove the aperture ring (Fig 5). Note that if you only want to clean the aperture blades, and the ring turns fine you don't have to remove it. If you do remove it, note how it is placed: in the ring there is a protruding slotted lever that should be engaged into the angled lever on the lower "golden" stopper ring. (I don't have a photo of it).

Fig 5 - Aperture ring

Fig 6 - Aperture ring removed

If you do decide to remove the aperture ring, take care not to loose the small bearing ball used for setting the "clicks" of the aperture. It is located under the ring where you see a set of grooves (Fig 5 - under my index finger).

 Fig 7a - Note the small ball sitting on top of a spring inside a drilled hole

Fig 7b - The ball removed you can see the hole with the spring inside.

If you do remove the ring and the ball falls, you can hold it back in place by putting a small drop of oil or a tiny dot of grease on it so it will "stick" into the hole.

We are almost there, only left with removing the back lens elements. There are to groups both can be unscrewed quite easily. As always try not to touch the glass it self. Ones the two groups are removed, you can screw them together so less dast will setter. Later you can screw them back in together. (You can be lucky and remove the whole assembly of the two groups as one part).

Fig 8 - Back lens assemblies removed.

Now we are at the aperture level. First you need to close the blades. There is a upright pin you can play with (no pun intended) few times and the blades will close. For cleaning I use White Gas, but lighter fluid should be fine as well. Drop a few drops on the closed aperture, and wipe. Repeat few times. You will see the old oil being dissolved by the gas and picked up by the cotton stick. Next drop few more drops and play with the aperture so the gas will get on to all the blades. Not drown the thin in gas. I use a syringe to drop 2-3 drops at a time.
You should also clean around the barrel where the levers are.

Fig 9 - Removing oil from the blades.

Ones you are happy with the result (it can be a good idea to level the lens in a worm room for a day, so more old oil will flow and will be removed) you just track back the steps and assemble the lens. If you did remove all 8 back screws, it can be smart to first assemble the back rings so you don't have to mess with the small spacers and balls.

Fig 10 - The back ring assembly. This is what you should end up with if you don't removed all 8 screws. 

Fig 11 - Don't you just love a snappy aperture?!