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The Minolta Auto-Rokkor 58/1.4 Striptease
© Alin Ciortea     2011-11-06 Categoria: Tutoriale \ Toate

Because I striped it naked and it teased me two days for that. This Rokkor had a very stiff focusing ring. So stiff that after a couple of hours outside on a November day it was barely moving at all. Since I a also have an MC Rokkor 58/1.4 I decided I might as well give it a go and try servicing it. The first step was to remove the trim ring from the front of the lens. Ideally I should have used a spanner wrench but since I don't have one, I used a vernier caliper instead. A lot of care is needed because any mistake in handling it will lead to either a scratched black ring (and usually these scratches tend to be deeper than the paint layer) or worse, a scratched or even chipped lens.

After the trim ring has been removed, you can see three more rings holding in place the entire optical group to the helicoid, the front optical group to the aperture system and the lenses from the front optical group together (resectively). For the moment just unscrew the first ring (the brass-looking one).

You can now remove the entire optics (and aperture system) from the lens body. You will notice a brass ring to the front of the assembly. It does not screw into place so if you take it off don't forget to put it back before reassembling the lens. You'll also notice that I've made some marks in order to know exactly how much to screw the front and rear lens groups back to the aperture system (this image was taken sometime in the middle of the process and that's why the lines are perfectly aligned). If the aperture system is working properly and there isn't any dust in there, you might as well leave it be. Otherwise it's time to unscrew the optical groups from the aperture system. To clean the apertures just drop some lighter fluid (I have a Zippo can) on the blades while working them back and forth with the aperture pin. Repeat this a couple of times and then leave it alone to dry. Don't pun anything on the blades. They only need to be perfectly clean to work.

Here you can see what thingy goes where, for when you'll be reassembling this toy. The aperture pin must engage the brass connector on the lens body and the tab on the aperture system must engage the slot in the inner helicoid (as marked in the image below). While replacing the optical group back to the body you'll have to push it just until the pin engages the connector and then, if needed, rotate the whole assembly until the tab also aligns with the slot. Only now can you push the assembly the whole way.

Now here comes the tricky part: the disassembly and especially reassembly of the focusing system. If you do it my way (just take it all out without paying attention), you'll probably spend a good few hours putting all back together the right way (or just follow this tutorial :)) ). So it is extremely important to mark things with a sharp thingy (a screwdriver in my case) and be very attentive to what you're doing. In this image you can see the focusing ring on it's infinity position. The ring is held in place to the helicoid by means of three screws (see the last image) that you can reach with a small screwdriver (slotted, no.2) between the inner side of the ring and the outer side of the helicoid (while looking at the lens from the front). Unscrew them and remove the ring by a pull-and-twist motion (the ring has a small cutting that will allow it to pass a screw on the lateral of the helicoid).

Notice the position of the brass helicoid related to the focusing scale. You'll see one of the screw holes needed to connect the focusing ring aligns with the white triangle on the focusing scale. Now mark this position on the brass helicoid (I scratched a triangle)

Also mark the position of the aluminium helicoid (the one with the black tube) related to the brass helicoid (a pair of lines in my case)

The third thing needed for reassembly is knowing when to screw the aluminium helicoid (let's call it helix2) in the brass helicoid (helix1). Now, I have no idea how they did it in the Minolta factory and I'm sure there must be a better way than mine, but this is all I could come up after many hours of fiddling with it. First of all the principle> the helicoid system must convert a rotation movement (the focusing action) into the linear back and forth movement of the optics. This is done by two interconected concentric screws (the helicoids), one of which screws into the body of the lens and the other screws into the first but is not allowed to rotate and therefore forced into only moving in and out. Helix1 is just a cilinder that screws conventionally on the lens body (clockwise) and has another thread on the inside, allowing helix2 to be screwed in. Helix2 has an outside counter-clockwise thread and connects to the inside thread of helix1. The optics are connected to it by means of a ring. The two helicoids have different thread steps and this means that when interconected, for only a small movement of helix1 (rotation) there's a big movement of helix2 (translation). Before disassembly make sure you've set focus at infinity! Because both screw into place, marking the position in which they are aligned to the body is not enough as at every turn the markings will align, but at a different depth related to the lens mount (and therefore the film plane). The infinity position for the brass helicoid is the most screwed it position minus just a bit more than a third of a turn (it's the position for which the holes in the focusing ring and helix1 are aligned. You can't really determine this position with the helix2 in place because it will not allow helix1 to go all the way down. I pretty much guesstimated the position and it turned out it was the correct one. Helix1 has a normal screw thread so it doesn' really matter where you start screwing it as it will always end up in the same place (nevertheless, I marked the position where it dettaches from the body with a cross aligned with the white triangle on the focusing scale). Helix2 is a true helicoid and therefore has a number of possible 'entries' on the inner thread of helix1. The position where it dettaches from helix1 has also been marked with a cross aligned with the cross on helix1. When reassembling, I first screwed all the way in helix1, than unscrewed it that third turn until my triangle marking aligned the white triangle. Then aligned the cross on helix2 with the one on helix1 and started screwing helix2 in position. This step is easier done with the metal tab that keeps helix2 from rotating unmounted. You can see this tab in the forth image from the bottom (it's the T shaped metal piece with three screws - in the image there are only two in place). As you screw helix2 in place, it will get stuck at some point so you'll have to unscrew helix1 while keeping helix2 from rotating just enough as to raise helix2 above the aperture connector. Screw helix2 beyond the connector then, while keeping it from rotating again, return helix1 to it's infinity mark. Theoretically they are now in the correct position. Keep helix2 steady again and unscrew helix1 until helix2 has raised enough to allow you to connect the T shaped tab. You can now rotate helix1 back to infinity as helix2 will stay in it's correct position. Replace the focusing ring and check that the screw holes are aligned with the holes in helix1. Replace the optics (don't forget the brass ring), and screw in the ring that keeps it in place. Go outside and focus on a very distant object. You should have a perfect infinity focus with the focusing ring aligned properly at the infinity mark. The other method that I can think of (a more scientific method) but haven't tried it while working on this lens is this> while disassemblying the helicoids, unscrew helix1 just until helix 2 is released from the metal tab, while counting the turns of helix1 ("n" turns). Now start unscrewing helix2 just until it dettaches from helix1 and mark the position on the two helicoids. Screw back helix1 the number of turns you just counted (it's now at infinity position) and now further until it stops, while taking note of how much further it rotates ("x" of a turn - where x should be aprox 1/3, just a bit more). At reassembly, first you screw in helix1 all the way, then unscrew it "x" of a turn and a further "n" number of turnes determined above. Now place helix2 into place (following the markings) and screw it in. It should get aligned just right with the T tab. This method sounds easier than mine but I just thought about it now so it's untested :)

These are the two helicoids.

You can see here the T shaped tab keeping helix2 from rotating and the aperture connector (along with the rest of the lens mechanism that is of no concern right now).

This is the lens into pieces. From upper left to lower right> trim ring and decorating outter ring, the ring keeping the optics in place, the brass threadless ring, the front optics, the back optics, (second row) the aperture system, helix2, helix1, focusing ring, (third row) aluminium protection ring on the base of the lens (on the mount), lens body and the my little "screw trays".

The helicoids after a good cleaning with ligher fluid. For relubrication I've used some lithium-calcium grade 2 multipurpose grease, diluted in mineral oil for fine mechanisms (undiluted it was much to thick and focusing was still far from a smooth experience). I've used a very small amount of grease. Very small! A q-tip soaked in the grease and then wiped of in a tissue is more than enough for all helicoids.

This is the completely screwed in position of helix1 related to the focusing ring detached from the lens but in it's infinity position. Note that misalignment of the holes, giving you that "x" of a turn mentioned above.

Put it all back in reverse order and go out and burn some film ;) Of course, I'm just an amateur so I stand corrected.

© Alin Ciortea     2011-11-06
Toate drepturile asupra articolului apartin autorului. Reproducerea integrala sau partiala a textelor sau a ilustratiilor din aceasta pagina este posibila numai cu acordul prealabil scris al autorului. Pirateria intelectuala se pedepseste conform legii.
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