fixing loose aperture pin on Minolta MD Rokkor 135mm f/3.5

[olek]

Hello

First post here

Greetings to everyone !!!

Pardon me if I am violating any rules, I asked the question on other forum site, and got no replies, so I am coming here hoping to find more vibrant community

Recently I got new (to me) Minolta MD 135mm lens. While removing its rear lens group to clean some dust/fungus a loose aperture pin/post was encountered. And it is driving me nuts. When lens is assembled and focused on infinity (shorter barrel), aperture is about half a stop too narrow. With lens focused nearby (much longer barrel), aperture is about 4 stops too narrow. That is easy to explain because loose aperture post is leaning instead of transferring the adjustment force, and it is a looooooong one.

I tried rotating pin itself in attempt to tighten it to no avail. Looks like it may be tightened by some small bolt/nut on the other side of the aperture mechanism.

Any hints on how to pull that mechanism out in order to get access to the pin's mount?

Or should I just apply some cyanoacrylate glue like somebody else have done with the similar problem? I personally dislike that solution and would prefer to fix it the right way...

This is my first lens that I am taking apart, but I have a feeling that it will not be last...
Please help me to turn my first experience with lens repairs from screw-up to success story!

[hinnerker]

Hi,

it would be a nice idea to show some detailed pictures of the mechanic inside.

After converting some Minolta lenses in the past, i realized Minolta aperture mechanics are different depending from the lens. Some of them use do use a "Fork" to transfer the aperture values from outer aperture ring to the aperture plate.. some use a radial cam on a long lever..

I noticed different kinds of mechanic. Please post some pictures..

Cheers
Henry

[trifox]

[olek]

I will get the picture of mechanism today, but what i can tell immediately, it is a 'fork' type of linkage.

I think that Minolta used 'cam' type linkage for lens that do not have much of the 'focusing travel', and 'fork' for lenses that have a lot of it.

I could be wrong at that, but this lens certainly travels a lot, it becomes about an inch longer when refocused from infinity to nearby, which is making the problem much worse...

Here are the photos. Lens shown in partially gutted state for better view (rear lens group missing). Look at the angle between the aperture pin and the vertical line inside the barrel next to it. They are supposed to be parallel, but they are not because post is leaning to the right.

And yes, I know I will need to de-dust the lens before final reassembly

[hinnerker]

Hard to see, whats the problem itself.. so i have some more questions..

1. It seems that the the aperture pin itself is in correct shape.. looks like
the most i have seen on Minolta lenses. I dont know the english expression for the sliding slot, shown in your picture. Normally the slider is hold with two screws,... are they thight?

2. are the apterure blades "oil-free" and snappy?

3. the sliding slot itselfs needs some grease, becouse i can see a lot of marks in it... that makes me thinking of not tighten screws..
please check this first..

[olek]

Thank you for attention, hinnerker.

Well, suspect I know what the problem is, I just do not know how to fix it
If you look at the pin itself, and slide you eye down to the place where the spring connects to it, and slide you eye ever more down, you will see the place where pin connects to some 'flat' part inside aperture assembly. That is the joint which is not tight at all, and is causing aperture pin to wobble (end of it can be moved good 4mm without aperture moving at all).
Since it is so wobbly, it is not precise at all at transferring position of the aperture ring. I need to tighten it, or to glue it solid, hoping that it will glue in the upright position. Question is how to get to the other side of the aperture assembly, where hopefully there is some tiny bolt that I can tighten.

One way or another I think I need to place an order for good spanner wrench, and use it to disassemble the lens from front, and see if I can get to anything interesting there...

This is the one I am planning to get: http://www.skgrimes.com/span/index.htm

[hinnerker]

Ok.. the Pin itself.. ok.

Most of the aperture pins i know are riveted on a ring. So the chance to fix it, its small.

I would give a try to an auxilliary sleeve socket, which is as small as the pin itself from the inner diameter and use a very small portion of 2k metal glue.. this is a simple workaround and looks like this scheme..




if you dont want to find the original spare part for the lens (could be a long search) this maybe could work. I did this with some cheap M42 lenses and it works for years.. Be carefull with the glue and take only small portions. Before using the glue, clean the area with acetone

The best way should be, to dismantle the lens completely and fix on the ring itself.. but this is a lot of work.. you need an new screwable pin with same lenght, diameter etc..

For me, this works most of the time.

Cheers
Henry

[hinnerker]

Maybe a good choice could be a ballpoint pen.. cut the cone point as short as need it to not disturb the moving aperture pin area and not disturbing the focus if infinity is neccesary. .



Cheers
Henry

[olek]

Riveted.... Darn... so much for hoping to fix it the 'right' way...

Thank you, Henry for that insight, it is very helpful.

You saved me a good chunk of time that I do not have to spend now tearing apart that lens.

I will look at the option of using a ballpoint pen's cone (it is indeed pretty much perfect diameter at the opening, right around 2.5mm, just as that pin).
If it will not fit in the slot in which pin is moving (it is pretty tight, and thick), I will attempt to simply glue the pin in place. With some luck, that might work.