Minolta MC Tele Rokkor 300/4.5: Slow aperture fixed

Heavy quality, still in a good design, this tele has been sitting unused in the closet for a long time:






The aperture blades move slowly when the aperture ring is turned; when the open aperture lever is pushed, the aperture remains closed:




When you look at it from the front, you can see slight traces of oil on the aperture register ring; the slats are clean.

I will remove the rear assembly and rear optics assembly and treat the aperture blades with Zippo lighter fluid. This should make the aperture work again.


About the lens on Artaphot (German):

Minolta 300mm 1:4.5 (SR/MC/MD)


Matt Bierner shows the dissection on YouTube:




Everything depends on a spring

We started with the aim of making the slow aperture fast moving again.










The path to aperture mechanics.




Oil under the moving parts.

The aperture blades can only be moved against slight resistance.




This is followed by applying lighter fluid and moving the mechanism through.

Then dab and dry with the Dremel Versatip butane gas burner.

Now the mechanics are becoming smoother.

I treat the aperture blades the same way even though there is no oil visible there.

Then I add some graphite powder to the moving parts to reduce friction there.

The mechanics now move noticeably easier for the quite long mechanical transmission path of the aperture position that runs through the tube.

A test assembly shows that the aperture now closes quickly but only opens with a delay. On the XD-7 the aperture only opens halfway.

So I look at the spring that is responsible for opening the aperture.




The spring doesn't have enough force to open the aperture quickly.

I disassemble it and bend it apart so there is more tension.

Some attempts follow.




I bend the end of the spring round so that coupling with the open lever is easier when inserting the ring.

Finally it works.








The spring hook helps to mount the spring in the tube.


Now the aperture opens with more force, but still not completely.

Therefore I solder the spring to its holder.

Now the panel is moved directly by the mechanics, without spring force in between.

The spring has become a rigid eyelet into which the open-dazzle lever couples.

Now the aperture works smoothly as it should.

However, the soldering doesn't hold.

Therefore, I attach the spring to its holder using enamelled copper wire.

The toggle is then soldered:

This holds now. The aperture works as intended.


Since a few grains fell through the aperture onto a lens due to the introduction of graphite powder, I removed the front lens block for cleaning:






A pretty heavy chunk.

The graphite dust can be easily blown off.

The catch on the tube, the catch ball, its spring and the aperture ring are cleaned with Zippo and lightly greased:














The catch proves to be service-friendly.

The ball can be inserted from the outside via a screwed „service hatch“ in the aperture ring.

This eliminates the usual playing with the ball:








The lens also has its own stop-down button.

It works when the lens is mounted to the camera:







Ready


+++

All information provided without guarantee and use at your own risk.

I wonder how a repair shop would solve this problem today. Service at Minolta with original spare parts has been no longer possible since a long time.

The order would probably be rejected on the grounds of „no spare parts“. Therefore, we DIY people have to find solutions ourselves. Even if it changes the original design. The main thing is that the lens works again and can be used for photography

Another excellent project. A Minolta lens lives on!

Bearman said:

Another excellent project. A Minolta lens lives on!

Click to expand...

Hopefully forever!

The fact that I made something rigid out of a flexible spring makes me think. Minolta's engineers will have had their reasons for making it the way it was. But as long as it works this way and nothing breaks, I think it's acceptable.