Tokina RMC 25-50/4: Zoom blockage fixed, plain bearings replaced, helicoids greased

This Tokina Zoom with a connection for Minolta MD came to me for just EUR 10,-.

Externally in beautiful condition, but with a blocked zoom ring and severe lens clouding.

I wanted to see what I could achieve.




Decorative ring off




After partial dismantling I saw the cause of the blockage.

The two opposing plastic plain bearings in the tube, which hold the focus ring vertically in position, have fallen into pieces and the associated screws have broken off.

An intact - longer - screw below.




After the area has been cleaned of the fragments of the plain bearings, only the remaining screw residue is a reminder of more mobile times.

This is what it looked like in both screw holes.




However, the two diametrical plain bearings for the focal length adjustment are intact.

But they just can't hold the focus ring alone.




Using the Dremel, a metal drill and a small milling knife, I worked my way through the remaining screws.

At first I just drilled a small hole because I wanted to insert a new smaller screw.

Then I discovered two plain bearings with screws in my stash and increased the diameter of the bore.

Freehand and by eye.




And it worked!

The two plain bearings could be screwed into the new holes.

Perfect fit - as if they were original spare parts.

The height was also right.

I was lucky.

All four plain bearings were then lubricated in their guides with Ballistol silicone oil.






After understanding the focusing mechanism, I separated the helicoids.

Previously I marked the infinity and minimum distance positions.

Likewise the point at which the helicoids seperate.

To avoid chemically stressing the plastic bearings, I cleaned the helicoids and the inside of the tube with isopropyl alcohol, the mildest solvent in my box.

Greased with Castrol LMX.

This allows the focus ring to slide smoothly as desired.

The assembly worked, with the markings providing orientation.






Rear and front lens units before …






... and after cleaning with surgical spirit.

Rear lens unit

The two lens blocks at the back and front can be unscrewed. I was able to loosen the screw connection with a rubber cloth und could clean most of the cloudiness.




I could loosen the rearmost screw connection with a little acetone.






Haze and cloudiness are gone, some dust came in, but that doesn't affect the image quality.




Overview




This time, cleaning swabs with foam heads were used for the first time.

I am very impressed.

When dry, the sticks absorb dirt and also resist acetone for some time.




A guide rail in the tube ensures that the maximum aperture opening is adapted to the respective focal length.

Here I was too hasty in loosening the adjusting screws while exploring.

But the remains of the sealing wax showed me the original adjustment position.

Lucky for the second time today






Aperture f = 4.0 at focal length 25 mm ...






... and at 50 mm.






The notches for the aperture settings also get some grease.

The locking ball drived by a spring behaved well when removing/attaching the aperture ring. It stayed at the table.




Assembly




Tokina back to action

The infinity position also fits.

Mission achieved


Conclusion

• I was able to get the zoom working again.
• Drilling out the remaining screws could be done freehand. Of course, a drilling table would be better so that the drill head can be guided exactly vertically.
• Coincidence gave me two suitable plain bearings with the corresponding screws.
• I chose the right drill bit by thumb. I didn't cut a thread into the hole either, but trusted that the metal screws would grip. Which they did. Of course, doing it properly also means measuring beforehand to ensure that the new plain bearing is in exactly the same place as the old one. But even with a sense of proportion and freehand it worked well.
• For me it was a nice teaching project and gained some experience again.
• Would I approach a valuable lens with this problem myself? I don't know, the accuracy that can be achieved at the kitchen table with the tools available is probably not enough. But that doesn't matter, just the fact that I can estimate what's working DIY and what isn't is already a win

+++

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