The Mamiya Six IV B is a medium format (6x6 cm) folder released in 1955. Interestingly, in this camera focusing is achieved by moving the film gate back and forth rather than the lens.
In the following I will explain how to remove the camera top, clean the viewfinder, and take apart the lens. If you are a novice camera tinkerer I suggest you read the introductory text on camera repair first to get an exposure to the challenges involved in an undertaking like this.
To begin with, evaluate the condition of the camera. Check the accuracy of the range-finder.
To remove the camera top:
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The ASA dial on the top of the film advance knob does not have to be removed. The film advance knob is a complex unit, which stays with the top plate.
The shutter release button will become loose when you lift off the camera top.
Watch out! The distance scale disk comes off easily. Make note of its position relative to other parts before you remove it (or before it accidentally falls off).
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Spend a few minutes on studying the range-finder and distance setting mechanism. The following diagram will help you understand how the range-finder works.
M1 is a semi-transparent mirror. The light ray coming from O' gives us the viewfinder image. The light ray that comes from O" and is reflected from mirror M and M1 is the measuring ray. It forms the range-finder spot. The concave lens between M and M1 deflects the measuring ray to a greater or lesser extent. Note, that only one half of that lens is actually used. Can you see that when we focus on a subject at infinity, the measuring ray goes through the middle of the lens? After disassembly (which comes next) identify and match the parts with the labels on the diagram.
Disassembly of the range-finder:
The range-finder unit is fastened to the camera top with only two black screws. These are easy to remove. The optical elements are glued on with shellac and at places painted over with black lacquer. Use acetone and/or methylated spirits to dissolve the glue. Take care, because the semi-transparent mirror is coated on the inside and is extremely delicate.
The picture below shows the range-finder unit completely disassembled. The amber colored semi-transparent mirror is on the left. Below it is the viewing lens. (It is important to remember when re-assembling that this lens bulges out very slightly towards the outside.) In the middle there is a small rectangular frame with a concave (diverging) lens in it. (By now you know what its function is.) Then there is a small mirror on the right. Its enlarged copy is shown in an inset; notice the fungus growth on its surface.
When setting out to clean the optical elements (if they need cleaning) weigh up the likely improvement against the possibility of accidental damage. The semi-transparent mirror is especially susceptible to harm. Blowing off the dust with a hand blower is probably all that should be done to it. If there is fungus infection on some of the other parts, ammonia usually works well against that.
Re-assembling and adjusting the range-finder:
Glue back the optical elements with shellac taking care not to smear any shellac onto their useful surface. After re-assembly the range-finder might not be accurate due to the mirrors not put back in exactly the same angle as before. A slight misalignment can be corrected with the fine tuning screw. (This is found at the opposite end of the arm that moves the concave lens back and forth. It is under the distance scale disk.)
If there is gross inaccuracy, the concave lens needs to be re-positioned in its socket. (This, of course, requires ungluing and regluing.) To be successful in this undertaking you really need to understand how the range-finder works.
Putting back the camera top:
This is fairly straightforward, but re-fitting the film advance knob does pose some challenges. There are two spring loaded stoppers (hooks) which get in the way. The smaller one is connected to the red plastic disk which pops up in the viewfinder after a picture has been taken. I tied a sewing thread to this hook and pulled it out of the way as I put the camera top back. Then I just pulled out the sewing thread. You might want to try this trick, too.
Removing the lens-shutter assembly:
Open the camera back and find the retaining ring at the front of the bellows. This needs to be unscrewed with a spanner wrench (or something similar). Be very careful not to tear the bellows. This is really all there is to it. The front and rear lens elements are easily removed from the shutter afterwards.
The bellows can be separated from the metal bracket at the front, but not at the back. At any rate, cleaning the front part of the camera is easily accomplished when it is brought to the stage as it is on the picture below.
The film gate can also be removed. Two pieces of wire need to be unhooked. While working in this area you can observe how the focusing is adjusted in the camera. Now it occurs to me to ask whether this kind of focusing is superior to the one accomplished with the lens. I can think of some advantages, but there seems to be at least one significant disadvantage: the film moves together with the film gate and its tension between the rollers changes. I think this would affect film flatness and result in the degradation of sharpness. It would be interesting to test this.
For other articles of this nature visit my camera and lens restoration page.
On a different topic see Roland FP-30 'secrets'; playing GS instruments on the Roland FP-30 digital piano.
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