Mamiya 645 S1000 AE prism viewfinder aperture pin

I have a Mamiya M645 S1000 with an AE prism viewfinder. This viewfinder has a pin that should click into the aperture lever of the lens.

Normally there is pressure on this that pushes the pin down and fixes it well in the handle.

Unfortunately this does not work properly any more. Sometimes the pin is pushed down properly, but usually it stays pressed against the top of the viewfinder when a new lens is inserted and the aperture ring is turned to lock with the pin. It appears the spring is not working as it should anymore...

Anyone have any idea how to open the viewfinder? And then further?

IIRC the cover-plate over the coupling pin can be removed separately; allowing access to the spring and pivot.

However, you may not need to do any dismantling at all. It's possible that the pivot of the coupling arm has simply got stiff with dirt or hardened grease. In which case a small amount of thin oil will probably free it up. By small, I mean the amount that can be carried on the tip of a fine sewing needle and rubbed into the pivot.

Another possibility is that the coupling arm has been bent or pushed back into the housing. In this case gently easing it forward might free it up.

Lastly, the spring might have come adrift from the arm, and it might be possible to relocate it by 'fishing' for it with some fine wire - maybe a straightened paperclip or similar.

Whatever. I'd explore those non-invasive measures before wading in and taking the lid off the prism.

IIRC the cover-plate over the coupling pin can be removed separately; allowing access to the spring and pivot.

 

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Thanks for your help Rodeo, really descriptive!

Just today I had a closer look at the finder and the mechanism. I noticed that when I pushed the pin in, to a certain point it would still spring back correctly.

Here I noticed that the pivot arm was very slightly bent... and when pushed further in, pressed against a plate inside the finder. It was at this point that it stopped working and I felt increasing resistance.

A careful push in the opposite direction to remove the bent, and that did it!

Now a happy owner of a beautiful M645!

Now a happy owner of a beautiful M645!

That's great!

I thought it would be an easy fix. Those old metal Mamy 645s are practically indestructible.