NOTE:
THE IMAGES IN THIS POST KEEP GETTING LOST SO I'VE NOW POSTED THIS ON MY BLOG AT:
Contax S2 shutter adjustment
IT HAS BEEN UPDATED AND HAS HAD THE NIKON REFERENCES REMOVED BUT IT HAS THE SAME INFORMATION.
This is a rewrite of my original post due to the pictures going missing and to include more information that I've learnt since writing the original. Some of the pictures are of other, similar, shutters so may not be exactly as you find in the camera.
The Nikon FM2 shutter is a mechanical shutter with speeds from 1 sec. to 1/4000 sec. plus B plus an extra speed of 1/200 sec. which was the original flash sync. speed of the camera. It originally used titanium blades which were etched with a honeycombe pattern to lighten them. Later, after some improvements were made, it was possible to achieve flash sync. at 1/250 sec. so the 1/200 sec. position was blocked off though still exists. Later still, the blades were replaced with aluminium alloy types which have a smooth finish instead of the honeycombe pattern.
The shutter is made by Copal and is a variant of the Copal square shutter though Nikon appear to claim to be the developers of it.
The same shutter was used in the Contax S2 introduced in 1992 which was after the aluminium alloy blades were introduced so none of the Contax cameras have the honeycombe patterned blades. A variant of the shutter was also used in the Yashica FX-3 and some other Cosina made cameras, initially with speeds only up to 1/1000 sec. but later up to 1/2000 sec.
The shutter curtains are released by a cam rotating around a shaft driven by a spring. The spring has a small amount of adjustment to its tension. The position of the cam at which the second curtain is released is fixed but the position at which the first curtain is released is moved when changing speeds. This is the only method used to adjust between the top four speeds (1/500 - 1/4000 sec.). slower speeds use an escapement and a number of linkages to hinder the rotation of the cam and so decrease the shutter speed. The two curtains are driven by individual springs each with its own tensioner. Once the curtains are released, they travel independantly and have no interaction.
As mentioned, the main spring has some adjustment which will cause the main shaft to rotate more or less quickly. This affects all shutter speeds but the affect is mostly to the slow speeds with minimal affect to the high speeds.
There is an adjustment that alters the timing between the release of the first and second curtains. This affects the high speeds with no affect to the slow speeds.
The curtain tensions can also be altered to increase or decrease the travel time - this will also affect the high speeds with no affect of the slow speeds.
Adjustment of the main spring should be possible with the shutter in-situ (my experience from working on Contax S2 cameras, Nikons may be different) but any other adjustment requires the shutter to be removed from the camera. Image 1 shows the main operating levers that will be used to cock and trip the shutter if it's removed from the camera.
Image 2 shows the location of the adjustments.
Image 3 shows the location of the curtain springs and the slow speed escapement.
When adjusting the shutter, adjust for the slow speeds first, then the high speeds. Only adjust the curtain tensions if absolutely necessary as it can be difficult to get both curtains to run at equal speeds - necessary to prevent tapering. Note that this is a mechanical shutter and the speeds will not be perfect. All the speeds should be checked after each adjustment and some compromise may be necessary to get all speeds to within acceptable limits.
The main shaft has a disk attached to the top with a number of slots in it. The end of the main spring sits in one of these slots. See image 4. To adjust the slow speeds, move the end of the main spring to a different slot to either increase or decrease the tension as required. I've found the fingers forming the slots get pinched together by the factory to stop the spring end from slipping out so the fingers need straightening first to allow the spring to be moved. A flat bladed screwdriver pushed between the fingers usually does it. BEFORE moving the spring, wedge the tip of a screwdriver behind the spring end so that, if it slips out of the slot or you lose your grip on it, it won't fully unwind.
Note: The disk on the top of the main shaft that has the slots in it has a slotted hole that fits over a matching raised part on the end of the shaft. In theory, it would be possible to remove the disk and refit it turned by 180 degrees. See image 5. This would allow the spring to be tensioned an extra half-turn. I've never had to do that but if a spring was particularly weak, it might be a solution. The screw holding the disk in place is left hand threaded so turn clockwise to unscrew.
To adjust the high speeds, remove the shutter from the camera. Look for the high speed adjuster. See images 2 (above) and 6. It looks like the head of a screw with a slot in it but it's only visible side-on and the slot may not be facing you so may not be visible. Because it's not possible to engage a scewdriver in the slot in the normal way, it's necessary to use the tip of a flat bladed screwdriver pushed sideways into the slot to turn it.
If the slot isn't facing you then you need a screwdriver with the tip bent 90 degrees to get into the slot. See image 7. Only a very small movement is required.
Just to reiterate, check all the high speeds after adjusting the high speed ajuster and check all the slow speeds after adjusting the main spring as a compromise may need to be made. Image 8 shows the typical speeds after adjustment. I fire the shutter six times at each speed and plot the highest and lowest values to check for consistancy. This was a good shutter - you may not get the speeds this close to ideal. But they should be within the upper and lower limits (red and green lines). Note that 1/250th (flash sync speed) has it's tolerance all on the over-exposure side because, if it runs high (under-exposure) then cut-off may happen when using flash.
