Contax S2 shutter adjustment

I was reminded recently of a post I made, some years ago, on Rangefinder Forum describing the adjustment of the Nikon FM2 shutter (later used in the S2). I thought posting it here might be a good idea. It's been edited from the original to remove the Nikon references and change a few specifics based on more recent experience.

Note: Some of the pictures are of other, similar, shutters so may not be exactly as the S2 shutter but they serve to show specific features.

The S2 shutter is a mechanical shutter with speeds from 1 sec. to 1/4000 sec. plus B. The shutter was made by Copal and is a variant of the Copal square shutter.

The shutter curtains are timed 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 independently 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 but this isn't recommended.

Adjustments can be made with the shutter in-situ but the high speed adjustment is easier with the shutter removed from the camera. Note that the mirror box of the camera needs to be removed to access the high speed adjustment. 

Image 1 shows the main operating levers that cock and trip the shutter.

Image 2 shows the location of the adjustments.

Image 3 shows the location of the curtain springs and the slow speed escapement.

Check all the speeds before making any adjustments. Only adjust what is necessary which is usually just the high speeds.

If you need to, adjust the slow speeds first then the high speeds. Note that this is a mechanical shutter and the speeds will not be perfect. It's quite typical for the top speed to be a little slow. All the speeds should be checked after each adjustment and some compromise may be necessary to get all speeds to within acceptable limits. Note that the 1/250th always runs slow to ensure flash synchronisation is acquired.

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, identify the high speed adjuster. See image below. 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 screwdriver 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.  Note that, usually, only a very small movement is required.

Just to reiterate, check all the high speeds after adjusting the high speed adjuster and check all the slow speeds after adjusting the main spring as a compromise may need to be made. The below image shows the typical speeds after adjustment. I fire the shutter multiple times at each speed and plot the highest and lowest values to check for consistency. 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.