SRT Meter Function

SRT Shutter Operation Notes

SRT Shutter Operation

On this page, I will attempt to explain how the horizontal cloth focal plane shutter on Minolta SRT series of cameras work. You can observe much of this behavoir simply by opening the back of the camera and removing the lens. Try cocking and releasing the shutter while observing the activity from the front and rear of the camera. It is truely marvelous how these complex mechanical contraptions work reliably and consistantly in such a wide range of conditions. I think that some of Rube Goldberg's wackey cartoon inventions could have been inspired by mechanical camera design such as seen in the Minolta SRT series.

The Shutter Basics

The shutter itself is made up of only a few components. This is a simplified diagram of these components as they would look separated from the camera.

The first curtain is composed of a piece of light proof cloth that blocks light from hitting the film when the shutter is cocked. It is connected to a couple of ribbons of cloth that will allow light to reach the film.
The second curtain is composed of the same components, except it is arranged in reverse order so that the the light is blocked by this curtain after the shutter is fired.
Two take up rollers on one side of the camera with embedded springs that provide the tension needed to pull the curtains across the focal plane when each is released. When the shutter is released the curtains are rolled onto these rollers.
Two take up rollers on the opposite side of the camera that are used to roll the shutters onto, when the camera is cocked. These shutters are connected by gears to the timing and winding mechanisms which will be discussed in the next section.

Putting the components together and into the cocked position.

Cocking the camera will unwind the shutters from the spring rollers and onto the other rollers, adding tension to the shutter springs in the process. The solid part of the first shutter will prevent any light from striking the film at this time. Once cocked, catches hold each shutter in the cocked position against the pressure of the springs, which would otherwise cause them to return onto the take up rollers..
shutter opening

When the shutter release button is depressed, the following sequence takes place.

Catches are released that allow springs to flip the mirror up and the f-stop diaphram on the lens to be positioned as selected by the operator.
When the mirror reaches the up position, it will release a catch that holds the first shutter curtain in the cocked position.
Springs on the take-up curtain shaft will start the pulling the first curtain across the plane of the film, starting the exposure.

The timing mechanism (to be described later) will eventually release the second shutter curtain, which will start the end of the exposure. This series of diagrams depicts what will happen with an exposure of 1/125 of a second. At this point, it should be clear why flashes will only expose part of a frame when used at speeds higher than 1/60 of a second. At speeds of 1/60 of a second or slower, the timer actually doesn't release the second curtain, until after the first curtain has completely opened the film to exposure. At 1/1000, it is possible to have a mal-adjusted shutter that releases the 2nd curtain too soon, so that no exposure at all occurs. It is important that both shutters travel at the same speed or the exposure on one side of the film will be different than that on the other.

Eventually the first shutter completes it's movement across the film plane, leaving only the second curtain to completely end the exposure.

A cam on a gear connected to the second shutter spring shaft hits a lever that allows the mirror and the F-stop diaphram to return to the ready position. This occurs just before the second curtain completes it's run.
Finally both shutters have come to rest, completing the exposure.

When cocking the shutter, both curtains move in concert. Thus there is no gap between them, that might allow light to expose the film.

The Shutter Timing Mechanism

This is the breakdown of the parts that comprise the shutter control mechanism for speeds of 1/60 second and faster. Critical parts and their functions are identified here.
top view shutter mechanism

Parts 2163, 2160, 2154 through 2158 make up the control system for the ASA setting. This assembly only affects the meter and has no affect on the rest of the shutter control mechanism. Changing the ASA only rotates the top portion connected to string 4081, which drives the meter through a pulley system.
Parts 2017, and 2018 make up the click stop system that ensures that the proper shutter speed is selected as the dial is rotated. The notches in 2017 are lined up with the key 2018 and holds the shutter speed at the selected setting.
Part 2020 lies up against the guide 2013 (or 2014 at speed 1/1000), and is used to transfer the shutter speed to the control mechanism that hangs below the base plate. This lever rides against the guide and as the speed is changed it pushes the lever out or allows a spring to push it in towards the axis of the wheel.
The two shutter speed eccentric screws 9208 and 9207 adjust this system. Screw 9207 only affects the 1/1000 speed, but screw 9208 affects all speeds from 1/60 through 1/500.

The levers hanging below the plate are the key operational parts of the system.
shutter mechanism

When the shutter is cocked, lever 2029 is held against a stop in a cam connected by gear to the second shutter. Spring 2028 ensures that this lever is engaged against the stop when the shutter is cocked. Until lever 2029 is pushed away from this stop, the second shutter remains stationary.
Lever 2026 is is held by spring pressure against a cam connected to the first shutter roller by a gear.
As the first shutter advances, the cam will rotate and push out on 2026, which will eventually push on lever 2029 and release the second shutter.
The speed setting transfered by bar 2021, axle 2024 and bar 9420 to lever 2026 determine the timing of when the second shutter is released.
However at speeds slower than 1/60 second, a secondary timing mechanism is engaged (which is not described here). This secondary mechansim must be implemented at speeds slower than 1/60 second because the first shutter has run it's course before the second shutter is started. The cam that lever 2026 is riding against, stops turning when the first shutter stops with the completion of the first shutters run. Lever 2026 can no longer be used to kick lever 2029 to start the second shutter curtain.

It may be clear, if you understood this description, what the key to proper shutter timing is. Shutter timing is primarily dependant on the speed of the first curtain as it travels across the camera body. This is because the timing system is controlled by a cam connected to the first shutter shaft by a drive gear. This is why the first step in any shutter tuning is setting the speed of the shutter curtains.

This image gives you a very rough idea what the assembled system looks like, though you will get a much better idea if you remove to top of your camera.

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