SRT Meter Operation Notes

SRT meter adjustments

The SRT meter has two adjustable components. One is the electronic circuit that controls the needle in the finder. The adjustment for the electronic portion is easily reached by removing the bottom cover of the camera. The other adjustment is the mechanical mechanism that controls the circle in the finder. This adjustment is a bit harder to reach as the top cover of the camera must be removed. I will describe here how the electronic portion of the meter works.

Overall SRT meter electrical circuit diagram and operation

meter curcuit
Here is a diagram of the electronic meter circuit in the SRT 101. The circuit is composed of only a few components. These few components control the behavior of the needle in the viewfinder.
  1. A couple of switches that disable the meter if the depth of field preview is pressed AND the lens is stopped down below maximum aperture. Note that one of the cameras I have, was built without the last switch adjusted, so that as long as the depth of field preview is engaged, the meter would be disabled, regardless of the f-stop setting on the lens.
  2. Two CDS cells that vary resistance depending upon the amount of light hitting them.
  3. A variable resistor that is used to adjust meter sensitivity. This is accessible by removing the bottom cover from the camera.
  4. Finally a meter that measures the current in the the circuit and which controls in the pointer seen in the viewfinder.
  5. A battery that powers the circuit.
The circuit itself is essentially a current measurement system that automatically adjusts the current depending upon the amount of light falling on the CDS cells mounted in the viewfinder. A variable resistor is used to fine tune this circuit. In order to fine tune the meter circuit on the SRT, it is important to understand behavior in different lighting conditions.   Here is some actual measured data from a SRT's CDS cells under different light conditions.   Note how the CDS cell's resistance and LUX intensities are essentially logarithmic functions.
light resistance curve


If the adjustable resistor was set to add 4k ohms resistance to the circuit, the resulting plot is varies quite a bit from the basic measurement in brighter light.  This demonstrates that the variable resistor adjustment has a much larger impact in brighter light than in dimmer light.  This is an important factor to consider when adjusting an SRT's meter.

Overall SRT meter mechanical diagram and operation

meter diagram
Here is a simplified not-to-scale diagram of the mechanical portion of the SRT's meter.  This system controls the behavior of the follower needle in the viewfinder.  This is the circle in the viewfinder that moves when you change ASA, shutter speed or F-stop.

Basically, the follower is controlled by two strings connected by a pulley mechanism and held under tension by a spring.
  1. One string is connected to the lens by means of the ring on the front of the camera.  This provides the system the selected F-stop.
  2. The other string is connected to the wheels that control shutter speed and allows you to input your ASA setting into the meter.
The overall effect is that the follower needle goes down if you either, decrease ASA, speed up the shutter or close down the selected F-stop on the lens.  The green arrows on this diagram show this effect.

The needle goes up if you either, increase ASA, slow down the shutter or open up the selected F-stop on the lens.  The red arrows on this diagram show this effect.

There is also some adjustment built in, either by adjusting the strings or the meter follower pivot.

In the actual mechanism, the strings are wound around wheels and are guided through the camera by a number of pulleys.  My reassembly page has the Minolta factory drawings of the string configuration.  The ramp shown here is actually constructed as a spiral mounted on a wheel.    The spring that essentially holds everything together is a constructed like a watches mainspring, so it fits convieniently under the wheels and keeps the strings under tension.  This is the same spring that holds the aperature follower on the front of the camera over to the left (when facing the front of the camera).  By the way, without this tension the strings will fall off the pulleys.  There is even one pulley in the camera that is held on it's post only by the tension of the string on it.  There is another smaller spring not shown on this diagram that keeps the follower needle guide against the ramp, so it is sensitive to movement in either direction.



Back to Mike's Hobby Home Page