A Shutter Analizer for Film Camera's

 

 

 

Highland Manor
164 South Main St.
Carbondale PA, 18407-2655
kendricksellen@hotmail.com

                                                Build An Electronic Shutter Analyzer

   One of the things that positively stumps a professional photographer is whether or not his expensive equipment is operating as it should. This is particularly true of one of the most basic elements in a film camera, the shutter. Shutter bothers have a habit of developing so slowly that the user is not aware of the problem until every other possible reason for substandard photos has been eliminated. The analyzer shown here gives an immediate visual picture of shutter performance. It reveals not only shutter-speed accuracy, but also the effects of dirty shutter leaves, the presence of �bounce,� and other irregularities.
   The instrument is particularly useful in large studios (especially those specializing in portraiture), photography schools, and new-papers where several cameras are available and a photographer may use a different one for each assignment. Unless the actual shutter speed is within its permissible tolerance, a photographer may find himself way out in left field. This device can provide a rapid, accurate check. 
   The unit, was assembled from an old oscilloscope salvaged from the service bench, using the cabinet, chassis and CRT mount brackets ect. The CRT in the old scope can be burned out because it will not matter, it will be replaced by a new CRT with a long-persistence (type P7) phosphor. Let�s build the analyzer. The front face plate has to be drilled for the mounting the reset button, the speed selector switch, and a three conductor socket and mating plug for the sensing-unit. A Stereo jack and plug is perfect for this.
If you use a salvaged scope the brightness and focus controls, as well as the two vertical-horizontal position controls be left there. If you build from scratch you have to drill these holes also.
   The sensing unit itself, which is positioned by hand inside the camera when checking a shutter, is made from a �dummy� socket mounted in it. Two cadmium-sulphide photocells are glued in position behind two small holes drilled in the dummy socket. A 6-foot 3-conductor cable, terminated in an Stereo plug, permits the sensing unit to be located far enough from the light source to keep stray light from falling on the CRT screen. 
   In the salvaged scope, the original CRT was a 5UP1, It was replaced with a 5UP7 (yellow), which is identical physically and electrically but has a long-persistence phosphor. The P7 phosphor holds the image long enough to permit viewing the entire shutter action from start to finish. A single horizontal sweep can be seen for several seconds in a darkened room.
   The circuit is showen schematicily in (Fig. 1) shows the analyzer circuit. Parts salvaged from the scope are marked with an asterisk. Potentiometer R14 is the brightness control, R16 the focus control, R3 the vertical position control, and R11 the horizontal position control. Resistors R1 and R2 were in the original power supply, but the value of R2 is changed to produce enough starting voltage to operate voltage regulator V3. Switch S3, the speed selector, is a double-pole 5-position rotary switch. The five potentiometers R19 through R23 and five capacitors C7 through C11 are used to control the sweep speeds are mounted on a subpanel under the chassis. (The analyzer has only five rangers. You can add more or select others by choosing proper R and C values. Table 1 lists R and C values for different ranges.) Diode D, filter C3, and trigger-sensitivity control R8 are mounted on the main chassis. 
   Pin 9 of the CRT, one of the horizontal-deflection plates, receives its voltage from the appropriate capacitor (C7-C11) in the speed control circuit. When the capacitor is fully charged, its potential is at 150 volts, regulated by V3. R11 adjusts the voltage applied to pin 10, the other horizontal deflection plate, to position the spot at its starting point at the left side of the CRT. Once adjusted, this voltage remains constant. Therefore, when the voltage on pin 9 is reduced by discharge of the capacitor, the spot on the CRT moves to the right side of the screen. 
   Thyratron tube V4 is used to discharge the capacitor, D rectifies the heater voltage and applies a negative voltage to the control grid of V4, keeping it cut off. PC1 is a photoresistive cell in the sensing unit which is placed inside the camera. When the shutter opens during analysis, light strikes PC1, lowering its resistance, which reduces the bias and allows V4 to conduct. R6 protects the photocell from the grid-current surge that occurs when the thyratron fires. Potentiometer R8 adjusts the bias voltage and determines the sensitivity of the trigger. When the V4 conducts, the timing capacitor is discharged through the tube in series with the calibration potentiometer, moving the spot across the 
CRT from left to right at a rate determined by the time constant of the series R-C network selected by S3. 
   Vertical deflection occurs at the same time, because light passing through the open shutter also reduces the resistance of PC2, the second photocell mounted in the sensing unit. This cell is connected so it changes the vertical-centering voltage applied to pin 6 of the CRT. With a fixed voltage on pin 7, the spot is deflected upward. 
   After the shutter has tripped and produced a trace on the screen, the spot is returned to its starting position at the left side of the screen by pressing RESET button S2 momentarily. The thyratron stops conducting. When the shutter is opened again, the tube remains cut-off and timing capacitor (C7-C11) recharges to 150 volts, returning the spot to the left side of the screen again. 
   Before the unit is calibrated, the CRT screen must be marked to indicate the correct starting and ending positions for the trace. This can be done with India ink on a piece of clear plastic film fastened to the CRT face with Scotch tape (see photo). Marking the screen is a very simple operation, if it is done before the sweep speeds are calibrated because any desired pattern can be used. The sweep is then adjusted to make the trace fit the design. 
   Each sweep speed is calibrated by adjusting its potentiometer against an external standard such as the 60-Hz line. An easy way to do this is to use a large neon bulb such as the NE-30 to trigger the sensing unit. The lamp is taped to the sensing unit so it is in direct contact with the surface containing the twe holes and so no outside light strikes the two photocells. Connect the lamp in the circuit shown in Fig. 2, so you can turn on and off easily. With the diode shorted, the lamp will flash 120 times per second; the diode cuts the speed down to 60 flashes per second. 
   Turn on the analyzer and allow it to warm up for about 10 minutes before calibrating it. 
While waiting, you can adjust the controls to bring the spot to its starting position and to obtain the best focus. Don�t leave the intensity turned up high for more than a few minutes while the spot is stationary, because the high intensity of the electron beam will burn the screen. Also adjust trigger-sensitivity control R8 so the light from the neon bulb just trips the horizontal sweep.
   With the spot at its starting position and vertical-gain control R4 set at maximum, switch on the neon lamp and observe the number of �pips� in the trace on the screen: Fig. 3 illustrates wate to expect. The lamp must be turned off to reset the spot, but take care not to switch off the lamp before the trace is completed. Adjust each potentiometer to obtain the proper number of cycles between the START position and the OK mark. Notice that you start counting from the second pip. The diode is used to reduce the number of pips that must be counted on slow speeds; on faster speeds, it can be removed from the circuit. The number of pips for any speed is found by multiplying the fraction by 60 when the diode is used and by 120 when the diode is switched out. (See Table 2). 
   Using this method, you can calibrate the sweep for any other speed you may need, and by using a switch with more positions, you can have as many speed as you want. 
      On slow speeds, you may want to recycle the spot a few times and count some of the pips during each sweep. It helps, when counting a large number of pips, to darken the room to make the trace more visible. Fortunately, extreme accuracy is not necessary, and you may find it feasible simply to calibrate against another shutter that you know works well. Other cameras then can be compared to the �standard� shutter. 
   We added the +-10% tolerance marks to help us decide which shutters needed overhauling. A small error of 12-15% of the distance from START to OK can be tolerated in any shutter. If you want to place the 10% marks accurately, here is how to do it. 
   Use the setup that you�d use to check a shutter at 1/30 second, using a 60-Hz calibrating signal. You�ll see two cycles of 60-Hz signal as in Fig. 4. The line going up from the 
START position is slanted because the shutter takes a small amount of time to open wide enough to energize the vertical-deflection photocell. This is not important. The second cycle of the signal is expanded because the horizontal sweep is faster on the right side of the screen. This is good because this is the area where critical measurements are to be made. 
   Adjust the analyzer controls so the peak of the wave is on the OK line. (Note that this differs from the procedure for shutter-speed measurements in  which the wave crosses the OK line at the 50% point.) Two cycles equal 720* as shown in Fig. 4. When the calibrating wave is centered properly, the distance between A and B-measured along the zero axis of the wave- equals 90*. Thus 10% of the full sweep time (not distance) is one-tenth of 720, or 72*. The 72* point is easy to find. It is simply 4/5 of 90.(Note: *=degree)
Testing a shutter: Allow time for the analyzer to warm up, and use the positioning controls to set the spot at its starting position. Plug in the sensing unit and place the photocells inside the camera-or behind the shutter, if you are testing an unmounted shutter. Open the diaphragm to maximum aperture. Set up a light source-a 50-watt light bulb, for example-so that, as the shutter opens, light will pass through the lens and strike the photocells. Keep stray light on the CRT to a minimum, because ambient light reduces the apparent persistence of the trace. Note:The intensity of the light, or the distance between the light and the lens, does not affect the calibration in any way. 
   Cycle the shutter through several trial runs, resetting the spot after viewing each trace. If horizontal or vertical deflection fails to occur, check to see that the sensing unit is held directly behind the lens opening so the light strikes both photocells. Be sure the diaphragm is open wide. Remember, to reset the spot, the sensing unit must be in complete darkness. 
   Using the vertical-gain control, adjust the height of the trace on the CRT screen. For the 
calibration pattern used in this model, the line marked 50% indicates the point at which the shutter is half-closed. Since different shutters vary in the linearity of their opening and closing rates, all calibrations are based on the time when the shutter is Using the vertical-gain control, adjust the height of the trace on the CRT screen. For the calibration pattern used in this model, the line marked 50% indicates the point at which the shutter is half-closed. Since different shutters vary in the linearity of their opening and closing rates, all calibrations are based on the time when the shutter is half-closed. The trace of a perfect shutter crosses the junction of the dotted line marked OK and the 50% line. 
   A shutter should be checked several times in succession to permit observation of different characteristics on each trial. In addition to timing, which should be within 10%, you can see evidence of dirt in the shutter leaves: it causes irregularities along the slanted part of the trace while the shutter is opening and closing. �Shutter bounce,� the tendency of a shutter to close slightly just after it has opened, makes a telltale dip in the horizontal part of the trace, usually near the beginning. 
   Focal-plane shutters also can be checked in a similar manner with one precaution: the sensing unit must be placed at the back of the camera where the film is normally held. In timing focal-plane shutters, the sensing unit must be held perfectly stationary with respect to the camera, and the two holes must be in a horizontal plane. 
   This instrument has a few shortcomings- it won�t tell you the actual shutter speed, only indicate an out-of-tolerance condition, for example- but it�s a mighty handy device for photographers who have many cameras. Institutions and newspapers also should find practical applications for the unit in maintenance and quality control. Since shutters have no simple do-it-yourself speed control anyway, knowing when to send a bad one to a camera-repair shop can mean a great deal to anyone who depends on the accuracy of fractional seconds for his living.