Maintaining Your 16mm Projector (Chapter One- Page 3)

Frequency Response and Gain

Several adjustments can be made on most projectors which will affect frequency response and gain. Use your projector's service manual for location of adjustment points and screws. If the manual is not available then upon a reading of the following material and a close inspection of your machine you should be able to discover the various points of adjustment.

High frequency signals appear as small modulations on the sound track. Since they are small, they will only be reproduced if the light beam falling on the track is very narrow. A wide beam cannot pick up a narrow modulation. The width of the slit mask is fixed. In the Eastman 25 it's .0003 inch. The slit must be focused sharply on the track. Here's the best way to focus:

Sound Focus Adjustment

1. Your projector has a focusing lens adjustment which may be a focusing rod or screw. It will have a locking nut, lock sleeve or a clamp. Loosen the locking mechanism so that you can move the focusing adjustment.

2. Make a 6 to 8 foot continuous loop from a section of sound focusing test film (5,000 or 7,000 cyle per second square wave track). Thread the projector with the emulsion side towards the focusing lens. Connect a 30W 8 ohm dummy resistor to the speaker output jack. With the projector running, observe the audio output on a VU meter (an AC voltage meter connected across the dummy load resistor). Observe the meter while slowly moving the lens up and down and slightly rotating until the maximum voltage is achieved. Reclamp or lock down the sound lens.

Azimuth Adjustment

If it's impossible to obtain a peak reading during the focus adjustment, the azimuth adjustment may be off. The slit of light must fall on the track squarely, otherwise if tilted it willl be reading a portion of the the track preceeding or following the desired area. Your projector may not have any adjustment for azimuth and its locked in. Where adjustment is provided, the sound optic carrying the slit can be tilted side to side by loosening one of the azimuth adjustment screws and tightening the other. The adjustment is made using the same test film in the projector as for the focus adjustment. Both of the azimuth adjustment screws should be tight when the final test is made.

Buzz Track Adjustment

The slit of light must fall on the sound track in the correct lateral position so that it will be modulated by the full width track but yet the sound track should not be scanned beyond its edge. This is important for maximum gain as well as signal to noise ratio. A buzz track test film for checking this adjustment is available from the SMPTE. On this test film, the normal track area is blank, but, on one side of the track area, there is a 1,000 cycle signal and, on the other a 300-cycle signal. Thus, if one or the other of the tracks is audible, it is apparent thast the light slit is off center. A buzz track adjustment screw controls the lateral position of the slit. Some projectors will have two screws butting up against one another but most have one. (See p1 graphic)..

1. For the test and adjustment, thread the projector with a continous loop of a buzz track test film. If either the 1,000 or 300 cycle track is audible, the slit is off center. The frequency of the note reproduced will determine whether or not the optics should be moved in or out.

2. The adjustment is a bit of trial and error and several attempts will probably be made before the exact point is located where neither the 1,000 cycle nor the 300 cycle track is audible. As a guide, understand that the 1,000 cycle track is on the outside and that you would turn the adjusting screw clockwise (in) until the 1000 cycle tone is inaudible and a 300 cycle tone becomes audible. Then back out the screw adjustment until neither tone is heard. If you have a two screw adjuster, then first loosen the outer adjusting screw so as to allow adjustment to made solely with the inner adjusting screw. When the exact location has been determined and the setting made, tighten the outer adjusting screw against the inner screw and recheck to see that the position has not been altered.

Prism, Field Lens, and Photoelectric Cell Adjustment

After the light slit strikes the film, depending upon your projector, it may pass through to a prism, then a collector lens and onto the photoelectric cell. Most projectors have no adjustment as the component(s) are locked in place during manufacture. The Eastman 25 allows for adjustment. Whether your projector provides for adjustment or not, for one reason or another, they can be jarred out of position. Here are some guidlines to help you reposition them.

1. The soundtrack must fall exactly on the face of the prism.

2. There must be several thousanths of an inch clearance between the prism and the sound drum during the full rotation of the drum.

3. When the prism is position properly, a circular light beam should fall on the center of the collecting lens. If it does not fall on the center, move the prism forward or backward from its present position. After this adjustment, recheck to make sure that the sound track falls on the front face of the prism and that the sound drum clears the prism. The film should not touch the the front surface of the prism as it travels over the sound drum.

4. Check the position of the light beam on the collecting plate of the photoelectric cell. If necessary adjust the cell in order to collect the entire spot of light.