90. Optical System.-The optical system is a very delicate piece of mechanism and its adjustment should never be changed by the projectionist. It has been very accurately adjusted with the use of delicate instruments, and if it is moved even a few thousandths of an inch the quality and volume of reproduction will suffer. In the optical system used in type PS-1 sound heads a small vertical illuminator is furnished for the use of the RCA service man. The position of this illuminator is controlled by a slotted knob on the side of the lens barrel. (See Figure 60.) The slot should always be in a vertical position, as the amount of light reaching the photo-cell is greatly reduced if it is at any other angle. The optical system was designed to have the illuminator left in at all times, and it should not be removed as this allows dirt to enter the optical system. Pull out the light slit slide occasionally and blow out any dust that may have collected.

When the optical system is focused, a condition is reached whereby every frequency that it is possible to record will be reproduced. If this focus is changed the quality will be very poor; all frequencies in the upper range will be lost, and all other frequencies will be distorted.

91. Sound Gates Used in Type PS-1 Sound Heads.-Two types of gates are used in the type PS-1 sound heads. These are illustrated in Figures 61 and 62. There are two differences between them. The pressure plate is easily removable in the gate shown in Figure 62. Although the pressure plate may be removed from the gate shown in Figure 61 by removing a screw, it should not be removed by the projectionist. The other difference is in the size of the light aperture in the stationary section of the gate. The aperture of the gate shown in Figure 62 is the larger of the two.

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