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156
BROWDER
August
possible to effect this change merely by moving the recording lamp from one position to the other. Fig. 5 is a photograph of the modulator with the light valve in place and the lamp moved up to the position for recording negative sound track. Fig. 6, in which the modulator is mounted in its recorder, shows the lamp moved down in position to record direct-positive sound track. The lamp bracket is mounted in a dovetail slide equipped with adjustable stops at each end of its travel. After the stops have been adjusted and locked for a particular lamp, the transition from direct-positive to negative recording may be
accomplished in a matter of seconds merely by moving the lamp.
The light valve is a completely self-contained component of the modulator, the mounting being arranged so that it may be removed readily from the modulator proper for inspection or replacement. Registry of the face of the light valve with the modulator mounting plate is accomplished by constructing this plate of soft steel so that the leakage magnetic flux from the valve serves to hold the valve face tightly against this plate. A single milled slot in the valve face and a dowel in the mounting
Fig. 5-Modulator and light valve. PMe loCate the Valve in the
right and left directions while
registry tabs at the back of the mounting plate bear against the rear edge of the valve to locate it fore and aft. After the valve is pushed home against the registry tabs, a clamping lever is swung into place and tightened to augment the magnetic clamping force further. Release of this lever then^ allows the valve to be slid across the steel mounting plate and finally lifted clear.
The objective cylinder lenses are mounted in a cylindrical cell which is spring-loaded against a threaded ring. Calibrations on the periphery of this ring serve as a reference in making film tests to establish the optimal focus setting. Provision is made for a limited range