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1950 LARGESCREEN TV PROJECTION 517
required the minimum wall thickness of the sleeve under the scanning yoke is 6.5 mm, and the problem of supplying sufficient scan current in these coils is very difficult, particularly at 15 kc, which is the line frequency for 625 lines, and also as the coils appear at the end of a long cable.
15. Is the focus coil. This is a complicated and costly unit. It consists of a long solenoid embracing the electrode system of the tube and having eight parallel windings so as to give the minimum practical resistance. The focus current is modulated at line and frame frequency, a worth-while improvement for high-voltage tubes having large deflection angles.
16. Is two sets of deflection coils, one used to center the beam in the focus coil and the other to center the scanned area or raster, on the face of the tube with the optical system. This electrical method of centering removes the need for accurate mechanical adjustments and greatly facilitates the realignment of tubes when these have been changed. In the same way we have found that only one in four of the tubes requires readjustment for the centering in the optical system to allow for the axial alignment of the tube neck to face. When the center of curvature of the tube has been placed on the axis then the adjustment to center the raster on to the center of the viewing screen is very slight and can be made electrically.
17. Is the correcting plate. This is held in four grooved rollers, the upper two of which are spring loaded and the other two are on eccentric rollers so that the plate can easily be centered on the optical axis while maintaining its parallelism to the mirror. In setting up these large Schmidt systems we have found that the spacing between the mirror and the correcting plate is uncritical, and we may in future make the correcting plate easily removable for tube changing, as this would simplify the internal arrangements of the tube mount.
18. The tube is held in position by being pressed into the polythene sleeve in the front and lightly clamped by the screws at the rear. Thus tube and focus coil mount are inserted into the optical system as one unit when a change is necessary. The focus coil mount is carried in the optical system on two girders made to be very rigid in the vertical plane, but designed to obscure as little light as possible. These girders are supported at the ends outside the optical path on adjustable supports.
Construction of Cathode-Ray Tube
The next most important item of the projector is the cathode-ray tube. Continuous improvement has been made in the performance