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kinescope and the necessity of ensuring that the tube fulfill its life expectancy made electronic protection a must item. Scanning failure could cause the faceplate of the tube to be burned so as to make it unusable and in the extreme case, with high-voltage beam concentrated on a single spot, that is without either vertical or horizontal scanning, it could melt a hole in the faceplate of the tube.
Protection to the kinescope has been provided for the following contingencies:
1. Open or shorted deflecting yoke,
2. Lack of drive due to tubes,
3. Loss of supply voltages,
4. Overdrive of kinescope (positive
grid).
In operation, primary power is first applied for all electron-tube heaters and to the bias supply. A relay on the bias supply then closes, connecting primary power to the plate power supplies.
Scanning will now be generated, but a series of interlocks must now be closed before the high voltage can be applied.
The following protection circuits must be functioned properly: the horizontal scanning; the vertical scanning; the electromechanical interlock on the projector cover must be closed; the vault high-voltage access door must be closed; then the high-voltage control circuit may be actuated. During operation excess video drive could damage the kinescope if it were not protected by an instantaneous bias control.
The most important consideration in the design of the protection system is the speed with which failure can be detected and corrective measures taken and, in addition, the circuits must 'fail safely. The circuits operate in such a manner as to drive the kinescope to beam cut-off (and this must be accomplished in a matter of less than 50 n sec) and then the relays operate to remove the high-voltage power.
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Fig. 8. Vertical deflection chassis, inside. March 1951 Journal of the SMPTE Vol. 56