Journal of the Society of Motion Picture Engineers (1930-1949)

voltage. The charge on C4 is added to the already duplicated incoming pulse voltage and V3 rectifies this potential to produce a charge on capacitor C5 of three times the incoming pulse voltage. In the case of this unit, the original pulse voltage is somewhat over 10,000 v so that the output of the voltage multiplier system is approximately 33,000 v. The type of voltage tripler which was just described is conventional and is used on some other types of television projection devices; however, practical experience has shown that Rl and R2 have such high voltage gradients that it is next to impossible to obtain resistors which will have long life and which will retain resistance stability for a reasonable period. Therefore, in the interests of maximum reliability, these resistors have been replaced in the Trad unit by two additional highvoltage diode rectifier tubes V4 and V5, as shown in Fig. 7, functioning as thermionic resistors and having longer life and greater stability. This was possible because the high voltage across the resistors was direct current; therefore, the vacuum tube could be connected in such a manner that it would present a high impedance to voltages of that polarity while permitting currents in the reverse direction to flow with relative ease. Thus the vacuum tube provides a higher inverse resistance than would have been practical with resistors and still have the advantage of a low impedance in the reverse direction. Figure 8 shows this circuitry embodied in a completely enclosed plastic housing which provides large margins of safety with respect to arc-over or strike-over of high voltage between the circuit elements and/or ground. The simple and clean design which has been achieved can readily be seen in this high-voltage unit. It is interesting to note also that the five-tube tripler unit can be readily removed from the balance of its circuits for routine maintenance, and that this construction also provides the maximum ease of replacement in the event of any form of failure in the unit. In Fig. 9, the capacitors Cl, C2, C3, C4 and C5 can readily be seen. It should be noted that there are no wires or other protuberances in the high-voltage compartment which would induce breakdowns. Figure 10 shows the actual circuitry in the high-voltage and sweep chassis. The circuit diagram shown herewith is fairly conventional, but may be worthy of a few words. The composite video signal has had the synchronizing information stripped from it in the associated low-voltage power supply unit, shown in Fig. 11. This synchronizing information is fed into the unit shown and separated into both horizontal and vertical synchronizing pulses by the 6SN7 vacuum tube. A blocking type of oscillator is used for the vertical deflection, whereas a synchro-lock type of horizontal oscillator circuit for maximum synchronizing stability is used for the line frequency. The five-tube tripler, which was explained above, is seen in this diagram, together with its associated circuitry. Figure 11 is the schematic circuit diagram of the low-voltage power supply and video chassis. It will be noted, a two-stage video amplifier with series and shunt peaking is provided, and that a d-c restorer of conventional design as well as a synchronizing stripper are incorporated on this chassis. The unconventional part of this unit is given on the right-hand side of the d-c restorer tube which, with its associated circuitry shown herewith, rectifies the video signal which appears across the plate load resistor of the second video amplifier stage and supplies this potential to the accelerator grid of the picture tube. This portion of the d-c restorer and its associated circuitry is an automatic brightness control. 130 August 1952 Journal of the SMPTE Vol. 59