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562 RICHARDSON'S BLUEBOOK OF PROJECTION
the a. c. power line, which is a short and simple circuit. The switching arrangements near the fuses, however, remain to be considered.
D-l, labelled "Door Switch," is obviously a safety arrangement, a single-blade, single-throw cut-out that opens when the door of the amplifier is unlatched. Its function is to protect the projectionist against the high voltage generated by the secondary of T-5. At this point it may be well to remark that no safety switch offers complete protection. Not in an amplifier. True, that T-5 no longer generates high voltage when switch D-l is opened ; but the voltage previously generated is stored in the condensers of the rectifier filter, and remains there after the safety switch is opened. To render this amplifier wholly harmless the filter condensers must be discharged. This is done by short-circuiting them using a screw driver blade with an insulated handle.
The double-throw switch, D-2, to the right of D-l, removes the line power from the two power transformer primaries and throws it over to the terminals at the extreme bottom left of the drawing, marked "110V. A.C. Chg." This switch, then, turns off the amplifier and turns on the battery charger. To complete our understanding of the workings of this switch we must look at the 12-volt d. c. terminals immediately above the charger terminals.
The d. c. output of the battery charger, 12 volts positive and negative, is wired to the terminals so marked, just above the 110-volt charger input terminals. Now, when switch D-2 is thrown to the right the positive side of the charger's d. c. output is open-circuited at switch D-2, and we know also that the 110-volt input to the charger is interrupted. When switch D-2 is thrown to the left, input power is supplied to the charger, and the positive side of its d. c. output is connected, through the upper blade of D-2, to the 12-volt positive battery terminal, just above the 12-volt positive charger terminal. The drawing also shows a permanent common connection between the negative of the battery and the negative of the charger output, therefore no switching is needed in the negative line.