Each strobe light has four inputs from the rest of the system: (1) 240 volts ac, (2) ground, (3) 120 volts ac timing pulses at the rate of two per second, and (4) a dc voltage connection to the monitor system. These inputs are plugged into a cable through a four-pin connector (No. 10). The unit steps up the 240-volt ac input voltage to 1,460 volts ac with a transformer (No. 15) and passes this voltage through a full-wave rectifier circuit of vacuum tubes (No. 13). The resultant 2,000 volts dc is applied to the electrodes of a flashtube and across the flash capacitor (No. 4).

The xenon-filled flashtube will fire only when ionization is initiated by a trigger pulse of about 5,000 volts applied to its third electrode. This pulse is supplied by a trigger coil. At the same time that the flash capacitor is storing its charge, the trigger capacitor is also being charged by the primary of the trigger coil, which is an autotransformer, and cuts the bleeder resistors in series out of the circuit. When the l20-volt ac timing signal arrives, it is applied to the coil of the trigger relay (No. 9), thus closing the relay contacts, allowing the trigger capacitor to discharge through the primary ofthe trigger coil. That generates the necessary trigger pulse in the secondary of the trigger coil, the flashtube fires, and the flash capacitor discharges across the flashtube electrodes. The flash capacitor discharges down to the deionization potential of the flashtube, at which point the tube becomes a nonconductor. The light-producing arc ceases, and the charge cycle begins again.

The charge stored in the flash capacitor is a potential safety hazard. To make sure that the capacitor is discharged when the light unit is shut off, provide a discharge circuit by a series of bleeder relays. The bleeder relay (No. 5) closes this discharge circuit when the power to the transformer is turned off.

The current that charges the flash capacitor creates a pulse voltage in a surge resistor twice each second. A part of this voltage is applied to a silicon rectifier through a tap-off of the surge resistor. The rectified voltage is then filtered and applied to the monitor relay. The value of this voltage is sufficient to keep the monitor relay energized when the unit is flashing normally. When the unit stops operating because of a component failure in the unit, the absence of the pulse voltage at the surge resistor will allow the contacts of the monitor relay to close. This action completes a circuit from the monitor connection through a monitor resistor of 22 kilohms to ground. The monitor and control chassis react to the ground by warning the operator.

Master Sequence Timer Cabinet

The master sequence timer cabinet has all of the controls for the strobe light system except the tower control unit. The cabinet is supplied from a 240-volt, phase-to-ground circuit. Our discussion of how the system operates is keyed to the numbered items in figure 6-42.

LOCAL/REMOTE CONTROL UNIT. - The local/remote control unit (No. 1) gives you a way to turn the system on locally or give control to the tower. In the center of this unit is a control knob with three positions: REMOTE/OFF/LOCAL-ON. There are two red indicator lights above the control knob and two green lights below it. When the control knob is in the LOCAL-ON position, the system is turned on, and red lights will glow to indicate that the system is on LOCAL CONTROL. The green monitor lights should burn unless there is a fault in the system; in which case, they will go out. When the control knob is placed in the REMOTE position, the system can be turned on and off at the tower control unit. The red indicator lights will go out, but the monitor lights will continue to work as before. You should remember that the tower has no control except when the switch is in REMOTE.

MONITOR AND CONTROL CHASSIS. - The monitor and control chassis has several functions. They are as follows:

1. It de-energizes the monitor lights in both control units when a set number of lights stop working.

2. It has a step-down transformer to supply the voltages needed for control and indication.

3. It has a diode rectifier that supplies direct cur- rent for relay operation.

4. It has the fuses that protect the master sequence timer, the indicator circuits, and other components.

The main power transformer in the monitor and control chassis is energized all the time from a local 240-volt ac supply. The secondary voltage from this transformer energizes the indicator lamp transformer and the transformer of the dc circuit. The indicator lamp transformer supplies 12 volts ac to the indicator lights in the local/remote control unit. The transformer for the dc power will supply 95 volts ac to a bridge rectifier that supplies 120 volts dc to the dc monitor circuit. As long as the master control switch is on, power is fed to the tower csontrol unit no matter what position the local/remote control unit switch is in. When the