6.  Operation - Master Caution Lights
A.  The master caution lights inform the pilots that a system fault indicator light has illuminated on overhead panel P5 or the P8-1 engine and APU fire control module. The system master caution annunciator informs the pilots which system has a fault. Pressing either the pilot's or copilot's master caution light extinguishes the master caution lights and the system master caution annunciators; however, the system fault indicator light on the P5 or P8 panels remains illuminated until the fault is corrected. The pilot's may recall faulted system indications on the system master caution annunciators by pressing either annunciator. All annunciator lights will illuminate while the annunciator is pressed. When released, only those systems with illuminated fault indicators will cause the corresponding system master caution annunciator to remain illuminated.
B.  Nine overhead P5 panel modules and the P8-1 fire protection module are connected to the master caution lights. Each module is also connected to one of the ten corresponding light sections of the system master caution annunciators. The master caution triggering and reset circuits are part of each system module. (See figure 3.) Two types of system indicator lights are used on the modules; a ground seeking type and a power seeking type. However, the basic caution circuits are similar. The majority of the lights are ground seeking. Referring to figure 3, and the typical ground seeking indicator light circuit, when a system fault is detected, a ground is applied to illuminate the system indicator light and is also applied to gate G1. More than one system indicator light and detection circuit is usually connected to gate G1 in each module. Power for the module is either from a system circuit breaker or from the DIM & TEST circuit breaker. Again referring to figure 3, gate G1 is normally off until a fault causes the input to gate G1 to go low. Gate G1 output then goes high, causes a pulser to trigger the SCR and biases transistor S1 on. A ground is then completed to turn on the system master caution annunciator and master caution light. Momentarily pressing the master caution light removes power from the SCR causing it to go off; thereby, extinguishing the master caution lights and annunciators. Pressing the system master caution annunciator completes a ground to gate G1 causing the pulser to operate again and retrigger the SCR. The SCR then turns on the system master caution annunciator again if the system fault is not corrected and the system indicator light is still illuminated.
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7.  Operation - Master Dim and Test
A.   All the system indicator lights in the control cabin may be dimmed or tested with a LIGHTS switch. The switch is spring loaded from the TEST position to BRT position. (See figure 4.) Holding the switch to the TEST position applies a ground to master test relay R33. The master test relay then applies either a ground or DIM & TEST 28 volts dc to the system indicator lights. (See figure 3.) The lights then illuminate until the LIGHTS switch is released.
B.  Power for system indicator lights is from three circuit breakers on 28-volt dc bus-1, 28-volt dc bus-2 and the 28-volt dc battery bus. (See figure 4.) A diode network (gates G1, G2 and G3) in master dim module M564 provides a dual power circuit to each distribution line. Zener diodes are used for decreasing the voltage and dimming system lights. Secondary circuit breakers distribute power for the majority of the system indicator lights. Refer to the Wiring Diagram Manual for specific circuit details. A few lights use system power for their operation, in which case either a transistor and zener diode circuit (gates G4 through G9 in annunciation and dimming module M469) is used to dim the light, or a ground and zener diode is applied directly to the light by master dim relay R34 or a dim relay in the master dim module. Auxiliary master dim relay R87 applies either DIM & TEST 28 volts dc or a ground for operation of the transistor and zener diode dimming circuits in the annunciation and dimming module. The dim relays are energized when the LIGHTS switch is positioned to DIM. Master dim relay R34 and the dim relay in the master dim module then insert zener diodes into the lights circuits to dim the lights. The dim relay also inserts resistors for dimming lights on the P5-10 air conditioning module. Auxiliary master dim relay R87 removes a ground from gates G4, G5 and G6 and applies power to gates G7, G8 and G9 causing them to go low and insert a zener diode in each light circuit. Gates G10 and G11 are used for testing the pilots' MDA lights and are operated by the minimum decision altitude trip established on the radio altimeter indicator.
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