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D.  Power for the lights in the master dim and test circuits is from three 28-volt dc circuit breakers labeled INDICATOR MASTER DIM BUS - NO. 1 DC, NO. 2 DC and BAT. The power sources are cross-fed to the lights (through diodes) to ensure power is still available in case one of the busses is de-energized. The three power lines are then connected through the master dim relay to six section circuit breakers where power is distributed to the various system indicator lights within the control cabin. A DIM & TEST circuit breaker provides 28 volts dc to the dim and test relays and to certain system indicator lights as well as the master caution circuits. All circuit breakers are on load control center P6-3.
Master Caution Light and Annunciator
A.  The master caution light consists of an amber light cap labeled MASTER CAUTION - PUSH TO RESET, a bulb and a base with an internal switch. Pressing the light cap operates the momentary switch and interrupts power to the master caution circuits causing the lights to go off. The light is easily relamped after pulling off the light cap.
B.  The system master caution annunciator is composed of six sections with two bulbs in each section, a base with an internal switch and a light cap divided into six sections. The light cap sections on the pilot's side are labeled FLT CONT, ELEC, APU, FUEL and OVHT/DET; one is a spare and is not labeled. The light cap sections on the copilot's side are labeled ANTI-ICE, HYD, OVERHEAD, DOORS, and AIR COND; one is a spare and is not labeled. Pressing the light cap operates the momentary switch which applies a ground to master caution circuits causing all lights to illuminate. When released, only those systems with detected faults will have indicator lights on and the corresponding system master caution annunciator on. The annunciator is easily relamped after pulling off the light cap.
543 
33-15-0 Page 4  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Feb 15/73 

543  Master Caution Annunciator Power Schematic 
Feb 01/74  Figure 2 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  33-15-0 Page 5 

3.  System Indicator Lights
A.  The majority of the system indicator lights on the various modules throughout the control cabin are composed of a light cap, two bulbs and a base with two diodes, a fuse and a momentary switch. Pressing the light cap operates the momentary switch which causes the light to illuminate (push-to-test). The annunciator is easily relamped after pulling the light cap assembly from the base.
B.  Two types of lights are used, one is a ground seeking type which requires a ground controlled circuit to turn it on; the other is a power seeking type which requires a power controlled circuit to turn it on (Fig. 3). The majority of the lights are of the ground seeking type. A few lights contain only one bulb and no diodes , and are not connected to the master caution circuits; however, they are operated by the master dim and test circuits. Diodes for these lights are usually in annunciation and dimming module M469. Refer to the Wiring Diagram Manual for specific circuit details of each system.
4.  Annunciation and Dimming Module
A.  The annunciation and dimming module consists of three printed circuit cards, a transistor circuit and diodes mounted within a box. Two electrical connectors are on the side of the box and a mounting plate is on the front. The module is installed with screws on the left side of the control stand P9 panel. Card A3 contains diodes which isolate individual system control of the system master caution annunciators from the master caution lights and provide diodes for various systems indicator light circuits. Cards A1 and A2 contain transistor circuits with zener diodes which cause certain system indicator lights to dim when the master dim and test switch is operated. The separate transistor circuit and zener diode is connected to the master caution lights and causes the master caution lights and system master caution annunciator to dim.
Master Dim Module
A.  The master dim module consists of a printed circuit card with a relay, diodes and a transistor circuit. The card is installed in a card rack in load control center P6. The relay connects zener diodes and resistors in the light dimming circuits to the P5-4 ac systems generator and APU module and to the P5-10 air conditioning module. The diodes provide isolation between dual power sources for the master dim and test circuits. The transistor circuit is not used.
543 
33-15-0 Page 6  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  Feb 01/75 

543  Typical System Indicator Light Circuit 
Feb 01/75  Figure 3  33-15-0 
Page 7 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 
　
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