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A 737-300/400/500MAINTENANCE MANUAL

HI CHIME 

LO CHIME 

65-54499-7,-12,-15 AWM AIRPLANES WITH 65-54499-7,-12,-15 AWM

AIRPLANES WITH PARK BRAKE WARNING
Aural Warning System Simplified Schematic
Figure 2 (Sheet 3)

C96975
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A 737-300/400/500MAINTENANCE MANUAL

C42974
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A
737-300/400/500MAINTENANCE MANUAL
 D.  The circuit board A2 chime circuit is capable of being keyed from a steady 28-volt dc voltage or from a 28-volt dc pulse. A logic and hold-on circuit operates the chime generation (phase oscillator) circuits each time the hi circuits are keyed by 28 volts dc from the flight and ground crew call system. The hold-on circuits keep the chime circuit operating and control the chime decay after the initail 28-volt dc keying pulse. Horn and chime tone controls are on the printed circuit boards and are not accessible from outside the box.
 E.  A priority is established within certain circuits to prevent different sounds from occurring at the same time and becoming garbled. The clacker and bell are not in the priority circuits; therefore, they may operate with each other or with any of the other sounds. Those sounds in the following priority list will be turned off by control circuits of sounds in a higher priority.
 PRIORITY________ SOUND 1 Intermittent Horn 2 Steady (Continuous) Horn 3 Wailer 4 Buzzer (if used)
 F.  The high chime can be operated along with any of the sounds listed above.
 G.  The mach airspeed clackers in the aural warn devices box are connected directly to the captain's and first officer's MAS indicators.
 H.  AIRPLANES WITH A 69-78214-1 SERIES AURAL WARNING MODULE; The aural warning module has two redundant channels, channel A and channel B. There are two aural synthesizers in each channel. One supplies the bell/clacker aural warnings and the other supplies the horn/wailer/chime warnings. These synthesizers make a combined audio signal input to the analog channel. The analog channel then outputs an audio signal to the speaker amplifier. Each channel can be tested independently using the BITE.
 3.  Landing Gear Warning System
___________________________
 A.  A steady (continuous) warning horn sounds when the landing gear position is incorrect for the current operating conditions. Switches, operated by the movement of engine thrust levers or the flap follow-up drum, control the warning horn. Secondary control is provided through the landing gear module to ensure that the horn will not sound when the landing gear position is correct for the current condition. The warning horn is mounted on the warning devices panel located on the right side of the aisle stand (Fig. 1).
 B.  Landing Gear Uplock and Downlock Sensors - The landing gear uplock and downlock sensors (proximity switches) provide signals to the landing gear warning system (Fig. 1).
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 C.

D.
E.
F.
A
737-300/400/500MAINTENANCE MANUAL
The main landing gear downlock sensors provide signals to the corresponding solid-state circuits in the landing gear module when the main landing gear is down and locked. The solid-state circuits will switch to a grounding condition when furnished a signal from the downlock sensor and thus provide a ground to illuminate the green indicator light for the individual main gear. The downlock and uplock sensors provide signals to the solid-state circuits so that the appropriate circuit will provide a ground to illuminate the red warning light and provide aural warning signals when the individual main gear is not locked or until the main gear position agrees with the landing gear control lever position.The nose gear sensors provide a signal to solid-state circuits in the landing gear electrical module when the nose gear is either up and locked, or down and locked. The sensor signal will cause the circuit to remove ground from a red warning light and the light will go off. The absence of a signal from a nose gear sensor (nose gear not up and locked, or down and locked) will cause the circuit to provide a ground to illuminate the red warning light and provide aural warning signals. The nose gear downlock circuit also provides a ground to illuminate a green indicator light when the nose gear is down and locked.Thrust Lever Actuated Landing Gear Warning Switches - Two switches (one in each autothrottle servo assembly in the equipment center) are actuated when the thrust levers are retarded and will actuate before the thrust levers contact the idle stop. An actuated switch completes a circuit to ground and sounds the warning horn if any gear is not down and locked. Another set of two switches (one for each thrust lever) are located in the pilots' control stand and are actuated by thrust lever advance to interrupt the flap operated landing gear warning circuit for flap setting
　
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