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12. Clockwise rotation of the lever from 0 (zero) to GO starts the hands. Clockwise rotation of the lever from GO to STOP, stops the hands. Counterclockwise rotation of the lever from STOP to GO restarts the hands. Clockwise rotation of the lever from STOP to 0 (zero) resets the hands.
D.  The sweep second hand and the minute recording dial hand are controlled by a knob (PUSH) at the lower right edge of the clock. When the sweep second hand and minute hand are on zero, the hands may be started by pushing the knob. Pushing the knob a second time stops the hands. Pushing the knob a third time resets both hands.
(1)  A small flag visible through a circular opening located within the minute recording dial shows the operating condition of the sweep second hand the the minute recording hands. The flag shows all white when the hands are reset to zero. The flag shows half white and half black when the hands have been stopped but not reset. The flag shows all black when the hands are operating.

H31410
539  Captain's and First Officer's Clock 
Aug 15/72  Figure 1  31-25-0 
Page 1 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

AURAL WARNING SYSTEMS - DESCRIPTION AND OPERATION
1. General
A.  The control cabin aural warning systems provide characteristic audio signals to alert the pilots to: an abnormal takeoff condition, landing condition, pressurization condition, mach-airspeed condition, an engine, APU or wheel well fire and calls from the crew call system or selcal (if activated) systems. The aural warning and call devices are installed in an aural warning devices box. The box is on the right side of the forward section of the control stand in the control cabin (Fig. 1). On some aircraft the 65-54499 Series aural warning devices box has been replaced by the 69-78214-1 Series aural warning module.
B.  Inputs to the aural warning devices box are from: the flight and ground crew call system and selcal systems (Chapter 23), the mach-airspeed warning system (Chapter 34), the engine, APU and wheel well fire detection systems (Chapter 26), cabin altitude warning system (Chapter 21), the takeoff warning system, and the landing gear warning system (Fig. 2).
1A.  Aural Warning Module (69-78214-1 Series)
A.  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.
2.  Aural Warning Devices Box (65-54499 Series)
A.  The aural warning devices box contains solid state horn, chime, logic and oscillator circuits, mounted on a printed circuit board, which provide the horn audio and chime audio to a loudspeaker whenever the appropriate circuits are keyed (Fig. 2). A second printed circuit board contains the oscillators and solid-state switching circuits use d with a mechanical bell and clacker. The printed circuit boards may be easily removed from their connectors and replaced as needed. The power amplifier transistors are on heat sinks installed directly on the box. A single multipin connector on the backside of the box completes all circuits to the box. The components use 28 volts dc obtained through circuit breakers on load control center P6.
B.  Circuit board Al is used with the clacker and bell. The board contains a multi-vibrator, an oscillator and two solid-state switching (gate) circuits. Whenever the oscillator is keyed by a ground from the mach-airspeed warning system, an oscillator turns a gate on and off, thus causing power through the clacker to be completed to ground. The clacker sounds once when the circuit is turned on and once again when it is turned off; therefore, the clacker sounds at twice the oscillator rate. When the multi-vibrator is keyed by a ground from the fire detection system, the multi-vibrator turns a gate on and off, thus causing power through the bell to be completed to ground and the bell to sound at the multi-vibrator rate.
500 
May 01/98  BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.  31-26-0 Page 1 

Aural Warning Systems Component Location  501 
31-26-0  Figure 1  Jun 20/83 
Page 2 
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

C.  Circuit board A2 contains a multi-vibrator, horn sound (tone) generation circuits, chime sound (tone) generation circuits, amplifiers, oscillator and logic circuits required to produce appropriate sounds from the loudspeaker. The circuit board also contains circuits, which are not used at present but provide a buzzer, wailer and low chime sound capability when required. Whenever the steady horn circuit is keyed by a ground from the landing warning circuits, an oscillator circuit provides a horn sound, which is, coupled to a driver (amplifier), power amplifier and loudspeaker. When the intermittent circuit is keyed by a ground from the takeoff warning or cabin pressure warning circuits, the oscillator is turned off and on by the multi-vibrator circuit, thus producing an intermittent horn sound. The chime circuits are 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 or each time the hi-lo or hi-lo-repeat circuits are keyed by 28 volts dc from the selcal system (if activated). The hold-on circuits keep the chime circuits operating and control the chime decay after the initial 28-volt dc keying pulse. Horn and chime tone controls are on the printed circuit boards and are not accessible from outside the box.
　
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