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C.  An erase head in the recorder unit automatically erases previously recorded information prior to recording. A 30-minute length of closed loop magnetic tape then provides a record of the previous 30 minutes of conversation. An internal monitoring circuit ensures proper operation of the recording mechanism and circuits. Audible and visible indications derived from the monitoring circuits are presented at a meter and a jack when the test switch on the control panel is operated. The entire tape may be erased after the airplane has landed and the parking brake has been set.
D.  An underwater locating device (ULD) is attached to the voice recorder front panel, although not a part of the voice recorder system. The ULD is a beacon that automatically energizes when the unit is immersed in water, thereby assisting to locate the recorder in case of an accident in the ocean or other body of water.
E.  Operating power for the voice recorder system is 115 volts, 400 Hz ac obtained from the ac bus No. 1 through the VOICE RECORDER circuit breaker on P6.
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H45004
Voice Recorder System Component Location  580 
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

2.  Voice Recorder Control Panel
A.  The voice recorder control panel is located in the pilots' overhead panel P5. It contains a microphone and preamplifier, a meter, a test pushbutton, a headphone jack, and an erase pushbutton. The microphone and its preamplifier pick up the conversation in the flight compartment and supply audio to channel 4 of the voice recorder unit. When the TEST pushbutton is pressed, it activates a test circuit in the recorder unit. If all channels are recording properly, the meter needle indicates in the green area and a 600-Hz test tone is supplied to the HEADSET jack. The ERASE pushbutton will erase the entire tape in the recorder if held for a minimum of two seconds. However, the bulk erase circuit will operate only with the airplane on the ground, power applied to the airplane, and the parking brake set.
3.  Voice Recorder Unit
A.  The voice recorder unit is installed in the aft cargo compartment on the right side of the airplane. It is housed in a thermally insulated, impact resistant, 1/2 ATR short case. All electrical connections are made through a single multipin connector at the rear of the unit. The voice recorder unit contains a tape transport, a 65-kHz bias generator, four recording amplifiers, a monitoring amplifier, a test module, a power supply, a motor jack, a test meter, and five test pushbuttons. In addition to the tape drive mechanism and tape, the tape transport contains the recording heads, the erase head, the monitor head, and the bulk erase coil.
B.  On Fairchild model A100 cockpit voice recorders a meter, monitor jack and five test pushbuttons are located on the front panel of the voice recorder unit. Pushbuttons 1 through 4 are used to check the recording channels separately. The fifth pushbutton (marked ALL) is used to test all four channels in sequence. A 600 Hz tone is supplied to the MONITOR jack while each of the channel pushbuttons is depressed and, if the recorder is operating properly, the meter indicates in the green area. If the ALL pushbutton is pressed, the 600-Hz tone and the meter indications are available for 0.8 seconds at a time as the test module ring counter samples each channel in turn. All four channels are sampled in about 5 seconds. The test stops whenever the pushbutton is released.
C.  On Fairchild A100A model cockpit voice recorders only the monitor jack is located on the front panel. These recorders are meant to be tested from the recorder control panel. All channels can be monitored at the monitor jack.
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4.  Operation
A.  Functional Description (Fig. 2)
(1)  
The voice recorder system automatically records the latest 30 minutes of communications and conversation in the flight compartment. The system is in operation whenever 115-volt ac power is applied to the unswitched electronics bus. Communications and interphone audio from the first observer's, the first officer's, and the captain's audio selector panels is supplied to channels 1 through 3 respectively of the voice recorder unit through the interphone audio accessory box. Conversation in the flight compartment is picked up and amplified by the microphone and preamplifier in the control panel and supplied to channel 4.

(2)  
The 65-kHz bias frequency for all four recording amplifiers is supplied from a common bias generator. This frequency is also used to drive the erase head removing information from the tape before recording.
　
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