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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.

(3)  
The bias frequency is used in an audio frequency tape recorder to shift the audio band upward to a frequency range suitable for operation of ac amplifiers and inductive recording and playback heads. The bias frequency is introduced in the recording amplifier and removed in the monitor amplifier. Each recording amplifier drives an inductive (electromagnetic) recording head which produces a varying magnetic field. As the tape transport moves the tape past the recording head at a constant speed, the varying magnetic field produced by the head magnetizes the magnetic oxide film on the tape producing a magnetic track of varying strength. As the magnetic track moves past the playback head, it induces a small varying current in the coil of the head. The monitor amplifier amplifies this signal, removes the recording bias frequency, and furnishes the audio to the recorder output.

(4)  
The playback head and monitor amplifier are used to monitor information recorded on all tracks of the tape. The audio output of the monitor amplifier is available at the MONITOR jack on the front panel of the voice recorder unit and at the HEADSET jack on the voice recorder control panel. This audio is also used to drive test meters.

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H34493
580  Voice Recorder System Block Diagram 
Oct 20/84  Figure 2  23-71-0 
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BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 

(5)  
On Fairchild model A100 voice recorders, pressing one of the channel test pushbuttons on the voice recorder unit causes the test module to provide a 600-Hz frequency to the recording channel. The channel is then monitored. If it is functioning properly, the meters read in the green area and the 600-Hz tone is present at the jacks. Pressing the ALL test pushbutton on the voice recorder unit or the TEST pushbutton on the voice recorder control panel causes the test module to apply the 600-Hz frequency to each channel in sequence thereby testing all channels.
　
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