(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 - Unpublished Work - See title page for details.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.

(6)  
On Fairchild model A100A voice recorders, only the TEST Pushbutton on the voice recorder control panel is provided.

(7)  
The bulk erase module contains timing circuits, relays, and erase circuits which apply 115 volts, 400 Hz ac to the erase coil in the tape transport. The bulk erase circuits erase all audio on the recorder tape within 10 seconds after the ERASE switch on the control panel is released. The bulk erase circuits will function only when the airplane is on the ground and the parking brake is set because the control circuit must be completed through relay K4 in landing gear accessory unit module M338.

B.  Control
(1)  
The voice recorder system operates automatically whenever power is applied to the unswitched electronics bus.

(2)  
To erase all data recorded on the tape, proceed as follows:

NOTE:  The tape can be erased only with the airplane on the ground and the parking brake set.
(a)  
Press ERASE pushbutton on voice recorder control panel and hold for at least 2 seconds.

(b)  
Release ERASE pushbutton.

NOTE:  Operation of the erase circuits may be monitored by plugging a headset into the jack on the control panel. After the erase pushbutton is released, a 400-Hz tone will be heard for 5 to 10 seconds.
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VOICE RECORDER SYSTEM - TROUBLE SHOOTING
1. General
A.  The following trouble shooting procedures are based on performance of the system operational test and are presented in a tree-type format to aid in rapid fault isolation.
B.  When a test step does not check out, find the box containing the trouble symptom and perform the stated action. Continue to follow a single line by analyzing the results of each test step until the required corrective action is determined. Perform the specified corrective action, then repeat the step at which the failure was encountered and complete the test to check out the system.
C.  All trouble shooting procedures are based on the assumption that wiring is OK and that electrical power is available. If the corrective action in the procedure does not correct the problem, check wiring using the wiring diagram.
2.  Prepare for Trouble Shooting
A.  Provide ground electrical power to airplane.
B.  Ensure that voice recorder and flight interphone circuit breakers on P6 are closed.
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