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The audio processor sends the Morse code station identifier to the Morse decoder circuits. when there is an identifier, the Morse decoder sends it to the ARINC 429 transmitter. The identifier replaces the frequency on the NDs.
The receiver circuits send the bearing part of the RF signal to the bearing computer circuit. The bearing circuit compares the inputs of the loop and sense antenna elements to calculate bearing to the ADF station.
Bearing data goes to an ARINC 429 transmitter that has two outputs. For the left ADF receiver, output 1 goes to the left AIMS cabinet and output 2 goes to the right AIMS cabinet. For the right ADF receiver, output 1 goes to the right AIMS cabinet and output 2 goes to the left AIMS cabinet.
If you select the antenna (ANT) mode, the processor sends a signal to the receiver circuit that removes the inputs from the loop antenna elements. In the ANT mode, the receiver circuits use inputs only from the sense antenna.
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ADF SYSTEM -FUNCTIONAL DESCRIPTION

If you select the beat frequency oscillator (BFO) mode, the processor sends a BFO mode signal to the audio processor to enable a 1020 Hz oscillator. The oscillator permits the audio processor to identify the keyed morse code identifier.
Built-in-Test Equipment
The ADF receiver contains BITE that monitors ADF receiver operation and signal inputs. A fault memory keeps failures that occur by flight legs. A discrete from the AIMS cabinets sets the flight legs. If the input discrete from the AIMS cabinet fails, the ADF receiver uses the air/ground discrete from the air/ground relay to set the flight legs.
The central maintenance computer function (CMCF) in the left AIMS cabinet sends test signals to the ADF receivers. The processor sends test signals to the antenna control circuit and to a function test RF coupler.
Test
During a test, the antenna control removes power to the antenna amplifiers of the ADF antenna so that no RF signals go to the receiver circuits. A test RF frequency replaces the antenna RF input and the receiver sends the signal through its internal circuits. Test results show on the navigation displays.
Program Pin Inputs
The ADF receivers get discretes from program pin selections.
The bearing computer circuit uses quadrantal error correction (QEC) program pin inputs to adjust for signal distortion caused by airplane structures.
The source destination indicator (SDI) discrete identifies the ADF receiver as the left or right receiver.
A discrete identifies the antenna location as a top mounted antenna.
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777 AIRCRAFT MAINTENANCE MANUAL

ADF SYSTEM -FUNCTIONAL DESCRIPTION

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ADF SYSTEM -TUNING INPUTS

General
These are the two ways to tune the ADF receivers:
*
Manually by the left, center, or right CDU

*
By the CDUs as an alternate tune source.

Manual Tuning
You manually tune the ADF receivers from the left or right control display units (CDUs). The center CDU can tune the recievers if the left or right CDU fails. Manual tune inputs from the CDUs go through the left and right systems ARINC 629 buses, then to the left and right AIMS cabinets. Manual tune inputs from the left or right AIMS cabinet goes from the right AIMS cabinet to the ADF receivers.
Alternate Tune
The CDUs monitor the AIMS cabinets tune functions. IF the AIMS tune function fails, the CDUs send a frequency source select discrete to the ADF receivers that changes the inputs from the right AIMS cabinet, to the input that comes directly from the left or right CDU. The CDUs use 28v dc from a pull-up resistor to set the discrete.
In alternate tune mode, the left CDU alternate tunes the left ADF receivers and the right CDU alternate tunes the right ADF receiver. The center CDU has no alternate tune function.
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777 AIRCRAFT MAINTENANCE MANUAL

ADF SYSTEM -TUNING INPUTS

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ADF SYSTEM -AUDIO CONTROLS

General
The audio control panels supply control inputs to the audio management unit. This permits the crew to hear audio signals on the speakers and headsets in the flight deck.
Description
These are the controls on the audio control panel that let you to hear ADF audio:
*
VOR/ADF receiver selector

*
VOR/ADF receiver volume control

　
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