(d)  
The PTT signal is routed out of the VHF transceiver as a key event signal. This PTT signal is used by the flight data recorder system to record the time(s) during which the VHF transceiver is keyed.

GUN 051; Receive:

 (a)  
With no transmit dc to the antenna switch, the incoming signals are passed to the receiver rf section. Here they are amplified and mixed with a local oscillator signal from the synthesizer to produce the IF signal which is further amplified and the amplitude modulation detected. The detector's output is routed directly (not through the squelch) to the SELCAL system, and to the squelch circuit. When the incoming signal is strong enough to provide a sufficient agc signal, the squelch is turned on, gating the detected audio through to the output audio amplifier, and on to the flight interphone system and to the transceiver front panel jack.

GUN 001-050,052-999; Receive:

 (a)  
When the transceiver is not keyed, the output from the frequency synthesizer is switched to the receiver mixer and the antenna switch connects the antenna to the receiver input.

 (b)  
With no signal input, the receiver noise output from the detector is filtered and compared to a squelch threshold voltage in the squelch comparator. The comparator output turns off the voice audio output to the interphone systems.

 (c)  
When a signal is received, it is amplified in the rf section and fed to the first mixer. Also in the rf section there is a negative feedback to the variable attenuator to prevent overloading the rf amplifier when there are excessively strong signals. In the first mixer, the frequency synthesizer output is beat with the incoming signal to produce a 20.025 MHz IF. The IF is filtered, amplified and mixed with 9.325 MHz to produce a third IF of 10.7 MHz which is filtered, amplified and detected. Now the detector output is strong and the squelch comparator will gate the audio output through to the voice output amplifier.

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 ALL  ú ú 05 Page 9 ú Mar 20/96
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
 B.

C.

D.

A
757
MAINTENANCE MANUAL

 (d) The data output is not squelch controlled and supplies tone
 sets to SELCAL directly, without filtering or gating.GUN 051; Self-Test
_________
 (1)  The VHF transceiver has a self-test, including breaking the squelch, which is initiated by pushing the front panel SQ/TEST switch. The microprocessor performs an analysis to isolate faults to the module level, and displays the number of a faulty module on an internal hexidecimal display (used for component maintenance only). In addition, all the front panel lights illuminate for 3 seconds (for a lamp test), and extinguish for 3 seconds while the self-test is in progress. If there are no faults detected, or in-flight faults registered, the green COMM light illuminates for 3 seconds and then extinguishes. If there is a fault detected, or in-flight faults registered, the appropriate failure light illuminates for 3 seconds and then extinguishes. The red COMM light comes on for a transceiver fault, and the red DATA IN light comes on for a frequency tuning data input failure.
GUN 001-050,052-999; Self-Test
_________
 (1)  
The transceiver has a self-test system that is initiated by pushing the test switches on the front panel of the transceiver. The SQL/LAMP TEST switch is used to disable transceiver squelch, and to check the front panel lights prior to system self-testing. The red CONTROL INPUT FAIL and green LRU PASS lights come on when the SQL/LAMP TEST switch is pushed.

 (2)  
The TEST switch is pushed to initiate the CONTROL INPUT FAIL and LRU PASS tests. The CONTROL INPUT FAIL test checks that the transceiver is receiving valid frequency tuning data from the control panel. If the tuning data is not valid, the red CONTROL INPUT FAIL light comes on indicating a control panel failure. The light remains on for about four seconds after TEST is pushed.

 (3)  
The LRU PASS test is initiated at the same time as the CONTROL INPUT FAIL test. The LRU PASS test identifies a failure in the transceiver. After pushing the TEST switch, the green LRU PASS light should come on for about four seconds. At the same time, the display should show a value of less than 3.0. This value is the voltage standing wave ratio (VSWR). If the LRU PASS light fails to come on, a problem may exist in the transceiver.

 (4)  
After the CONTROL INPUT FAIL/LRU PASS tests are completed, turning the RFL/OFF/FWD (power test) switch will cause power values to show on the display. When the RFL/OFF/FWD switch is in the FWD position, a minimum value of 25 for forward power should show on the display. When the RFL/OFF/FWD switch is in the RFL position, the display should show 25 percent or less of the forward power value. This is the reflected power.
　
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