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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
757
MAINTENANCE MANUAL
3. Operation (Fig. 2)
_________
A. Functional Description
______________________
(1) General
(a)
IN DUAL HF INSTALLATIONS; the left and right HF systems each function as independent systems with the single antenna as the only common component. The functional signal flow is similar for the left and right systems including the X-Y crossconnections between the antenna couplers. The antenna coupler connections to the single antenna assembly are also similar.
(b)
Each HF system has a 115v ac three-phase circuit breaker on P11. One phase of the 115v ac provides power directly to the control panel. The mode control switch on the control panel operates the power relay in the transceiver.
(c)
The 115v ac is applied to the transceiver power supply, the transceiver blower motor and the antenna coupler which has its own power supply. The transceiver power supply provides +5v dc, +28v dc and the other voltages required by the transmitter and receiver circuitry. The +28v dc is also provided to the antenna coupler.
(d)
The blower motor runs all the time that the transceiver is powered.
(e)
The frequency selector knobs on the control panel are connected to the frequency display and to the binary encoders. Parallel serial shift registers process the frequency and mode data to generate the 32-bit digital word with the HF comm address label in ARINC 429 low-speed format.
(f)
The HF comm 32-bit word is received at the port B decoder of the transceiver. The decoder monitors the input word for proper address, parity, sign status matrix, and interval. The control circuits tune the transceiver to the desired mode and frequency via the synthesizer.
(g)
GUN 001-050, 052-999; The operating frequency band of the transceiver is between
2.800 MHz and 23.999 MHz. Whenever the frequency selector knobs are selected below 2.800 MHz or above 23.999 MHz the HF system will not function. If an out-of-band frequency is selected, a 1 kHz tone will be heard through the headset when transmission is attempted.
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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757
MAINTENANCE MANUAL
SELECTOR PANELS
HF TRANSCEIVER (AUDIO OUTPUTS
1 AIRPLANES WITH DUAL HF INSTALLED
FROM TRANSCEIVER)
HF Communications System Schematic (Typical)
Figure 2
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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757
MAINTENANCE MANUAL
(h)
GUN 051; The operating frequency band of the transceiver is between
2.000 MHz and 29.999 MHz. Whenever the frequency selector knobs are selected below 2.000 MHz or above 29.999 MHz the HF system will not function. If an out-of-band frequency is selected, a 1 kHz tone will be heard through the headset when transmission is attempted.
(i)
Whenever a new frequency is selected at the control panel, the control circuits in the transceiver output a "rechannel" pulse to the antenna coupler. The rechannel input causes the antenna coupler tuning elements to go to the "home" position. Tuning of the antenna coupler to the selected frequency is initiated by a PTT input to the antenna coupler. While the antenna coupler elements are tuning, the transceiver transmits a low wattage AM output. The "tune-in-progress" 1 kHz tone is the sidetone output. When the antenna coupler tuning is complete, the transceiver returns to the receive mode.
NOTE: A continuous or pulsed tone indicates that the coupler
____ is tuning to a new frequency. The coupler tune tone will sound no longer than 15 seconds. The average coupler tune time is approximately 1 to 7 seconds.
NOTE: Some coupler types are able to tune quickly when
____ previously used frequencies are selected, in which case the tune tone may be only a momentary beep.
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