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 C.  To receive: the mode switch, on the HF control panel, is switched to USB, LSB, or AM mode and the desired frequency is selected. The HF system must be selected on the audio selector panel. The RF SENS and VOL (volume) knobs, on the HF control and audio select panels, may be adjusted to obtain a comfortable listening level.
 D.  To transmit: the HF system microphone selector switch, on the audio selector panel, must be pressed. Pressing the microphone PTT (push-to-talk) switch operates control circuits in the HF transceiver which disconnects the receiver circuits and connects the transmitter circuits to the antenna. Transmission begins when audio signals are present on the microphone circuits. Sidetone is returned to the interphone system so that audio being used to modulate the transmitter can be heard.
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 ALL  ú ú 09 Page 7 ú Nov 12/01
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
 E. Collins 628T-2 HF Transceiver (Fig. 2)
 (1)  
The transmitter output is nominally 400 watts PEP (peak effective power) with a maximum average power of 125 watts. The HF transceiver has a dual-conversion receiver that produces 100 mw received voice audio output, 100 mw of sidetone output and 500 mw of SELCAL audio. Operating voltages are produced by internal power supplies that operate from a 3-phase, 115-volt, 400 Hz primary source.

 (2)  
Power Control

 (a)  Turning the HF control panel's mode switch from OFF energizes the ON/OFF relay; which provides 3-phase 115v ac to the power supply. Phase C operates the relay driver and the blower which provides cooling air during transmit operations.

 (3)  
Frequency and Mode Selection

 (a)  
The operating mode and frequency are selected on the HF control panel and supplied to the HF transceiver. This parallel data is serialized then sent to the tuning control circuits where it is converted into bcd (binary coded decimal) data. The bcd data is used to tune the frequency synthesizer. The USB, LSB, or AM modes of operation enables either the 69.3 or 70.3 MHz injection oscillator, respectively.

 (b)  
Within the frequency synthesizer, the bcd frequency data is applied to variable dividers in a dual phase-locked loop. The 71.8- to 99.7999-MHz output signal from the vco loop is the local oscillator signal to a mixer in the receiver/exciter.

 (c)  
When a change of operating frequency is selected on the HF control panel a rechannel signal is sent to the HF antenna coupler. This signal causes the HF antenna coupler to return to its home position. Before the HF transceiver can operate on this new frequency, the HF antenna must be tuned.

 

 (4)  
Transmit Mode

 (a)  
In transmit mode, the HF transceiver will produce nominally 400 watts PEP RF output in SSB operations and a nominal 125 watts average RF output in AM operation.

 (b)  
Pressing the microphone key or PTT switch initiates tuning of the HF antenna coupler and activates the transmitter circuits in the HF transceiver.

 (c)  
During tuning, the HF antenna coupler supplies a TIP (tune-in-progress) signal to the HF transceiver and holds the key line down until tuning is complete. Within the HF transceiver the TIP signal produces the following results: the transmitter is placed in the AM mode with low-power output (72 watts), and the tune tone oscillator produces a 1000-Hz modulating audio tone. When tuning is complete, the TIP signal is removed from the HF transceiver, the HF antenna coupler unkeys the transmitter, and HF transceiver returns to the receive mode. With the HF antenna coupler tuned to the selected operating frequency, the HF transceiver is then ready for transmission.

 

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 23-11-00
 ALL  ú ú 07 Page 8 ú Mar 15/92
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
T/R RELAYAM MECHRF
AMP COAX
UATOR INTERNALKEY LINE
FILTER  ATTEN-
AM AGC RF IF AGC
AGC
F5 RF SENS
PA ALC DIR.
RECEIVER/ ATTEN-WATTMETER 20V DC
EXCITER
UATOR ALC CONTROL INPUT FAIL IND
　
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