Communications - Radios
- Boeing - Stuck Microphone Transmit Switch
Radio Communication System
The Radio Communication System consist of:
HF Communication System
As installed
One or two independent HF radios are installed. These are designated HF L (left) and - as installed - HF R (right). The ACPs are used to control voice transmission and receiver monitoring.
The HF radio(s) are tuned on a dedicated control panel.
To tune the HF radio, rotate the frequency selectors on the control panel. The left knob changes the digits to the left of the decimal point, and the right knob changes the digits to the right of the decimal point.
The HF radio sensitivity can be adjusted by rotating the RF SENS control knob. Rotating clockwise increases the sensitivity to receive weak or distant stations. Rotating counter-clockwise decreases sensitivity to decrease noise and static.
Note: Decreasing sensitivity too far prevents reception, including SELCAL monitoring of the HF radio.
As installed
Both HF radios use a common antenna. When either HF radio is transmitting, the antenna is disconnected from the other HF radio, and it cannot be used to transmit or receive. Thus, both radios can be tuned to the same frequency without interfering with each other. However, both HF radios can receive simultaneously if neither is being used for transmitting.
When an HF transmitter is keyed after a frequency change, the Antenna Coupler tunes. While the antenna is being tuned, a 1000 Hz coupler tone an be heard through the audio system (tuning takes a maximum of 15 seconds).
VHF Communication System
As installed
Two or three independent VHF radios are installed, designated VHF L (left), VHF R (right) and - as installed - VHF C (centre). The ACPs are used to control voice transmission and receiver monitoring.
The VHF radios are tuned on:
- a dedicated control panel for each radio (-200)
- the Radio Tuning Panel (-300)
Each VHF radio allows the tuning of two independent frequencies, an active and a standby frequency. These can be interchanged with the Frequency Transfer Switch.
Radio Tuning Panels (757-300)
The Radio Tuning Panels (RTPs) are used to tune the VHF and HF radios. The panels are designated left, centre and right, and are normally associated with the respective VHF and HF radios. Any radio can be controlled by any RTP. Normally, the right RTP is used for tuning the right VHF or HF radio, the left RTP is used for tuning the left VHF or HF radio and the centre RTP is used for tuning the centre VHF radio. This is called 'on-side' tuning.
An off-side tuning light on each RTP indicates that a communication radio normally associated with that RTP has been selected by another RTP, or that RTP is selected to a radio normally associated with another RTP.
Each HF radio can be tuned by any Radio Tuning Panel. However, HF radio sensitivity can only be set on the on-side Radio Tuning Panel.
