Optional Pa/Speaker Volume Control; Mobile Microphone Selection And Bias - Motorola Astro XTS Instruction Manual

Optional PA/Speaker
Volume Control
Mobile Microphone
Selection and Bias
Data connection to the XTVA is made through a data cable connected
to DB25 connector J5. Once the data device is connected and turned
on, the data transfer operation is started. During data transfer
operation, the voice mode will be paused in the portable radio. Four
RS232 signals originate in the radio, and four others are produced by
the XTVA microprocessor, based on radio SB9600 signals.
Although the external speaker and associated audio cable are optional,
the supporting circuits are standard and part of every XTVA's printed-
circuit board (PCB) circuitry. A loop between pins 3 and 6 on jack J1 is
used by the XTVA to detect the presence of a speaker. When a short is
detected across these pins, the XTVA signals the radio to mute its
speaker and disable its volume knob. The volume control and all XTVA
audio circuits are powered by switched audio A+, which is active
whenever the radio is turned on. Amplifier circuits U4-2, -3, and -1
process the radio's audio output for use as the PA/speaker's input signal.
The radio's output is balanced. Therefore, amplifier stage U4-2
unbalances the signal and minimizes squelch transients. When audio
is squelched, the radio output has a zero Vdc level and when
unsquelched, it has a Vbatt/2Vdc level + audio. The PA/speaker has a
single-ended, capacitively-coupled, low input (2kW) impedance. The
buffer stage (U4-1) accepts signal from U4-2 for drive voltage to the PA/
Input. The PA/speaker drive signal is muted unless receiving.
The XTVA's volume control is:
• (rev. "A" consoles) in line with the audio signal path, or
• (rev. "B" or higher consoles) connected between regulated 5V
supply and microprocessor U5 input pin 49.
In rev. "A" consoles the volume control trims the audio signal level. In
rev. "B" or higher consoles, the volume control sends position
indication to U5 by a dc voltage. The dc voltage is converted to an
SB9600 message by U5, then sent to the radio to adjust the audio
signal level.
In the radio standby mode, U4-3 pin 8 is high and turns on transistors
Q7 and Q24, which mutes the audio input. When a received message
unsquelches the radio, the signal at U4-3 pin 8 goes low, which turns
off the transistors.
Resistors R47 and R14, and capacitor C94 form a muting delay circuit.
The delaying action in this circuit prevents any unsquelch dc voltage
transient originating in the radio from causing a "pop" sound in the
PA/speaker.
Microphone connection to the XTVA is made via the console
connector jack, J4, which accepts a standard- or enhanced-type
microphone.
The HHCH microphone has an internal microprocessor that uses
serial-data communication to the radio. The data path is put into use
by automatic switching of signal routing, performed by Q9 and U2.
The HHCH microphone draws supply current through R85, causing
Q9 to conduct and U2 to switch signal paths.
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