Transmit Audio - Motorola ASTRO XTL 1500 Detailed Service Manual

Theory of Operation: Controller Section
The CODEC D/A analog output signal is routed to the RX_FILT_AUDIO line at both J0401 and
J0402/J2 for legacy accessories and special applications products at an amplitude of 100 mV per
kHz of deviation. The signal amplitude is independent of volume setting. This output is also routed to
a multiplex switch, U0210, which is one source of audio to the audio PA speaker output. The
alternate PA source, AUX_RX, is an input from J0401 and is used with vehicular repeater systems
(VRS). The multiplexer output, normally received speaker audio, is routed to a volume control digital
programmable potentiometer, U0206, and then to the audio PA (U0204) input. The audio PA output
then drives the external speaker.
The audio power amplifier (U0204), is a DC-coupled-output, bridge-type amplifier. The gain is
internally fixed at 40 dB. Speaker audio leaves U0204 on pins 4 and 6. For dash-mount radios, the
audio is routed to the speaker via J0402, pins 19, 21, 23, and 25, and then to J2, pins 20 and 26. The
amplifier is biased to one-half of the A+ voltage. An audio isolation transformer must be used if
grounded test equipment, such as an audio analyzer or service monitor, is to be connected to the
speaker outputs.
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C a u t i o n
Normally, R0220 pulls up U0204, pin 8, to SW_B+ to enable the audio PA. When the radio is
squelched, the audio PA is muted by the microprocessor. Q0200 is enabled to provide approximately
4.5 V through a voltage divider (Q0200 and R0222) to U0204, pin 8, which mutes the audio PA.
When SW_B+ is turned off, the voltage from SW_A+ on U0204 falls below 2 V, placing the audio PA
in standby, which turns off the U0204 output transistors to the speaker.
voltages present at U0204, pin 8, during its various conditions.

3.14.10Transmit Audio

The mobile microphone connects to the front of the control head through connector P104.
Microphone audio (MIC_HI) enters the main board via J0401, pin 4, and is routed to multiplexer
U0209 (refer to
for the microphone's internal circuitry.
6815854H01-A
The speaker outputs must NOT be grounded in any way.
Table 3-13. PA Condition Voltages at U0204, Pin 8
Power Amplifier
Condition
Standby
Mute
Enabled
Figure 3-49 on page 3-74). Resistors R0200 and R0204 provide 9.6 Vdc bias voltage
Table 3-13
U0204 Pin 8
(Vdc)
0-2
3.3-6.4
8.5-17
3-73
shows the
June 15, 2005