Service manual

CIRCUIT DESCRIPTION
6-9
August 1993
Part No. 001-9750-001
Pin 10 of U200 is the output of an internal RSSI
(receive signal strength indicator) circuit. It provides a
high-impedance current sink that increases in conduc-
tion in proportion to increases in the IF signal level.
The result is that the voltage on pin 10 decreases as the
IF signal strength increases and vice versa. This signal
is buffered by U201B and then fed to comparator
U602C on the audio/logic board. The microprocessor
uses this indication and the noise squelch indication to
control receiver muting and data detection.
6.3.5 AUDIO POWER AMPLIFIER (U100)
The audio signal from the front panel volume
control (front mount) or digital volume control U600
on the audio/logic board (remote mount) is applied to
amplifier U100. This is a bridge amplifier which
provides up to 12 watts of power into a 4-ohm speaker.
CR100, R109, R110, and C115 provide the biasing
voltage, and the output is stabilized by R107/C118 and
R108/C119. When an external speaker is used, it is
connected by a wire harness to J101, pins 1 and 2.
When the internal 16-ohm speaker is used, a jumper is
installed between pins 1 and 3 of J101 and the speaker
is connected to J100. R120 limits power to this
speaker to approximately 5 watts to prevent damage to
the speaker.
6.3.6 13.8-VOLT SWITCH (Q100, Q103, Q106-
Q108)
The 13.8-volt switch circuit is controlled by the
front panel or remote control unit power switch. When
power is turned on, the power switch grounds the
emitter of Q402 on the audio/logic board. Then if this
transistor is turned on by the ignition sense line or the
4Q output of latch U409, a low voltage is applied to
the base of Q103 on the RF board. The 13.8-volt
switch circuit is then turned on and approximately 13
volts appear on the collector of series-pass transistor
Q100.
The 0.8-volt drop of this circuit provides about 30
dB attenuation of noise pulses less than 1.6-volt P-P
that are present in the 13.8-volt supply from the
vehicle. This lessens the amount of vehicle noise in
circuits powered by the 13.8-volt supply such as audio
amplifier U100.
6.4 TRANSMITTER CIRCUIT DESCRIPTION
NOTE: RF amplifier Q290 is described in Section
6.3.1.
6.4.1 POWER AMPLIFIER MODULE (U500)
The RF input signal to the transmitter on J501,
pin 18, is at a level of approximately +10 dBm. This
signal is applied to a 11 dB pad formed by R506-
R508 which sets the correct input level to U500.
Power amplifier module U500 amplifies this signal to
approximately 6 watts. VS1, VCTRL, and VS2 are
supply inputs to separate amplifier stages in U500.
Controlling the voltage on the VCTRL input controls
the power output of the transmitter. Various capacitors
on these supply inputs isolate the supply from various
AC signals present in U500.
6.4.2 DRIVER (Q500), FINAL AMPLIFIER (Q501)
Driver Q500 amplifies a 6-watt input signal to
approximately 20 watts. C558, C559, and C519
provide impedance matching with U500. Inductor
L500 provides self biasing of Q500. Supply current to
Q500 flows through R587, and the voltage drop across
this resistor is monitored by U504B in the power
control circuit (see Section 6.4.4). L501 is an RF
choke, and C517, C518, and C555 isolate the supply
voltage from AC signals present in Q500.
If the transceiver is a 15-watt model, the output
signal from Q500 is applied to the directional coupler
and Q501 is not used. If it is a 30/35-watt model, final
amplifier Q501 amplifies the Q500 output signal to
approximately 35/40 watts. Impedance matching
between Q500 and Q501 is provided by L501, C539,
and three sections of microstrip. L503 provides self
biasing of Q501. Capacitors and a section of micros-
trip on the collector of Q501 provide impedance
matching with the directional coupler.
6.4.3 ANTENNA SWITCH (CR502, CR506,
CR507)
The output signal of Q501 is fed through the
directional coupler (next section) and then applied to
an antenna switching network. This network connects
the antenna to the receiver in the receive mode and the
transmitter in the transmit mode.