SEA 157S, Предварительное руководство по обслуживанию

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This audio signal is then fed to the CODEC (U8) to be digitized.
The audio output is continuously monitored by the DSP, looking for
the DSC dot pattern.
Voltage Stabilization: U3 provides a stab5Vdc to active
mixer U1 and FM IF system and U2 provides a stab8Vdc for
the local oscillatorQ2 and other voltage sensitive receiver
circuitry.
5.4 TRANSMIT AMPLIFIER CHAIN:
GENERAL: Referring to Sheet 4 of the Mainboard Schematic Diagram,
the transmit amplifier chain of the SEA 157S consists of the
discrete RF amplifiers Q10 and Q9 and a two-stage hybrid RF power
amplifier module U16.
PRE-DRIVERS: The buffered output signal from the frequency
synthesizer is first amplified by Q10 and its output is coupled to
the input of Q9. Q9 further amplifies the signal and applies it to
the input (IN) of the power amplifier module U16. The RF signal
from Q9 is however only available to the input of the power
amplifier module when in the transmit mode and 13Vdc is present on
13V_TX bus to power Q10.
FINAL AMPLIFIER: U16 is a hybrid power amplifier module
containing two gain stages. When the radio is on, 13.6Vdc is
applied to the power amplifier module at all times. The power
amplifier module will however only produce RF power when the
radio is in the transmit mode and the RF signal from Q9 is
available at the module input (IN). The amount of RF output
produced by the power amplifier module is dependent on the level
of bias voltage available to pin 2(PA) of the power amplifier
module and to Q9 via R5, R6, and L5. This bias voltage is
controlled by the amplifier consisting of Q5, Q13 and their
associated components and the amount of control voltage available
at the 1W_25W bus.
5.5 ANTENNA INTERFACE CIRCUITS:
TRANSMIT/RECEIVE SWITCHING: Antenna changeover between transmit
and receive is accomplished by the PIN diode switches D33 on the
Mainboard and D1 on the Receiver Board. In the transmit mode,
voltage is applied to the 13_VTX bus and current passes through
R43, R72, R62, L7, R2, R2A, L1, and D33 on the mainboard and
finally to ground through L1 and D1 on the Receiver board. This
current through D33 causes it to become forward biased and pass RF
power from the power amplifier module to the low-pass filter
consisting of L2, L3, L4, and their associated capacitors. The DC
current flow though D1 also causes it to become forward biased and
short-circuits the input to the receiver. The short-circuit in
addition causes the input impedance of the 1/4 wave matching
section comprised of C1, L1, and, C2 on the Receiver board to
become high and effectively isolates the receiver from the
transmitter RF.
5.5