Abell TH308G, Руководство по обслуживанию

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ABELL TH-308G VHF Service Manual
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1
）
Transmit audio
The modulation signal from the microphone is amplified by IC100A, passes through a
preemphasis circuit, and amplified by the other IC100B to perform IDC operation. The
signal then passes through a low-pass filter (splatter filter) (Q101 and Q100) and cuts
3kHz and higher frequencies. The resulting signal goes to the VCO through the VCO
modulation terminal for direct FM modulation.
2
）
QT encoder
A necessary signal for QT encoding is generated by IC302 and adjusted by
FM-modulated to the PLL reference signal.
3
）
VCO and RF amplifier
The transmit signal obtained from the VCO buffer amplifier Q205, is amplified by Q206
and Q405. This amplified signal is passed to the power amplifier, Q404 and Q403,
which consists of a 2 stage FET amplifier and is capable of producing up to 3.5W of RF
power.
4
）
ANT switch and LPF
The RF amplifier output signal is passed through a low-pass filter network and a
transmit /receive switching circuit before it is passed to the antenna terminal. The
transmit /receive switching circuit is comprised of D400, D501 and D500. D501 and
D500 turned on (conductive) in transmit mode and off (isolated) in receive mode.
5
）
ATC
when the temperature of thermistor(TH400) reaches to 80 , the protection Circuit
℃
turns on the Q409 to protect the final amplifier(Q403) from over heating.
5
．
Power supply
A 5V reference power supply [5V] for the control circuit is derived from an internal
battery. This reference is used to provide a 5V supply in transmit mode [TXC], a 5V
supply in receive mode [RXC], and a 5V supply common in both modes [PLC] based
on the control signal sent from the microprocessor.
6
．
Control system
The IC302 CPU operates at 18.0178 MHz.