Abell TH-308GB UHF, Руководство по обслуживанию

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TH-308GB UHF SERVICE MANUAL
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A necessary signal for QT/DQT encoding is generated by IC2 and FM-modulated to the
PLL reference signal. Since the reference OSC does not mudulate the loop characteristic
frequency or higher, modulation is performed at the VCO side by adjusting the balance.
3
）
VCO and RF amplifier
The transmit signal obtained from the VCO buffer amplifier Q45, is amplified by QY1. This
amplified signal is passed to the power amplifier, QY4 and Q29, which consists of a 2 stage
FET amplifier and is capable of producing up to 3W 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 D17, D19. D1 and D19 turned on
(conductive) in transmit mode and off (isolated) in receive mode.
5）
Terminal protection circuit
When the thermistor(TH2) reaches about 80°C, the protection circuit turns on Q50 to
protect transmitting final amplifier (Q29) from the 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 IC2 CPU operates at 7.37 MHz. This oscillator has a circuit that shifts the frequency
according to the EEPROM data.