Abell A720T, Service Manual

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A720T (VHF) Service Manual V1.0
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5.5.4 Voltage-Controlled Oscillator(VCO) and RF Amplifier
Signal comes from VCO buffer amplifier(Q605) will be amplified when through Q606, Q8, and Q10. This
amplified signal go through amplifier and Q3(RF MOSFET DOLULE), the RF power will be 5W if you set radio
as high power, will be 1.5w if your set radio as low power. Check chart 7.
ANT
Q8,Q10
AMP
Q4
RD01MUS2
Q3
PA AMP
D1 D3
ANT SW
LPF
ANT SW
D501 D502
RX
T5V
APC
FC
+7.5V
TH1
Temperature
Q9 Chart 7 Auto Power Control
AMP
RD07MUS2B
U8 NJM2904V
5T
5.5.5 Antenna transfer switch and LPF
Before RF signal reach antenna terminal, RF amplifier output signal , harmonic will be filtered when it going
through low-pass filter then this signal go through transmit/receive switch circuit(made of D1 and D3). D1 and D3
keep open(conductive) in transmitting mode, keep closed(isolation) in receiving mode.
5.5.6 Power Control
Power control circuit can stabilize the transmission output power through detecting drain current of final
amplifier field-effect tube. Voltage comparing circuit, U8 use the reference voltage configured by microprocessor
to compare with the voltage obtained from the last current. The different value of power control voltage and
U8output automatic detection voltage and reference voltage is in direct proportion. The output voltage can
control the power amplifier of field-effect tube, and maintain the constant of output power of transmission part.
The output power of transmission part can be changed by microprocessor, and the reference voltage can be
changed by microprocessor to control output power.
5.5.7 Temperature Protection Circuit
When the thermistor(TH1)reaches about 80
℃
, the protection circuit turns on to protect transmitting final amplifier(Q3)
from the overheating.