Kenwood TK-782 Service Manual page 39

TK-782
" Unlock Circuit
During reception, the TR signal goes high, the KEY signal
goes low, and Q10 turns on. Q11 turns on and a voltage is
applied to the collector (8R). During transmission, the TR
signal goes low, the KEY signal goes high and Q13 turns on.
Q12 turns on and a voltage is applied to 8T.
The CPU in the TX-RX unit (B/2) monitors the PLL (IC300)
LD signal directly. When the PLL is unlocked during trans-
mission, the PLL LD signal goes low. The CPU detects this
signal and makes the KEY signal low. When the KEY signal
goes low, no voltage is applied to 8T, and no signal is trans-
mitted.
"
8R
LD
IC300
PLL
PLL lock
: LD “H”
TX-RX UNIT (B/2)
Fig. 7 Unlock circuit /
38
CIRCUIT DESCRIPTION /
8C
Q11 Q12
SW SW
Q10 Q13
SW SW
TR KEY
IC508
IC511
SHIFT
CPU
REG.
" Power Amplifier Circuit
The transmit output signal from the VCO is amplified to a
specified level of the power module (IC400) by the drive block
(Q203, Q204). The amplified signal passes through the trans-
mission/reception selection diode (D211) and goes to a low-
pass filter. The low-pass filter removes unwanted high-fre-
quency harmonic components, and the resulting signal is
goes the antenna terminal.
"
" APC Circuit
The automatic transmission power control (APC) circuit
detects part of a power module output with a diode (D27,
D30) and applies a voltage to Q21. Q21 compares the APC
control voltage (PC) generated by the D/A converter (IC5) and
DC amplifier (IC6) with the detection output voltage to con-
trol Q19 and Q20, generates DB voltage from B voltage, and
stabilizes transmission output.
Q17 turns the PC signal on or off using 8T so that the
circuit works only during transmission. With stability at low
power in mind, Q29 turns off to optimize the detection volt-
8T
age.
The APC circuit is configured to protect overcurrent of the
power module due to fluctuations of the load at the antenna
end and to stabilize transmission output at voltage and tem-
perature variations.
"
Q203 Q204
RF RF
D15
AMP AMP
8T
(TX : 8V)
IC6 Q17
PC
DC PC
IC5
AMP SW
23pin
TX :
PC SW ON
Fig. 8 APC circuit /
ANT
IC400 D211
POWER ANT
LPF
AMP SW
DB
Q19
D27,30
APC POWER
+B
DRIVER DET
Q20
PRI
DRIVER
Q21 Q29
APC H/L
CONTROL SW
VR1
H/L
IC7 5pin
25W (High power)
: “H”