Kenwood TK-885, Service Manual

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23
TK-885
Fig. 7 Unlock circuit /
CIRCUIT DESCRIPTION /
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 control unit monitors the PLL (IC300) LD
signal directly. When the PLL is unlocked during transmis-
sion, the PLL LD signal goes low. The CPU detects this sig-
nal and makes the KEY signal low. When the KEY signal goes
low, no voltage is applied to 8T, and no signal is transmitted.
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, Q205). The amplified signal passes through
the transmission/reception selection diode (D209) and goes
to a low-pass filter. The low-pass filter removes unwanted
high-frequency harmonic components, and the resulting sig-
nal is goes the antenna terminal.
Fig. 6 Transmitter system /
IC504 IC3 Q103 IC711 MIC
AF AMP
TA75S01F
MIC KEY
INPUT
AF AMP,
IDC, LPF
TC35453F
IC511
CPU
30620M8
-394GP
SUM AMP
TA75W558FU
X1
VCXO
16.8MHz
VCO
2SK508NV
(K53)
IC300
PLL
SA7025DK
Q106
RF AMP
2SC4226
(R24)
Q300
BUFFER
2SC4215
(Y)
Q202
RF AMP
2SC4093
(R27)
Q204
RF AMP
2SC3357
Q205
ANT
RF AMP
2SC2954
IC400
POWER AMP
M68762SL
Q7
BUFFER
2SC5110
(O)
IC508
SHIFT
REG.
IC511
CPU
Q10
SW
Q11
SW
IC300
PLL
Q13
SW
Q12
SW
LD
CONTROL UNIT
T/R
8C
8R 8T
KEY
PLL lock
: LD "H"
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-
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.