Kenwood TKR-851 Service Manual page 13

3-3. Driver and Final Power Amplifier Circuit
The transmit signal is generated by the TX VCO (A2), am-
plified by Q11, and sent to final unit (X45-374). This amplified
signal is amplified by Q1, Q2, Q3 and Q4, and is passed to the
FINAL stage. The RF power amplifier consists of MOS FET.
TX-RX unit (X57-696 A/2)
STT
MO
TX VCO
A2
(X58-481)
FCT
BUFF TCXO
MB
IC1 X3
16.8MHz
3-4. Automatic Power Control, Circuit and Transmitter
The automatic power control (APC) circuit stabilizes the
transmitter output power at a pre-determined level, and con-
sists of forward/reverse power detector circuits, and switch-
ing transistor Q8. The forward/reverse power detector cir-
cuits detects forward RF power and reverse RF power to DC
voltage, and consists of a CM coupling type detection circuit
formed by a strip line, RF detector D4/D5, and DC amplifier
IC3 (A/2).
The voltage comparator (IC3 B/2) compares the above de-
tected voltage with a reference voltage, set using the micro-
processor and IC6 located in the TX-RX unit. An APC voltage
proportional to the difference between the sensed voltage
and the reference voltage appears at the output of IC3. This
output voltage controls the gate voltage for the drive amplifi-
ers Q3 and Q4 and final amplifier Q5, which keeps the trans-
mitter output power constant.
3-5. RF Power Down Detect
When the transmitter output power has dropped to or be-
low the preset values (High Power : 10 W, Low Power : 0.4
W) in "RF Power Down Detect" adjustment, IC604 (CPU :
X57-696) detects the drop in power and outputs a Low-level
(default) signal to the AUX out port.
3-6. Fan Action Control Circuit
If fan action is set to "Temperature", the cooling fan is
turned ON or OFF according to temperature.
If the ambient temperature of the final unit exceeds
approx. 35°C, the output from the temperature detection IC1
changes from H to L. This signal is sent to the CPU (IC604)
by Q602 in the TX-RX unit B/2 (control section), and the out-
put from pin 13 (FAS) of IC7 in the TX-RX unit A/2 controlled
by the CPU goes H.
CIRCUIT DESCRIPTION
Final unit (X45-374 A/2)
Q1
Drive
AMP
Q11
ATT
Fig. 3 Driver and final power amplifier circuit
Q3 Q4 Q5
Drive Drive Final
AMP AMP AMP
TEMP
Q2
Vcont
Drive
AMP
IC3
DC
SW
AMP
POWER ADJ
REF
PC
AMP
IC3 VR1
This signal turns Q9 in the final unit ON to run the cooling
fan. It has a hysteresis of approx. 2°C by IC1. If fan action is
set to "Continuous", the fan operates continuously, but Q9
stays ON.
X45 A/2
FAN
CN4
FAB
+B
FAG
IC1
[Fan operation]
OFF
+20°C +30°C +35°C +40°C
Fig. 4 Fan action control circuit
TKR-851
CM COUPLER
D4 D5
REV
FWD
DET
DET
Q8
PDT
(to IC604 CPU)
X57 A/2
IC7
Q9
FAS
13
Q6
CK,
DT,
STB
TMP
Q602
ON
+50°C
TX ANT
X57 B/2
IC604
CPU
TMP
17
13