Kenwood TM-271A Service Manual page 4

TM-271A/271E
■ Unlock Circuit
During reception, the 8RC signal goes high, the 8TC signal
goes low, and Q34 turns on. Q33 turns on and a voltage is
applied to 8R. During transmission, the 8RC signal goes low,
the 8TC signal goes high and Q36 turns on. Q35 turns on and
a voltage is applied to 8T.
The CPU monitors the PLL (IC401) LD signal directly.
When the PLL is unlocked during transmission, the PLL LD
signal goes low. The CPU detects this signal and makes the
8TC signal low. When the 8TC signal goes low, no voltage is
applied to 8T, and no signal is transmitted. (See Fig. 7.)
Q33
8R
SW
Q34
SW
8RC
Fig. 7 Unlock circuit
MIC IC201
MIC/IDC
MIC KEY
INPUT
IC161
D/A
CONVERTER
X401
TCXO
16.8MHz
4
CIRCUIT DESCRIPTION
8C
Q35
8T
SW
Q36
SW
8TC
LD
IC101 IC401
CPU PLL
PLL lock
: LD "H"
IC202 IC203 IC161
SPLATTER SUM
AMP CONVERTER
FILTER
IC101 IC161
D/A
CPU
CONVERTER
Q406 Q410
VCO
BUFFER
Q404
PLL RF AMP
IC401
Fig. 8 Transmitter system
Q501 Q502
PRE
RF
D409 DRIVE
AMP
AMP
IC203
(PC)
DC
IC161
AMP
(PC/TVO)
Transmitter System
■ Outline
The transmitter circuit produces and amplifies the desired
frequency directly. It FM-modulates the carrier signal by
means of a varicap diode.
■ Power Amplifier Circuit
The transmit output signal from the VCO passes through
the transmission/reception selection diode (D409) and ampli-
fied by Q501, Q502 and Q503. The amplified signal goes to
the final amplifier (Q504) through a low-pass filter. The low-
pass filter removes unwanted high-frequency harmonic com-
ponents, and the resulting signal is transmitted through the
antenna terminal. (See Fig. 8.)
■ APC Circuit
The automatic transmission power control (APC) circuit
detects part of a final amplifier output with a diode (D606,
D607) and applies a voltage to IC501. IC501 compares the
APC control voltage (PC) generated by the D/A converter
(IC161) and DC amplifier (IC203) with the detection output
voltage. IC501 generates the voltage to control Q503 and
Q504 and stabilizes transmission output.
The APC circuit is configured to protect over current of
Q503 and Q504 due to fluctuations of the load at the antenna
end and to stabilize transmission output at voltage and tem-
perature variations. (See Fig. 9.)
IC203
D/A
BUFFER
IC402 Q411 Q501
1/2
RF AMP RF AMP
DIVIDER
D602,D603
Q503 Q504 D605
ANT
DRIVE FINAL
AMP AMP
IC501
VR601
APC
CONTROL
Fig. 9 APC circuit
Q502 Q503
PRE DRIVE
DRIVE AMP AMP
ANT
LPF
SW
POWER
D606
DET
D607
ANT
Q504
FINAL
AMP