Icom IC-M412 Service Manual page 13

TX AMPs
The TX amplifi ers consist the YGR and power amplifi ers, and
amplify the TX/CH70-RX VCO output to the transmit output
power level.
The buffer-amplifi ed VCO output from the TX/CH70-RX VCO
is applied to the YGR amplifier (Q10) as the TX signal via
LO SW (D7), and amplified to the level need to drive the RF
power module (IC3). The amplified TX signal is applied to
the RF power module which provides the 25 W of RF output
with 10 mW input.
The power-amplified TX signal is passed through the TX
power detector, ANT SW and LPF (as a harmonic filter),
then applied to the antenna.
• TX AMPs AND APC CIRCUIT
RF power AMP IC3
R102
270
TX mute SW1
Q18
KRC404
TX mute SW2
KRC404
Q17 R107
22
HV
HV
T5V
Q15
XP1501
R109 R111
10K 10K
APC AMP
APC CIRCUIT
The APC (Automatic Power Control) circuit stabilizes transmit
output power to prevent the transition of transmit output
power level, which is caused by load mismatching or heat
effect, etc. The APC circuit also protects the TX amplifier
from the refl ected waves caused by load mismatching.
The power detector rectifi es a portion of the TX signal and
converts it into DC voltage which is in proportion to the
transmit output power. The rectifi ed voltage is applied to the
base terminal (pin 3) of dual transistor (Q15; as a differencial
AMP). The TX power setting voltage (divided voltage of T5V)
is applied to another base terminal (pin 3) as the reference.
The differencial AMP compares the rectified voltage and
reference voltage, and the difference of these voltages is ap-
peared on the corrector terminal (pin 5) to drives APC driver
(Q14). The APC driver controls the gate bias of RF power
module to reduce/increase the gain for stable TX output
power.
The TX muting is carried out by applying voltage ("TMUTE")
to the TX mute SWs (Q17, 18). When the PLL unlock, etc. is
occurred, "TMUTE" becomes to "High" and TX mute SWs are
ON to stop supplying voltage to the RF power AMP.
ANT SW (TX)
R125
L20 D14
LW-25
68 L308CC B
L19
LA-253
TX power SW
Q16
R114
100
KRC402
4 - 7
LPF (harmonic filter)
C126 L21 L22
0.001
LA-243 LA-243
C128
4P
L31
LA-243
ANT SW
TX: BPF
TX power detector
RX: LPF
R127
100K
CP15
To ANTENNA
CP20
C150
0.001
C151
22P
Trap
(BEF)