Icom IC-M412 Service Manual page 8

1ST IF CIRCUITS (MAIN-RX)
The 1st IF circuits consist of 1st mixer, 1st IF fi lter and 1st IF
amplifi er. And it down-converts the RX signals into the 1st IF
signal then fi lters and amplifi es.
The filtered RX signals from the 2-pole tuned BPF are
applied to the 1st mixer to be converted into the 1st IF signal,
by being mixed with the 1st Local Oscillator (LO) signals from
the RX VCO (Q72, D51–53) via buffers (Q74, 76) and LPF
(L12, 13, C77).
• 1ST IF CIRCUITS (MAIN-RX)
R8V
C204
0.01
FL-412
R186
C195
0.001
1.2K
Q23
KTC3880S
1st IF AMP
2ND IF AND DEMODULATOR CIRCUITS
(MAIN-RX)
The 2nd IF circuits consist of 2nd mixer, 2nd IF filter and 2nd
IF amplifier. It down-converts the 1st IF signal into the 2nd
IF signal, then filters to fi lter 2nd IF signal only and amplifi es
it. The demodulator circuit converts the 2nd IF signal into AF
signals. The IF IC "TA31136FNG" contains major sections of
2nd IF and FM demodulator circuits in its package.
IF IC (TA31136FNG) BLOCK DIAGRAM
8
Noise
Filter
2nd IF
FM
AMP
Detector
9
10 11
The amplified 1st IF signal from the 1st IF AMP (Q23) is
applied to the IF IC (IC2, pin 16), and converted into the
450 kHz 2nd IF signal at the internal 2nd mixer, by being
mixed with the 2nd LO signal from the reference frequency
oscillator (Q85, X1). The converted 2nd IF signal is output
from pin 3 and filtered by the external 2nd IF filter (FI3).
The filtered 2nd IF signal is backed to the IF IC (pin 5),
and saturation-amplified by the internal 2nd IF AMP.
The amplified 2nd IF signal is FM-demodulated by the
quadrature detector with a discriminator (X2).
The demodulated AF signals are applied to the RX AF
circuits.
FL-368
FI2 FI1
R185
C189
0.001
1.2K
1st Mixer
1st IF filters
1st LO
2
7 5 3
2nd
Noise
Mixer
AMP
Noise Det.
RSSI
12 14 16
The converted 1st IF signal is passed through two 1st IF
fi lters to remove unwanted signals. The fi ltered 1st IF signal
is amplifi ed by the 1st IF AMP.
The amplified 1st IF signal is then applied to the 2nd IF
circuits.
Q22
3SK293
L13 L12
0.12 U 0.1U
LPF
• 2nd IF AND DEMODULATOR CIRCUITS (MAIN-RX)
R224 R223
10K 15K
IC11
CP5
NJM2125F
CP15S
Noise AMP
LTM450EW
FI3
I O
2nd IF filter
Squelch threshould
adjustment pot
Noise filter
(active BPF)
4 - 2
5
1
4
3
2 R205 Discriminator
1.5K
X2
IC2 JTB450C24
TA31136FNG
1 16
OSCIN MIXIN
2
15
OSCOUT GND
3 14
MIXOUT N-RE C
4 13
VCC N-DET
5
12
IFIN R SSI
6 11
DEC IFOUT
7 10
FILOUT QUAD
8
9
FILIN AFOUT
R211
470K R219
470
DETM
R5V
R
C205
100P
CP3
CP15S