Icom IC-M412 Service Manual page 11

RX AF CIRCUITS (MAIN-RX only)
The RX AF circuits consist of AF filter, AF SW, AF
amplifi er and AF power amplifi er, and amplify and fi lter
the AF signals demodulated by the demodulator circuits.
The demodulated AF signals from the IF IC (IC2) are
passed through the AF line SW (IC4, pins 13, 14) which
toggles the AF lines between MAIN-RX and CH70-RX,
and passed through the de-emphasis circuit (R231,
C232) to obtain –6 dB/oct of frequency response.
• RX AF CIRCUITS (MAIN-RX only)
AF Power AMP
MMZ1608Y301C IC9
5
R353 C353 EP403
1
470K
1
2
3
R354
R357
15K 1
R358
1
SQUELCH CIRCUIT (MAIN-RX only)
The squelch circuit cuts the AF output signals off when no
RF signals are received. Filtering noise components in the
demodulated AF signals, the AF mute SW (IC4) disconnects
the AF line from the AF AMP.
A portion of FM-demodulated AF signal from the IF IC
(IC2) is passed through the noise filter to filter the noise
components (approx. 30 kHz signals) only. The filtered
noise components are rectified to be converted into the
pulse-type signal by noise detector to produce DC voltage
corresponding to the noise level ("SQL" signal).
NOISE SQUELCH DIAGRAM
To RX AF circuits
Fc=30 kHz
R214 Noise filter
From IF IC (Pin16)
The "SQL" signal is amplified by noise AMP (IC11), then
applied to the CPU (LG: IC1, pin 62). If the CPU interprets
that the applied noise level is higher than preset one, the
CPU sends the "RMUTE" signal to the AF mute SW (IC4) to
cut the RX AF line off.
C356
4
470
LA4425A
MPZ1608S221A
Noise
"SQL"
AMP
Noise detector
(Rectify)
The de-emphasized AF signals are passed through
the BPF (Q31, 32) and the AF mute SW (IC4, pins
15, 1) then applied to the VR-A UNIT. The AF signals
are adjusted its level by the variable resister (VR: R1),
and backed to the MAIN-A UNIT and applied to the AF
power AMP (IC9). The AF signals are power-amplified to
obtain more than 2 W of audio output power. The power-
amplified AF signals are applied to the internal/external
speaker.
VCC
MPZ1608S221A
EP341 CP16
SP+
SP+
CP15S
To EXT SP
EP342 CP17
SP-
CP15S
• SQUELCH CIRCUITS (MAIN-RX only)
R224
10K
CP5
NJM2125F
CP15S
Noise AMP
LTM450EW
FI3
I O
2nd IF filter
Squelch threshould
adjustment pot
Noise filter
(active BPF)
4 - 5
W2
1
To SQL BOARD
2
3
4
5
R651
6
0 R652
7
47
8
OPC-1264A
R223
15K
5
1
4
3
IC11
R205
2
1.5K
X2
JTB450C24
IC2
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
R219
470K
470
DETM
5
To VR BOARD
Discriminator
C205
100P
CP3
CP15S