Icom IC-R6, Service Manual, Page: 10 / 36

4 - 2
• 1ST IF AND 2ND IF CIRCUITS
• 3RD IF AND DEMODULATOR CIRCUITS
1ST IF CIRCUIT (RF UNIT)
The RX signal from the RF circuit is applied to the 1st IF
mixer (IC1) to be mixed with the 1st LO signal from the 1st
LO VCO (Q528, Q529, D105 and D106), resulting in the
266.7 MHz 1st IF signal.
The 1st IF signal is filtered by the 1st IF filter (FI1), and then
applied to the 2nd IF mixer (Q523).
2ND IF CIRCUIT (RF UNIT)
The 1st IF signal is applied to the 2nd IF mixer (Q523) to be
mixed with the 2nd LO signal from the 2nd LO VCO (Q6 and
D17), resulting in a 19.65 MHz (for FM and AM modes) or
19.95 MHz (for WFM mode) 2nd IF signal.
The 2nd IF signal is filtered by the 2nd IF filter (FI3; for FM
and AM modes, or C507, C714 and C715; for WFM mode),
and then applied to the 2nd IF AMP (Q5).
The amplified 2nd IF signal is applied to the 3rd IF circuits.
X2 X2
LPF
BPF
IF
AM P
BPF
XTAL
BPF
MODE
SW
SW
MODE
LO
SW
SW
LO
RF UNIT
To the 3rd IF circuits
From the RF circuits
1st LO VCO
2nd LO VCO
IC1
FI1
Q523
D58,D59
FI3
D60,D61
Q5
D47,D50,D52
Q31 Q31
D42-D44
266.7 MHz
266.8-1099.695 MHz
FM,AM: 247.050/286.350 MHz
19.650 MHz
1st Lo
2nd Lo
19.950 MHz
WFM : 246.750/286.650 MHz
PLL
IC
X1
BPF
CERAMI C
SQ L
AMP
IF IC
SW
MODE
SW
MODE
RF UNIT
SW
AF line
From the 2nd IF circuits
To the AF circuits
IC12
D56
D55
FI2
19.200 MHz
3rd IF
IC14
3rd Lo:19.2 MHz
FM,AM: 450 kHz
NOISE
IC13
WFM: 0.750 MHz
3RD IF AND DEMODULATOR CIRCUITS
(RF UNIT)
The signal from the 2nd IF AMP (Q5) is applied to the IF de-
modulator IC (IC12) which contains the 3rd IF mixer, 3rd IF
AMPs and FM/AM demodulator circuits, etc. in its package.
The 2nd IF signal is applied to the 3rd IF mixer and mixed
with the 3rd LO signal from the reference frequency oscil-
lator (TCXO; X1), resulting in a 450 kHz (for FM and AM
modes) or 750 kHz (for WFM mode) 3rd IF signal.
The converted 3rd IF signal is passed through or bypassed
the ceramic BPF (FI2) to eliminate unwanted signal, and
amplifi ed by the 3rd IF AMP, then demodulated by the FM/
AM detector.
The FM detector employs a ceramic discriminator for the de-
modulation for FM mode and LC discriminator for the WFM
mode as a phase shifter. The AM detector employs a diode
detector circuitry.
The demodulated AF signal is applied to the AF circuits on
the LOGIC UNIT through the AF line SW (IC14).