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4 - 5
(3) AM MODE
The 3rd IF signal is output from the FM IF IC (IC17, pin 3),
and passes through the mode switch (D134) and ceramic
bandpass filter (FI8). The filtered signal passes through the
mode switch (D136), and is then applied to the IF amplifier
(Q544). The amplified signal is demodulated at the AM
detector (Q545). The AF signals pass through the ANL cir-
cuit (D139), and are then applied to the AF switch (IC21, pin
4) via the “AM_DET” line, and are then applied to the AF cir-
cuit (LOGIC unit) as “B_DET” signal.
4-1-7 AF AMPLIFIER CIRCUIT (LOGIC UNIT)
The AF amplifier circuit amplifies the demodulated AF sig-
nals to drive a speaker.
• A-BAND CIRCUIT
While in FM mode, AF signals from the demodulator circuit
(RF unit) pass through the AF filter (Q25), and are then
amplified at the AF amplifier (Q23).
While in WFM/AM/SSB/CW modes, AF signals from the
demodulator circuit (RF unit) bypass the AF filter via the AF
filter bypass switch (Q26), and are then amplified at the AF
amplifier (Q23).
The signals are applied to the electronics volume (IC7, pin
6) to control volume level. The signals output from pin 5, and
are applied to the pre-amplifier (Q12). The signals are ampli-
fied at the AF amplifier (IC5, pin 1), and then output from pin
6. The signals are applied to the internal speaker which is
connected with J8 via the external speaker jack (J3).
• B-BAND CIRCUIT
While in FM mode, AF signals from the demodulator circuit
(RF unit) pass through the AF filter (Q24), and are then
amplified at the AF amplifier (Q22).
While in WFM/AM modes, AF signals from the demodulator
circuit (RF unit) bypass the AF filter via the AF filter bypass
switch (Q27), and are then amplified at the AF amplifier
(Q22).
The signals are applied to the electronics volume (IC7, pin
11) to control volume level after passing through the mute
switch (Q19). The signals output from pin 12, and are
applied to the pre-amplifier (Q16). The signals are amplified
at the AF amplifier (IC5, pin 1), and then output from pin 6.
The signals are applied to the internal speaker which is con-
nected with J8 via the external speaker jack (J3).
The electronic volume control circuit controls AF gain, there-
fore, the AF output level is according to the [VOL] setting
and also the squelch conditions.
4-1-8 SQUELCH CIRCUIT (LOGIC AND RF UNITS)
• NOISE SQUELCH
The noise squelch circuit cuts out AF signals when no RF
signals are received. By detecting noise components in the
AF signals, the squelch circuit switches the AF mute switch.
A portion of the “A_NOISE” signals from the FM IF IC (RF
unit; IC2, pin 13) are applied to the CPU (LOGIC unit; IC3,
pin 5). The CPU analyzes the noise condition and outputs
the “AFON” signal (from pin 17) to the AF amplifier’s con-
troller (LOGIC unit; Q7).
• TONE SQUELCH
The tone squelch circuit detects AF signals and opens the
squelch only when receiving a signal containing a matching
subaudible tone (CTCSS). When tone squelch is in use, and
a signal with a mismatched or no subaudible tone is
received, the tone squelch circuit mutes the AF signals even
when noise squelch is open.
16
Limiter
amp.
X1
19.2 MHz
Q555
Q556
D171
RSSI
IC2 TA31136F
13
2nd IF (19.65 MHz)
from Q5
"B_RSSI" signal to the CPU
(LOGIC unit; IC3, pin 87)
11
10
9
8
7
5
3
2
Active
filter
Noise
detector
FM
detector
Noise
comp.
"B_NOISE" signal to the CPU
(LOGIC unit; IC3, pin 4)
12
R437
R436
C765
C764
R443
C772
C768
C769
AM
DET
IF
C770
R440
R439
R438
"B_WFMC"
L105
C880
C767
"DET_OUT" signal
to the AF switch
"B_AGC" signal to
RF amplifiers
"AM_DET" signal
to the AF switch
Mixer
3rd
Q543
AGC
REF
CONT.
REF
OSC
LO
LPF
AM/NFM
WFM
FI8
Q544
Q545
Q545
ANL
D139
• B-BAND 3RD MIXER AND DEMODULATOR CIRCUITS