Icom IC-R20 Service Manual page 14

(1) A-BAND CIRCUIT
A portion of the AF signals from the FM IF IC (RF unit; IC2,
pin 9) passes through the low-pass filter (LOGIC unit; C161,
C164, R169 and R175) via the "A_DET" line to remove AF
(voice) signals and is applied to the CTCSS decoder inside
the CPU (LOGIC unit; IC3, pin 89) via the "A_CTONE" line
to control the AF mute switch.
(2) B-BAND CIRCUIT
A portion of the AF signals from the FM IF IC (RF unit; IC17,
pin 9) passes through the low-pass filter (LOGIC unit; C160,
C163, R168 and R174) via the "B_DET" line to remove AF
(voice) signals and is applied to the CTCSS decoder inside
the CPU (LOGIC unit; IC3, pin 90) via the "B_CTONE" line
to control the AF mute switch.
4-1-9 AGC CIRCUIT (RF UNIT)
The AGC (Automatic Gain Control) circuit reduces IF and
RF amplifiers gain to keep the audio output at a constant
level. The receiver gain is determined by the voltage on the
AGC line (A-BAND circuit is Q4's collector, B-BAND circuit
is Q545's collector).
• A-BAND CIRCUIT
A part of the 3rd IF signal from the mode switch
(D111–D113) is amplified at the IF amplifier (Q1), and is then
applied to the AM detector. (Q4, pin 5). The signal is output
from pin 1, and is then applied to the AGC circuit of the same
one (pin 3). The signal is output from pin 4 as "A-AGC" sig-
nal, and is then applied to the 3rd IF amplifier (Q1), 2nd IF
amplifier (Q5) and RF amplifiers (Q14, Q35, Q36 and
Q505).
• B-BAND CIRCUIT
A part of the 3rd IF signal from the mode switch (D136 and
D137) is amplified at the IF amplifier (Q544), and is then
applied to the AM detector. (Q545, pin 5). The signal is out-
put from pin 1, and is then applied to the AGC circuit of the
same one (pin 3). The signal is output from pin 4 as "B-AGC"
signal, and is then applied to the 3rd IF amplifier (Q544), 2nd
IF amplifier (Q542) and RF amplifiers (Q24, Q25, Q548 and
Q549).
• AF AMPLIFIER AND IC RECORDER CIRCUITS
EXT. SP.
J3
INT. SP.
J8
Pre. amp.
Q12
AF amp.
IC5
Pre. amp.
Q16
Playing line
Sub CPU Memory
IC2 IC6
USB Interface
IC3
The A-AGC or B-AGC voltage is used for the the bias volt-
age of the IF and RF amplifiers. When receiving strong sig-
nals, the detected voltage increases and the AGC voltage
decreases. Therefore, the receiver obtain stable receiver
gain.
4-1-10 IC RECORDER CIRCUIT (LOGIC UNIT)
The IC RECORDER circuit is composed of RECORDER IC
(IC7), sub CPU (IC2), flash memory (IC6), USB interface
(IC3), etc.
(1) IN CASE OF RECORDING
• A-BAND CIRCUIT
The AF signals from the AF amplifier (Q23) are applied to
the REC board via the HJ8, pin 2 as "A_AIN" signal. The sig-
nals are applied to the RECORDING IC (IC7, pin 25) to com-
press the voice signals, and are then converted to digital sig-
nals. The signals are applied to the sub CPU (IC2), and are
then stored in the flash memory (IC6).
• B-BAND CIRCUIT
The AF signals from the AF amplifier (Q22) are applied to
the REC board via the HJ8, pin 3 as "B_AIN" signal. The sig-
nals pass through the AF mute switch (IC8, pins 1 and 2),
and are then applied to the RECORDING IC (IC7, pin 20) to
compress the voice signals, and are then converted to digi-
tal signals. The signals are applied to the sub CPU (IC2),
and are then stored in the flash memory (IC6).
(2) IN CASE OF PLAYBACK
The stored RECORDING data in the flash memory (IC6) are
applied to the sub CPU (IC2), and are then applied to the
RECORDING IC (IC7). The signals are converted to analog
signals, and are then expanded into the AF voice signals.
The AF signals output from RECORDING IC (IC7, pin 25).
The signals are applied to the LOGIC unit via the J6, pin 3
as "AOUT" signal, and are then applied to the electronics
volume (IC7, pin 11) to control volume level. The signals out-
put 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 inter-
nal speaker which is connected with J8 via the external
speaker jack (J3).
VR
IC7
SW
Q19
Rec. IC
IC7
REC UNIT
4 - 6
AF Filter
A_DET
Amplifier
Q23, Q25, Q26
AF Filter
B_DET
Amplifier
Q22, Q24, Q27
Recoding line