Icom IC-91A Service Manual page 11

• B_BAND 2ND IF AND DEMODULATOR CIRCUITS
2nd IF signal to the AM demodulator (Q203, Q204)
FM-demodulated signals
to the AF circuits
5-1-4 AF AMPLIFIER CIRCUITS
(MAIN AND LOGIC UNITS)
The demodulated AF signals from the demodulator circuits are
amplified and filtered in AF amplifier circuits.
- A_BAND -
• FM/WFM mode
The demodulated AF signals from the A_BAND IF IC (MAIN UNIT;
IC100, pin 9) are applied to the mode switch (MAIN UNIT; IC500,
pins 1, 2) and then passed through the AF filter (MAIN UNIT;
Q550, Q552).
• AM mode
The demodulated AF signals from the AM-demodulator circuit
(MAIN UNIT; Q104, Q105) are applied to the AF switch (MAIN
UNIT; D105) and then passed through the AF filter (MAIN UNIT;
Q550, Q552).
- B_BAND -
• FM/FM-N mode
The demodulated AF signals from the B_BAND IF IC (MAIN UNIT;
IC200, pin 9) are applied to the mode switches (MAIN UNIT; IC500,
pins 10, 11, IC700, pins 11, 10) and then passed through the AF
filter circuit (MAIN UNIT; IC501 pins 1, 2; 6, 7).
• AF CIRCUITS
from the AM
demodulator
circuit (Q104, Q105)
IC500
from the A_BAND
IF IC (IC100, pin 9)
1
3
from the B_BAND
IF IC (IC200, pin 9)
10
from the AM
IC700
demodulator
circuit (Q203, Q204)
11
Q202
8
7
Noise
Amplifier
Limitter
Quadrature Amplifier
Detector
10 11
9
MAINR3B
(+3V)
X200
Mode switch
W/N
Q201
Q550
FM/AM
AF
filter
2
4
11
10
(FM/AM)
BPF
D202
D203
FI200
45.9 MHz
2nd LO signal
BPF
FI201
(FM-N)
5 3 2
Noise
Detector
RSSI
12 13
16
"BNOIS" signal to the CPU (LOGIC UNIT; IC1, pin 28)
"BRSSI" signal to the CPU (LOGIC UNIT; IC1, pin 46)
• AM mode
The demodulated AF signals from the AM-demodulator circuit
(MAIN UNIT; Q203, Q204) are applied to the AF (MAIN UNIT;
D205) and mode switches (MAIN UNIT; IC700, pins 10, 11) and
then passed through the AF filter (MAIN UNIT; IC501 pins 1, 2;
6, 7).
The filtered AF signals are applied to the D/A converter (LOGIC
UNIT; IC303 for A_BAND; pins 21, 22; 23, 24/for B_BAND;
pins 13–16) and level adjusted. The level adjusted AF signals
are applied to the AF power amplifier (LOGIC UNIT; IC550, pin
2). The power amplified AF signals are then output from pin 6,
and applied to the internal speaker (CHASSIS; SP1) via [SP]
connector (MAIN UNIT; J700).
5-1-5 AGC CIRCUIT (MAIN AND RF UNITS)
A portion of the AM-demodulated signals are converted into DC
voltage by AGC (Automatic Gain Control) detector (MAIN UNIT;
Q104; A_BAND/Q203; B_BAND), and fed back to the IF and RF
circuits as the AGC signal.
MAIN UNIT LOGIC UNIT
Q552
AF
filter
WFM
IC501
2 AF 7
filter
5 - 4
Q450
X3
Q451
B_BAND IF IC (IC200)
1st IF signal from the 1st IF amplifier (Q200)
IC303
21 23
D/A
13 15
converter
IC550
AF
2 6
power
amp.
TCXO
X450
Speaker
[SP] jack