Icom IC-756PRO Service Manual page 9

The level shifted signal is applied to the DSP IC (IC2001) for
36 kHz digital IF filter, demodulation, automatic notch and
noise reduction, etc. The output signal is level shifted 3.3 V
to 5V at the level converter (IC2052), and is applied to the
D/A converter (IC2351) to convert into the analog audio sig-
nals.
The converted audio signals are passed through the active
filter (IC2371a), AF amplifier (IC2371b), analog switches
(IC2372, pins 14, 13 and pins 1, 15) then applied to the low-
pass filter (IC2401). The filtered signals are passed through
the analog switches (IC2372, pins 4, 3 and IC2473), high-
pass filter (IC2441A) and mixer amplifier (IC2471A), and
then applied to the MAIN unit via J2001 (pin 13) as the DTAF
signal.
3-1-11 TWIN PBT CIRCUIT (DSP BOARD)
General PBT (Passband Tuning) circuit shifts the center fre-
quency of IF signal to electronically narrow the passband
width. The IC-756PRO uses the DSP circuit for the digital
PBT function and actually shifts the both lower and higher
passbands of 3rd IF filter within ±1.8 kHz.
The twin PBT circuit in DSP IC (IC2001) controlled by the
[TWIN PBT] controller adjusts the 3rd IF passband width
and rejects interference.
3-1-12 AGC CIRCUIT (DSP BOARD)
The AGC (Automatic Gain Control) circuit reduces IF ampli-
fier gain and attenuates IF signal to keep the audio output at
a constant level.
The receiver gain is determined by the voltage on the AGC
line (IC2461, pin 4). The D/A converter for AGC (IC2461)
supplies control voltage to the AGC line and sets the receiv-
er gain with the [RF/SQL] control.
The 3rd IF signal from the level converter (IC2051) is detect-
ed at the AGC detector section in DSP IC (IC2001), and is
applied to the D/A converter for AGC via the level converter
(IC2052). The AGC voltage is amplified at the buffer amplifi-
er (IC2471b) and is applied to the MAIN unit to control the
AGC line.
When receiving strong signals, the detected voltage increas-
es and the AGC voltage decreases via the buffer amplifier
(IC2471b). As the AGC voltage is used for the bias voltage
of the IF amplifier (RF unit; Q721), IF amplifier gain is
decreased. And also AGC voltage is amplified at the AGC
amplifier (RF unit; IC871c) and applied to the ATT driver
(Q781, D784) to drive the PIN attenuator (D781, D783).
• AF amplifier circuit
IC301
5
DRAF
7
6
Squelch gate
DSP board MAIN unit
"SQLS" signal
IC311 IC331
1
AMP VCA
"AFGV" signal
3-1-13 S-METER CIRCUIT (MAIN UNIT)
The S-meter circuit indicates the relative received signal
strength while receiving by utilizing the AGC voltage which
changes depending on the received signal strength.
A portion of the AGC bias voltage from the DSP board is
applied to the differential amplifier (IC101a, pin 2) where the
difference between the AGC and reference voltage is detect-
ed.
The detected voltage is passed through the analog switch
(IC3631, pins 12, 14) as the SML signal and applied to the
main CPU (IC3501, pin 108) to activate the S/RF meter via
the sub CPU (IC401) on the DISPLAY board.
3-1-14 SQUELCH CIRCUIT (MAIN UNIT)
The squelch circuit mutes audio output when the S-meter
signal is lower than the [RF/SQL] setting level.
The S-meter signal is applied to the main CPU (IC3501, pin
108) and is compared with the threshold level set by the
[RF/SQL] control. The [RF/SQL] setting signal is applied to
the main CPU via the sub CPU (DISPLAY board; IC401, pin
91). The main CPU analyzes the compared signal and out-
puts control signal to the squelch gate (IC301, pin 5) via the
interface IC (IC3653, pin 19) to open or close the squelch as
the SQLS signal.
3-1-15 AF AMPLIFIER CIRCUIT (MAIN UNIT)
The AF amplifier amplifies the audio signals to a suitable dri-
ving level for the speaker.
The AF signals (DRAF) from the DSP board are passed
through the squelch gate (IC301) and amplified at the AF
amplifier section of IC311 (pins 2, 4) and volume is con-
trolled by the AFGV signal at the VCA section (pins 7–9).
The volume controlled AF signals are passed through the AF
mute gate (IC331, pins 1, 7), then applied to the AF power
amplifier (IC332, pin 1) via the ripple filter (Q331).
The amplified audio signals are passed through the
[PHONES] and [EXT SP] jacks then applied to the internal
speaker when no plug is connected to the jacks.
The AF mute gate is controlled by the [AF] control via the
sub and main CPUs.
"AFMS" signal
2
6
Q331
1
Ripple
7
filter
Mute switch
3 - 3
[PHONES]
AF
power
[EXT SP]
amp.
IC332
Int. speaker