Icom IC-7700 Service Manual page 8

MAIN UNIT
MIAN UNIT contains receive circuits; from demodulator
to AF circuits. The 2nd IF signal is demodulated by digital
audio processing circuits.
• DEMODULATOR CIRCUIT
The 36 kHz 2nd RX IF signal from the MAIN UNIT is
applied to the Analog to Digital Converter (ADC; IC1251) via
the IF mute SW (IC1201) and unbalance-balance convert
circuit (IC1211, 1231), to be converted into the 24-bit serial
data. The serial data is applied to the DSP (IC1001) and
processed as digital audio signals. The processed digital
audio signals are then applied to the Digital Audio Converter
(DAC; IC1501) to be converted into the analog audio
signals; as recovered AF signals.
The recovered AF signals are applied to the RX AF circuits
via buffer (IC1751).
• RX AF CIRCUITS
The recovered AF signals are amplified and level-adjusted,
then output from the audio interfaces. (The destination of AF
output signals is depends on the setting)
<OUTPUT FROM Int./Ext. SPEAKER>
The recovered AF signals from the buffer (IC1751) are
passed through the SQL SW (Q351), and applied to the
AF AMP (IC331). The amplified AF signals are splitted into
two AF lines; one is bound for the headphones jack (J2) on
the FRONT UNIT, another is bound for the internal speaker
(SP1)/external speaker via the external SP jack (J751) on
the MAIN UNIT. Both of AF signals on each AF lines are
applied to the 2-channel Voltage Controlled Amplifier (VCA;
IC311) whose gain is determined by applied control voltage
to be adjusted its signal level.
The level-adjusted AF signals are applied to the AF power
AMP (IC281) which power-amplifies to obtain audio output
power, via LPF (Q291/301).
The power-amplified AF signals are passed through the AF
mute SW (RL261), then applied to the internal speaker (SP1)
via the external SP jack (J751), or an external speaker on
the MAIN UNIT.
When the headphones are connected to the headphones
jack (J2) on the FRONT UNIT, the power-amplified AF
signals from the AF power AMP (IC281) are applied to the
connected headphones.
<TO THE AUDIO STORAGE>
When recording received audio, recovered AF signals from
the buffer (IC1571) are also send to the audio storage
(IC203) on the LOGIC UNIT to be recorded.
<OUTPUT FROM ACC PORT>
The recovered AF signals from the buffer (IC1571) are
amplified by AF AMPs (IC241), the amplified AF signals
are applied to the VCA (IC231) to be adjusted its signal
level. The level-adjusted AF signals are output from [ACC1]
socket.
SCOPE UNIT
The 1st RX/TX IF signal from the buffer (Q1352) on the RF
UNIT are passed through the scope IF SW (IC31), scope
ATT, LPF and BPF (dual tuned), and amplified by scope
AMP (Q2) then applied to the 2nd scope mixer (D101).
The 1st IF signal is converted into the 45 MHz 2nd scope IF
signal (S2IF), and filtered to remove unwanted signals, then
amplified by the 2nd scope IF AMPs (Q201 and Q202). The
amplified 2nd scope IF signal is applied to the 3rd scope
mixer (D4) to be converted into the 200 kHz 3rd scope IF
signal. The converted signal is amplified by the 3rd scope IF
AMP (IC401), filtered by the active LPF (IC403), level-limited
by the saturation AMP (IC404). The level-limited signal is
applied to the ADC (IC501) which samples the 200 kHz 3rd
scope IF signal (0–2.5 V) into digital signal, then applied to
the DSP (IC601) as the serial data.
The DSP processes input data, then output to the CPU to
display the frequency and relative signal strength of RX
signals on the spectrum screen.
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