Icom IC-2710H Service Manual page 8

4-1-8 AF AMPLIFIER CIRCUIT
The AF amplifier circuit amplifies the detected signals to
drive a speaker. For the separate speaker function, a
stereo power amplifier is used.
AF signals are passed through the AF switch (0158), then
the AF selector (IC22 pins 1 O, 11 ). AF signals from IC22
(pin 11) are applied to the active filter (0163, 0164) which
functions as a high-pass filter to suppress subaudible tone
signals for tone squelch operation.
The filtered signals pass through the volume control IC
(IC18) and are then applied to the AF power amplifier (IC15
pin 2). The output signals are applied to an external
speaker jack (J2) via the "SPA" line. When no plug is
connected to the jack, the signals are fed back to the UHF
audio input (IC19 pin 5) and combined with the UHF audio.
The mixed audio is applied to the other external speaker
jack (J1) and then to the internal speaker.
4-1-9 VHF NOISE SQUELCH CIRCUIT
A noise squelch circuit cuts out AF signals when no RF
signal is received. By detecting noise components in the
AF signal, the squelch circuit switches the AF mute
switches.
Some of the noise components in the AF signals from IC20
(pin 9) are passed through the active filter section (IC20
pins 10, 11 ), and then applied to the noise detector circuit
(0197, 0121 ). The variable resistor (R398) adjusts the
input level of the active filter, and the level is used for
. squelch threshold reference. The detected noise signals
are applied to the CPU (IC29 pin 95) via the "VSOL S" line.
The [VHF SOL) (CONTROL unit R38) controls the input
level of the sub-CPU (CONTROL unit IC2 pin 6) in DC
voltage. The sub-CPU reads the angle of the [VHF SOL]
rotation, then send the squelch data to the CPU incor­
porated in the RDATA. The squelch level is also controlled
from the HM-98; the CPU receives squelch data from the
HM-98 via the MDATA line.
The applied signals from the noise detector circuit and the
CONTROL unit or from the HM-98 are differentiated by the
CPU, and then the CPU controls AF mute switches (IC16,
01 91) via the "VA MUTE" line.
4-1-10 UHF RF CIRCUIT
The signals from the UHF antenna switching circuit
(075-077, 090) are applied to the RF pre-amplifier (085)
and are then applied to the RF amplifier (086) via the RF
attenuator circuit (078). The amplified signals are passed
through the bandpass filter (Fl4), then applied to the 1st
mixer circuit.
AF SIGNAL LIN E
�
V DET
AF selector
IC22
____!
U DET
10
filter
79 VAMUTE
CPU
IC29
80 UAMUTE
AF
4 - 2
4-1-11 UHF 1ST MIXER AND 1ST IF CIRCUITS
The amplified signals from the RF amplifier (086) are mixed
with a 1st LO signal at the mixer circuit (087) to produce a
57.65 MHz 1st IF signal. The 1st LO signal is the PLL
output which comes from the U-UHF VCO circuit (0123,
0107, 0145). The 1st IF signal is passed through a pair of
crystal filters (Fl3) to suppress out-of-band signals and then
amplified at the IF amplifier (078).
4-1-12 UHF RECEIVING VIA VHF DISPLAY
The transceiver has additional UHF RF and mixer circuits
for a U/U para-watch function.
Some of the RF signals from the RF pre-amplifier (085) are
applied to the U/U RF amplifier (057) via the attenuator
circuit (045). The amplified signals are mixed with a 1st LO
signal at the mixer circuit (058) to produce an 45.05 MHz IF
signal via the bandpass filter (Fl1 ).
applied to the UHF IF circuit (IC20 described in section
4-1-5).
The UHF VCO circuit (033) in the VHF PLL circuit
generates the 1st LO signal for the U/U para-watch
function.
4-1-13 UHF 2ND IF AND DETECTOR
CIRCUITS
The IC23 incorporates the 2nd mixer, 2nd local oscillator,
limiter amplifier, noise amplifier, quadrature detector and
S-meter detector circuits. The 2nd local oscillator section
and X4 generate 57.195 MHz for the 2nd LO signal.
The amplified 1st IF signal is fed to the FM IF IC (IC23 pin
16) where the signal is converted into a 2nd IF signal, then
AF signals.
The AF signals output from IC23 (pin 9) pass through the
AF switch (0168), AF selector (IC22 pins 2, 1) and then
active filters (0171, 0172).
4-1-14 UHF NOISE SQUELCH CIRCUIT
A portion of the AF signals from IC23 (pin 9) are passed
through the noise amplifier (IC23 pins 1 O, 11) and detected
at 0167 and 0123. The detected noise signals are applied
to the CPU via the "USOL S" line, for reference. Then the
CPU controls AF mute switches (IC16, 0190, 01 92) after
being differentiated by the squelch level setting.
The signal is then
EXT SP
jack
R353
7
SPEAKER