Icom IC-R8500 Service Manual page 12

4-1-15
AF
AMPLIFIER CIRCUIT (MAIN
UNIT)
The
AF
amplilier circuit
amplifies the
demodulated
AF
signals
to
drive
a speaker.
AF
signals
from the squelch
control
gate are pre-amplified
at
the
AF
amplifier
(IC20)
and are then applied
to
the
AF
power
amplifier (1C21)
after either
passing through
or by-
passing the
APF
circuit
(IC29-IC31,
IC39).
The power
amplified
AF
signals are applied
to
the
Internal
speaker
via
the
[EXT SP]
and [PHONE]
jacks
when
no plugs are con-
nected
to
the
jacks.
The
[AF
GAIN]
control signal
is
also applied to the
AF
pre-amplifier (IC20. pin
8)
via the
CPU
(IC33)
to
control
amplifying gain.
4-1-16
NOISE
BLANKER
CIRCUIT
(MAIN
UNIT)
The
noise blanker
circuit
detects pulse-type noises,
and
stops IF amplifier
operation during
detection.
A
portion of the 3rd IF signal
from the
bandpass
filter
(FIS)
is
amplified
at
the noise amplifier
circuit (Q9, 105,
Q11).
The
amplified signal is rectified at
the noise detector
circuit
(DiO)
for
conversion
into
DC
voltage.
The
DC
voltage
is
applied
to
the
NB
control
circuit (Q12,
Q13) to control the
NB
switch (Q14).
Some DC
voltage
is
fed
back
to
the noise amplifier
circuit
(ICS) via the
DC
amplifier
(QIO).
The
DC
amplifier function
as an
AGC
circuit to
reduce averaged
noise.
Therefore, the
noise blanker function shuts
off
pulse-type noise
only.
4-1-17
AGC
CIRCUIT (MAIN
UNIT)
The
AGC
(Auto
Gain
Control)
circuit
reduces
IF amplifier
gain
to
keep the audio output
at
a
constant
level.
A
portion of
the 3rd
IF
signal
from
the IF amplifier (Q18)
is
applied to
the
AGC
detector
circuit
(D20).
The
detected
signal
is
applied
to
the
AGC
control
circuit (Q20,
Q21
,
Q25,
Q27) and then applied
to
the
IF
amplifiers
(Q2,
Q4,
Q15-Q17). The
AGC
voltage
is
also applied
to
the
RF-A
and RF-B
unit
via the current-amplifier
circuit
(lC32b).
The
current-amplified
AGC
signal
is
applied
to
the IF
amplifier
and
AGC
control
circuit in
the
RF-A
unit (IF
amp.: Q8,
AGC
control:
Q3),
and
the buffer-amplifier
circuit in
the
RF-B
unit
(ICIa).
When
strong signals are received, the
detected voltage
increases
and
the output
level
of the
AGC
control
circuit
decreases.
The
AGC
voltage
is
used as the bias voltage
for
the IF amplifiers, therefore, the IF amplifier gain
is
de-
creased.
AGC
response time
is
controlled
by changing the time
constant
at
the
AGC
control
line
with resistors (R179, R180,
R503) and
capacitors (C134-C137).
R179. R180, Cl 34
and C135
are
used
for
AGC
slow,
and R503, C136 and
Cl 37 are used
tor
AGC
fast
mode's time
constant.
The
time constant
for
AGC
slow
is
connected
to
the
AGC
control
line-
while
AGC
is
set to slow.
However,
it's
disconnected
from
the
AGC
control
line,
and
the time constant
for
AGC
fast
is
connected
to
the
AGC
control
line
while
AGC
is
set
to
fast.
Both time constants are disconnected from the
AGC
control line
while
scanning
or
when
WFM
or
FM
mode
is
selected
for
faster
response
than
AGC
fast
mode.
4-1-18
S-METER
CIRCUIT (MAIN
UNip
The
S-meter
circuit
indicates the
relative
received
signal
strength while receiving
and changes depending on
the
received
signal strength.
A
portion of the
AGC
signal
is
applied
to
the
meter
amplifier
circuit (IC9a).
The
amplified signal
is
then applied
to
the
CPU
(IC33)
as an
SMAD
signal to
drive the S-meter.
*
The
SMAD
signal
is
also
used
for
noise
and S-meter
^
squelch operation
by comparison
with
the
[SQUELCH]
control setting level
and
receiving signal strength
at
the
CPU.
4-1-19
APF
CIRCUIT (MAIN
UNIT)
The
APF
(Audio
Peak
Filter)
circuit
boosts
a
specified
frequency
to pick
up
desired
CW
signals,
etc.
When
[APF]
is
turned
ON,
the
AF
signals
from
the squelch
control
gate (IC19) are applied
to
the
APF
circuit
(IC29-IC31, IC39).
The
[APF]
control adjusts the
peak
frequency
within
200-1 000
Hz.
IC29
is
a
triple
2-channel
analog switch IC
and
the
AF
signals are
either
passed
through
or
bypass
the
APF
circuit.
The
APF
circuit
functions
as a
tone
control
while
WFM,
FM,
AM
or
SSB mode
is
selected.
•
Noise blanker
and
AGC
circuits
4-5