Logic Circuit; Other Circuits - Icom IC-04A Service Manual

4-5-2
REDUCED VOLTAGE
DETECTING
CIRCUIT (MAIN
UNIT)
The
reduced voltage detecting
circuit
consists
of
IC105B, R168, R169, and R171.
A
regulated
5V
is
divided
at
R168
and
R169
and
a
voltage
of
approximately
1
.03V
is
applied to pin
6
of
IC105B. The
voltage of the
Vcc
is
divided
by
R1
70
and
R1 71
,
and
is
applied to
pin 5.
The
voltage
division
ratio
is
selected so that the voltage
at pin
5
is
1
,03V
when
the
Vcc
is
approximately 5.6V.
If
the
Vcc
is
greater
than 5.6V, the
voltage
at
pin
5
of
IC105B
is
higher
than
that at pin
6.
Pin
7
then
becomes
"HIGH".
If
the
Vcc
voltage decreases
to less
than 5.6V,
the voltage
at pin
5
is
less
than
that
at pin
6 and
the
out-
put voltage
at pin
7 and the output
of
IC105B
is
"LOW".
This information
is
fed to
MPU
IC401
,
causing the
BAT-
TERY CONDITION INDICATOR
to
appear on
the
FRE-
QUENCY
DISPLAY.
4-6 LOGIC CIRCUITS
The main
part of
the
logic circuits
is
MPU
IC401
.
This
in-
cludes
a
2k word ROM,
128 word
ROM,
160
byte
RAM,
and
a circuit to drive
FREQUENCY
DISPLAY
DS401.
The next 4-7 page
allocation
is
an explanation of opera-
tions
and
their I/O ports.
4-7
OTHER
CIRCUITS
4-7-1
LAMP
CIRCUIT (MAIN
UNIT)
The lamp
circuit
consists
ofQ131, Dll
5,
Dll
6,
and
other
components and
drives backlight
DS401
at
a
con-
stant current, ensuring that brightness does not
change
even
with a
change
of
power
supply
voltage.
When S106
is
turned
ON
current
flows
into
R173,
resulting in
the base voltage
of
Q131
being approx-
imately
Vcc
1.2V as determined by
D115
and
Dll
6.
The
emitter voltage of
Q131
is
then
Vcc 0.6V
and
the
voltage at
both ends
of
R172
is
kept constant.
The
result
is
a
constant
current
even
with
a
change
of
power
supply
voltage.
4-7-2 BEEP CIRCUIT (MAIN UNIT)
This
is
a
phase
shift oscillator
consisting of
IC103F,
R155-R158,
Cl
31,
Cl
32,
and Cl
34.
The
circuit
oscillates
when
the cathode of
D1 13 becomes
"HIGH".
The
oscillating
frequency
is
set at
approximately
2500Hz.
4-7-3
TRANSMIT/RECEIVE SWITCHING
CIRCUIT
(MAIN
UNIT)
When PTT
SWITCH
S101
is
pushed,
Q107
turns
ON
and
pin
13
of
IC103C
and
pin
1
of
IC102
become
"HIGH".
Pin
2
of
IC102C
remains
"LOW"
for approx-
imately
20msec.
via
time constant
circuits
R138
and
C122.
After
20msec.
pin
2
of
IC102C becomes
"HIGH", and then
pin
3
of
IC102C becomes "HIGH".
Thus
Q108
is
turned
OFF
and
Q109
is
turned
ON,
then
the
T +
5
and
TS5
lines
become
5V. "Transmit
mode"
information
is
sent to the
MPU
from
pin
12
of
IC103C
via
D107
with no delay time.
MUTE
signals
from IC401
are
"HIGH" and
applied to pin
13
of
IC102D from
MPU
IC401
for
approximately
60msec.
to
mute RF
output
signals,
preventing
unstable signals
from using
Q210
in
the
PLL UNIT.
When
SI 01
is
released after
20msec,
pin
3
of
IC102C
is
"LOW"
and
turns
Q108 ON
and and
Q109
OFF.
4-7-4
DTMF ENCODER
CIRCUIT (TONE
UNIT)
iifOS
version only)
IC501, the
DTMF
encoder, generates Dual
Tone
Multi-
Frequencies. While transmitting,
Q505
turns
ON,
ap-
plying voltage to
IC501
.
If
any keys on
the
KEYBOARD
are
pushed
at this time,
the proper frequency
dividing
ratio
for
the dividing
frequency
of
X501
(3.58MHz)
is
selected to output
one
set of
audio frequencies
cor-
responding
to
row
input
and column
input
from
pin
1
6
of
IC501.
Also,
a
"HIGH"
level
is
applied
from
pin
10
of
IC501
when
the
KEYBOARD
is
activated. This
level
has
a
time
constant of approximately
1
msec
for turning
Q506
ON.
Thus key
entries
can be
made
without holding the
PTT
SWITCH
down.
4-7-5
SUBAUDIBLE
TONE ENCODER
CIRCUIT
(TONE
UNIT) {#05 and
#09
versions)
When
a
tone
number
is
set,
data
is
sent to
IC502 from
MPU
IC401 on
the
LOGIC
UNIT.
IC502
converts
serial
data
from IC401
to
parallel data,
and
feeds
it
to
IC503.
IC503
divides
the frequency
of
X502
(3.
579545MHz)
corresponding
to data,
and
outputs
a
subaudible tone
from
pin
1
.
4-7-6
TONE
CALL
CIRCUIT (TONE
UNIT)
(#04 version
only)
The
TONE
CALL UNIT
generates a
1
750Hz
subaudible
tone
to
open
a repeater.
When
the
TONE BURST
SWITCH
is
pushed,
Q601
turns
ON
and
Vcc
is
applied
to IC601. IC601
divides
7.1680MHz
by 1/4096 and
outputs
1
750Hz
from
pin 4.
6