Kenwood TM-631A Service Manual page 5

TM-631A
CIRCUIT DESCRIPTION
Table 6 MCF (L71-0228-05) Characteristics
(144 MHz
TX-RX
Unit
XF1)
©
AF
Unit
The path from the
detector output
to the
AF
power amplifier
input
consists
of
two separate identical circuits
for the main
band and the subband. The audio signal is first
switched
by
an
analog switch into the
AF
preamplifier for its
corresponding
band. After amplification, the signal level
is
adjusted by
a
two-
channel electronic volume control
[C:M51523
which
is
con-
trolled by the MAIN
VOLUME
and
BALANCE controls on the
front panel. An
AF
low-pass
filter then removes unwanted high-
frequency components. Next the separate audio
signals
are
selected to speaker mixed or separate mode by analogue switch
circuit which
is
amplified by the
STEREO
audio power amplifi-
er
before being applied the speaker.
(See Fig. 3.)
@
Preamplifier
For main
IF
detect @ Squelch
amplifier
band
IF
detect ©
Preamplifier
@ Squelch amplifier
Fig. 3
AF
Section
®
Squelch Circuits
Independent squelch circuits
are
provided for the main band
and subband.
These circuits receive the output from the de-
tector, remove the
50 kHz noise component, amplify
the
sig-
nal
with two transistor
stages,
and rectify
it
by
means of
a
diode
to
generate
the
squelch control signal. After DC amplification,
the
control signal
is
used to
switch the main and sub pream-
plifiers on and off.
° S
Meter Circuits
Independent S meter circuits for the
144 MHz
and
220
MHz
bands receive signals from the
455
kHz ceramic filter, amplify
if
in
two-stage
meter amplifier, and rectify the resulting signal
to
generate
a
DC voltage. The microprocessor converts the ana-
log
DC
voltage
to
a
digital output that
is
used to drive
an
LCD
bar meter.
CFI
8c
DIO
R39
IOK
Item
Specification
Nominal center
frequency
(fo)
10.7 MHz
Pass bandwidth Max. fo
+7.5
kHz at 3 dB
Attenuation bandwidth
1)
Max.
+25
kHz at 40
dB
2)
Max.
+45
kHz at
60
dB
Guaranteed attenuation
70
dB or more
within
+1
MHz
40
dB or more
spurious
80
dB or more within
-900
kHz to
-920
kHz
Ripple
Max.
1
dB
Insertion loss
Max. 1.5
dB
Terminating impedance
3 kQ/0
pF
!
J
CPU
33pin
PTH01
t
ASS Y
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THI
M2
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NOY
C38:
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Q5
Le
P33.P30
Fig.
4
144
MHz S-meter circuit
¢
Switching of Balance Range
To disable the
BALANCE
control
in
the
single mode,
the
con-
trol
voltage range
of the
BALANCE
control
is
switched. This
switching
is
done by
a
Low signal
from the INH (Inhibit)
pin
of the microprocessor that turns on transistor Q7
in
the single
mode.
Since
the center
voltage
of the balance control resistor
is held to
approximately 4
V,
the
voltage
at the
BAL
pin
can-
not
exceed 4
V,
so even
if
the
balance
VR
is
moved
to the
SUB
position, the main band remains unattenuated. (See Fig. 5
and
6.)
In
dual band mode
In
single band mode
144M
LPF
For main band
AF
PA
For sub band
qn
220M
MN4066BS'
M51523AL
TC4053BF
AF
MAIN
SUB
MAIN
suB
os
AF
LPF
For sub
band
0
8
4
Voltage
at BAL
pin
(V)
Voltage
at
BAL
pin
(V)
Fig.
5
Fig. 6
During normal operation, the
two electronic volume controllers
in
the set operate using the M51523AL, but
when
the remote
control
unit
(RC-10)
is
connected
and used to adjust the out-
put level, they
are
controlled by the
LC7532M.
When
the remote control unit is
connected,
a
High switch con-
trol
signal
is
sent from the
CE
pin
of shift
register IC4 to force
the attenuation of the
M51523AL
to zero. (Normally the
CE
output
is
Low.) The
UP and DOWN signals from the shift
register then vary the attenuation of the
LC7532M. (See Fig. 7.)
4
TM-631A
CIRCUIT DESCRIPTION
Table 6 MCF (L71-0228-05) Characteristics
(144 MHz
TX-RX
Unit
XF1)
©
AF
Unit
The path from the
detector output
to the
AF
power amplifier
input
consists
of
two separate identical circuits
for the main
band and the subband. The audio signal is first
switched
by
an
analog switch into the
AF
preamplifier for its
corresponding
band. After amplification, the signal level
is
adjusted by
a
two-
channel electronic volume control
[C:M51523
which
is
con-
trolled by the MAIN
VOLUME
and
BALANCE controls on the
front panel. An
AF
low-pass
filter then removes unwanted high-
frequency components. Next the separate audio
signals
are
selected to speaker mixed or separate mode by analogue switch
circuit which
is
amplified by the
STEREO
audio power amplifi-
er
before being applied the speaker.
(See Fig. 3.)
@
Preamplifier
For main
IF
detect @ Squelch
amplifier
band
IF
detect ©
Preamplifier
@ Squelch amplifier
Fig. 3
AF
Section
®
Squelch Circuits
Independent squelch circuits
are
provided for the main band
and subband.
These circuits receive the output from the de-
tector, remove the
50 kHz noise component, amplify
the
sig-
nal
with two transistor
stages,
and rectify
it
by
means of
a
diode
to
generate
the
squelch control signal. After DC amplification,
the
control signal
is
used to
switch the main and sub pream-
plifiers on and off.
° S
Meter Circuits
Independent S meter circuits for the
144 MHz
and
220
MHz
bands receive signals from the
455
kHz ceramic filter, amplify
if
in
two-stage
meter amplifier, and rectify the resulting signal
to
generate
a
DC voltage. The microprocessor converts the ana-
log
DC
voltage
to
a
digital output that
is
used to drive
an
LCD
bar meter.
CFI
8c
DIO
R39
IOK
Item
Specification
Nominal center
frequency
(fo)
10.7 MHz
Pass bandwidth Max. fo
+7.5
kHz at 3 dB
Attenuation bandwidth
1)
Max.
+25
kHz at 40
dB
2)
Max.
+45
kHz at
60
dB
Guaranteed attenuation
70
dB or more
within
+1
MHz
40
dB or more
spurious
80
dB or more within
-900
kHz to
-920
kHz
Ripple
Max.
1
dB
Insertion loss
Max. 1.5
dB
Terminating impedance
3 kQ/0
pF
!
J
CPU
33pin
PTH01
t
ASS Y
©
THI
M2
c37
NOY
C38:
ol
Q5
Le
P33.P30
Fig.
4
144
MHz S-meter circuit
¢
Switching of Balance Range
To disable the
BALANCE
control
in
the
single mode,
the
con-
trol
voltage range
of the
BALANCE
control
is
switched. This
switching
is
done by
a
Low signal
from the INH (Inhibit)
pin
of the microprocessor that turns on transistor Q7
in
the single
mode.
Since
the center
voltage
of the balance control resistor
is held to
approximately 4
V,
the
voltage
at the
BAL
pin
can-
not
exceed 4
V,
so even
if
the
balance
VR
is
moved
to the
SUB
position, the main band remains unattenuated. (See Fig. 5
and
6.)
In
dual band mode
In
single band mode
144M
LPF
For main band
AF
PA
For sub band
qn
220M
MN4066BS'
M51523AL
TC4053BF
AF
MAIN
SUB
MAIN
suB
os
AF
LPF
For sub
band
0
8
4
Voltage
at BAL
pin
(V)
Voltage
at
BAL
pin
(V)
Fig.
5
Fig. 6
During normal operation, the
two electronic volume controllers
in
the set operate using the M51523AL, but
when
the remote
control
unit
(RC-10)
is
connected
and used to adjust the out-
put level, they
are
controlled by the
LC7532M.
When
the remote control unit is
connected,
a
High switch con-
trol
signal
is
sent from the
CE
pin
of shift
register IC4 to force
the attenuation of the
M51523AL
to zero. (Normally the
CE
output
is
Low.) The
UP and DOWN signals from the shift
register then vary the attenuation of the
LC7532M. (See Fig. 7.)
4