Technics ST-9600 - Page 9

@ SERVICE AID
7.
Servo
Tuning
Servo
tuning
is
circuitry
which,
if
the
tuned
frequency
should
for
some
reason
become
slightly
detuned,
detects
the
EM
discriminator
error
voltage,
amplifies,
and
charges
and
discharges
the
C311
capacitor.
The
bias
of
variable-capacity
diode
Di
is
then
changed
by
the
voltage
across
the
capacitor,
the
oscillation
frequency
is
regulated,
and
the
tuned
frequency
is
automatically
returned
to
the
perfectly-tuned
point.
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(Servo Tuning Circuitry)
Fig. 11
(1) If muting switch is OFF .
Because
the
muting
switch
(S3-1)
and
the
servo-tuning
switch
(S3-2)
are
interlocked,
the
servo
tuning
is
off
when the
muting
switch
is
off.
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As
shown
in
figure
11,
when
(S3-2)
is
in
the
OFF”
position,
the
base
of
TR315
is
near
ground
potential
and
is
cut
off.
Asa
result,
bias
voltage
is
applied
to
the
base
of
TR314,
turning
it
on.
When
TR314
operates,
TR313
becomes
off
because
the
collector
becomes
ground
potential,
and
TR311
becomes
on.
Asa
result
of
this
series
of
operations,
current
flows
from
R320,
through
R318,
to
D311,
and
through
R312,
becoming
the
emitter
current
of
TR311.
Because
there
is
no
change
in
the
voltage
across
C311,
there
is
also
no
change
of
the
oscil-
lation
frequency.
(A
voltage
which
is
always
constant
is
applied
to
variable-capacity
diode
D1.)
(2)
When
the
muting
switch
changes
from
OFF
to
ON,
the
timer
circuitry
operates.
(a)
When
the
muting
circuitry
operates,
the
muting
AND
circuitry
(see
note
below),
TR501
and
TR502,
becomes
on,
and
the
base
voltage
of
TR503
thus
is
turned
off
by
the
ground
potential.
In
other
words,
+B
voltage
is
applied
to
the
collector
of
TR503,
and
the
bias
of
D603
and
D313
connected
there
is
reversed.
That
is,
the
condition
of
D313
becomes
as
if
not
connected
to
the
base
of
TR315.
And
the
+B
voltage
pas-
ses
through
R324,
and
charges
C313.
When
the
voltage
across
C313
(VBE
of
TR315)
is
charged
to
0.6V,
TR315
becomes
on,
TR314
becomes
off,
and
the
timer
circuitry
functions.
The
time
until
TR314
becmoes
off,
as
determined
by
the
time-constant
of
R324
and
C313,
is
set
to
be
about
6
seconds.
When
TR314
becomes
off,
TR313
becomes
on,
and
TR311
becomes
off.
Because
TR311
does
not
operate,
the
standard
voltage
across
C311
is
affected
by
the
operation
of
TR312.
[Refer
to
item
(4).]
ST-9600
Note:
The
AND
circuitry
is
circuitry
which
includes
2
transistors,
TR501
and
TR50O2.
If,
when
a
narrow-band
IF
signal
is
applied
to
TR501
and
a
wide-band
IF
signal
is
applied
to
TR502,
both
inputs
do
not
exceed
the
pre-set
level,
neither
transistor
becomes
on.
In
this
unit,
2-step
muting
changeover
is
accomplished
by
the
level
applied
to
TR502.
(Refer
to
‘’1.
Muting
Circuitry”
on
page
7.)
(b)
If,
because
the
input
level
is
small,
the
muting
circuitry
does
not
operate,
TR503
becomes
on
because
TR501
and
TR502
are
off,
and
the
collector
is
ground
potential.
Voltage
is,
therefore,
applied
to
D313
and
D603
in
the
forward
direction,
and
current
flows.
However,
because
these
diodes
are
germanium
diodes,
no
bias
can
be
applied
to
TR315,
and
it
becomes
cut
off.
This
becomes
the
same
circuitry
as
when
_
the
muting
switch
is
off.
In
other
words,
the
timer
operates
when
the
muting
switch
is
changed
from
‘‘off”
to
"on"
by
input
which
is
higher
than
the
muting
operation
level,
or,
about
6
seconds
after
the
instant
when,
with
the
muting
switch
“on”,
a
strong
signal
is
received
during
reception
of
a
weak
signal
which
is
less
than
the
muting
signal
(or
no
signal).
(3)
When
frequency
deviates
to
a
frequency
slightly
higher
than
the
perfectly-tuned
frequency:
The
discriminator
center
voltage
from
the
sixth
terminal
of
!C301
is
applied
to
the
base
of
TR312.
Because
the
center
voltage
is
usually
OV
at
the
time
of
perfect
tuning,
TR312
does
not
operate.
!f,
however,
the
tuning
point
is
changed
to
a
frequency
slightly
higher
than
the
perfectly-tuned
frequency,
a
positive
voltage
is
produced
at
the
sixth
terminal
of
1C301
in
the
proportion
that
the
frequency
deviation
is
to
the
perfectly-tuned
frequency.
This
voltage
changes
TR312
to
the
"on"
condition,
and
the
electric
charge
of
C311
passes
through
R314
and
discharges.
In
other
words,
the
voltage
across
C311
decreases,
and
the
voltage applied
to
variable-
capacity
diode
D1
in
the
front-end
decreases.
When
this
happens,
the
capacity
of
D1
increases,
the
oscillation
voltage
becomes
tow,
and
the
perfectly-tuned
point
is
then
reached.
When
it
is
tuned,
the
discriminator
center
voltage
returns
to
OV,
and,
therefore,
the
operation
of
TR312
and
the
discharge
of
C311
both
stop.
(4)
When
frequency
deviates
to
a
frequency
slightly
lower
than
the
perfectly-tuned
frequency:
Because
the
discriminator
center
voltage
also
moves
in
the
negative
direction,
negative
voltage
in
proportion
to
the
frequency
deviation
is
produced
at
the
sixth
terminal
of
1C301.
Because
TR312
is
in
the
off’
condition
at
OV,
the
"off
condition
naturally
continues
with
the
negative
voltage.
As
explained
previously,
TR311
be-
comes
“off’’
when
the
timer
circuitry
operates,
and,
therefore,
+B
voltage
passes
through
R315
and
flows
to
C311,
thus
increasing
the
voltage
across
C311.
The
reverse
bias
of
D1
is
increased
by
this
charging
voltage,
and
the
capacity
decreases,
thus
causing
the
oscillation
voltage
to
become
high,
and
the
perfectly-tuned
point
is
reached.
The
charging
of
C311
then
stops.
mw
POWER
SUPPLY
SCHEMATIC
DIAGRAN
......
Set
for
Australia
[XAL]
F1100mA.T 1901 orG
|
METER LIGHT
AC 240V
DIAL LIGHT
BLK BLU
st-9600 [14]