Sony TC-730 - Acservo Circuit; Servo Amplifier Operation

FF, STOP
REW
right reel motor
self-locking {Turns ON when
contacts of \FWD button is pushed,
RY602
S607
Turns ON when FF
or REW button is
pushed
RY602
FF relay
Fig. 2. Reel motor circuit
4. AC SERVO CIRCUIT
The
Model
TC-730
employs
ac
servo
amplifier
circuit
and
keeps
tape
speed
constant.
4-1. Principle
See
block
diagram
in
Fig.
3.
Motor
speed
determined
by
voltage
Em
can
be
changed
by
voltage
Er.
E (fixed) = Em + Er
When
motor
speed
becomes
faster
or
slower
than
intended
speed
according
to
external
disturb-
ances,
the
intended
motor
speed
can
be
obtained
again
by
changing
voltage
Er,
or
by
changing
resistance
R.
The
frequency
generator
FG
in
the
motor
detects
motor
speed
deviation.
Servo
amplifier
changes
resistance
R
according
to
the
deviation.
TC-730
uses
impedance
R
between
collector
and
emitter
of
tran-
sistor
Q901
instead
of
resistance
R
as
shown
in
Fig.
4.
Impedance
R
can
be
changed
by
the
base
voltage.
The
motor
speed
can
be
kept
stable
by
controlling
the
base
voltage
for
the
motor
speed
deviation.
The
bridge
type
rectifier
comprising
four
diodes
makes
current
flow
through
transistor
Q901
in
the
direction
shown
by
the
arrow
in
Fig.
4.
Efconstant)
SERVO AMP
Fig.
3.
Servo
control
system
block
diagram
SERVO AMP
Fig. 4. Q9017 instead of resistance R
Outline of IC (CX-032})
Q2
Fig.
5.
Servo
amplifier
outline
Ac
4-2. Servo amplifier operation
Servo
amplifier
which
changes
impedance
R
operates
as
follows:
See
Fig.
5
and
Fig.
6.
1)
2)
3)
4)
Sine
wave
signal
generated
by
frequency
ge-
nerator
FG
is
transfered
to
bases
of
Q1
and
Q?2.
Since
base
bias
voltages
of
Q1
and
Q2
are
set
to
saturation
region,
sine
wave
signal
is
waveshaped
to
square
wave
signal
shown
by
(A)
in
Fig.
6.
Square wave signal from Q2 is amplified by Q3,
Q4
and
QS.
It
is
transfered
to
differentiating
circuit
comprising
Cl
and
R1
and
is
wave-
shaped to pulse shown by (B) in Fig. 6.
Then it is transfered to base of Q6.
De
voltage
Vs
stabilized
by
Q12
is
divided
by
R3
and
R4.
Then
it
is
transfered
to
base
of
Q8
and
keeps
the
emitter
voltage
Ek
of
Q8
(also
that
of
Q7)
constant.
Vs also charges C2 according to the time constant
decided
by
C2
and
R2.
The
charged
voltage
is
applied to collector of Q6.
When pulse shown by (B) in Fig. 6 is transfered
to
base
of
Q6,
Q6
repeats
ON
and
OFF
states.
C2
discharges
while
Q6
makes
current
flow
as
shown
by
the
arrow
(1)
in
Fig.
5.
Thus
sawtooth
wave
signal
shown
by
(C)
in
Fig.
6
is
obtained
at collector of Q6.
—10—
5)
6)
4-3.
1)
The sawtooth wave signal is supplied to base of
Q7. Since the emitter voltage Ek is kept con-
stant, Q7 turns ON only when the peak value of
sawtooth wave signal is greater than Ek. When
Q7 turns ON, the negative pulse shown by (D)
in Fig. 6, appears at collector of Q7.
This pulse is amplified by Q9, Q10 and Q11,
and its polarity is inverted. The positive pulse
is transfered to the integrator comprising R5
and C3, and is waveshaped as shown by (E) in
Fig. 6. After being amplified by Q13 and Q14,
it is transfered to base of Q901. And this base
voltage changes impedance between collector
and emitter of Q901 and controls motor speed.
Motor speed deviation from intended value
When motor speed becomes faster (Fig. 7);
Switching speed of Q6é becomes faster since
frequency of frequency generator FG increases.
Collector voltage of Q6 is grounded before it
becomes greater than emitter voltage Ek (con-
stant) because of short charging time for C2.
The peak value of the sawtooth wave signal at
base of Q7 is less than Ek. Thus Q7 turns OFF,
and base voltage of Q901 decreases since the
pulse is not supplied to base of Q9. When