Sony PS-2250 - Page 5

Stage/Control
Buffer/phase
inverter
Q09, Q010,
Q011
Low pass filter
/buffer
amplifier
De amplifiers
Q4, QS,
Q6
Function
Q09 and QO11 is an emitter follower
but Q010 acts as a phase inverter
providing a positive pulsating signal to
the following stages.
Buffer amplifier Q3 and an RC network
consisting of R15, C11, R16, C12, C13
R17 and C14 comprise a low-pass
filter having a sharp rolloff charac-
teristic. .
Notice that this stage acts as an
integrator, converting the input positive
pulses into a dc voltage proportional to
the input pulse width.
De output from the low-pass filter is
applied to the base of Q4. As Q4, Q5
and Q6 are directly coupled, ‘a change
in input dc voltage alters the con-
duction of Q6, controlling the voltage
applied to the motor.
_ Servo Operation
When, by any cause, the motor speed
becomes slightly faster or slower than
the specified value, the servo system
works as follows:
Referring to Fig. 1-3, assume that the
motor speed becomes faster. The FG
output signal frequency becomes
higher, resulting in a shorter interval
between pulses for triggering the saw-
tooth wave generator. The shorter
—_ 1 1 - ;
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ee
When motor speed becomes faster.
Stage/Control
Power supply
D8, D9
C17,C19
D7
Speed
selector
switch S1
normal
—— |
i —
When motor speed becomes slower.
Function
interval between trigger pulses causes
lower saw-tooth wave height, which in
turn yeilds a shorter “ON”? period for
comparator QO7. Therefore, the output
pulse width at the emitter circuit of
QO11 becomes shorter, reducing the
positive bias upon .Q4.
As a result, the collector-emitter
impedance of Q6 increases, reducing
the motor speed.
Conversely, if the motor speed becomes
slower, the collector-emitter impedance
of Q6 decreases, increasing the motor
speed.
A positive 12 volts for the system is
provided by the full-wave rectifier
consisting of D8 and D9, filter ca-
pacitors C19, C17 and zener diode D7.
Speed changeover operation is per-
formed by changing the saw-tooth wave
frequency as previously described.
Since the saw-tooth wave frequency is
determined by the RC time constant
in the collector circuit of Q06, a speed
selector switch is connected in parallel
with VR2 and R10. A smaller time
constant results in faster motor speed
and vice versa. So S1 is open when the
speed selector switch is. set to
33-1/3 rpm.
slower
FG output signal
collector of QO2
pase of Q06
collector of Q06
collector of Q07
base of Q3
base of Q6
Fig. 1-3. Waveforms on servo control circuit