Sony PS-X7 - Page 28

PS-X6/X7
1. Operation of Servo Amplifier Circuit (See Fig. 1-1)
During Motor Start
1. When the power switch is turned on, Q202 is
still turned off since an output signal from the
MG head is not applied to IC1-2. C204 will
thus charge up through R210, R209 and RV201
(or R208 and RV201 for 45 rpm).
2. Once the emitter voltage of Q203 (charging
voltage applied to the base of Q203) exceeds
+Vpp, this transistor turns on.
3. Q205 is thus turned on, thereby applying a
voltage to ICI-1.
Limiter Circuit
1. Once the turntable starts to rotate, an output
signal is produced by the MG head.
Although the MG head output signal is a sine
wave, there is some fluctuation in amplitude.
Therefore, above a certain level, this sine wave
is changed into a constant-amplitude square
wave.
Waveform Rectification
1. The IC1-2 output signal is amplified by Q201
to obtain a complete square wave signal. The
D201 is employed to maintain the base voltage
of Q201 at £0.6Vp-p.
to terminal No.1 of
1C2(phase control)
201 2scexza 0202 2SCE33A
2. This limiter circuit (ICi-2, Q201) serves to
prevent incorrect detection of amplitude fluct-
uations as frequency changes.
3. The square wave signal on the collector of Q201
is then applied to the phase control circuit.
Differential Circuit
The square wave signal on the collector of Q201
is changed to a triangular wave by C203, R206 and
R207. The differentiated (triangular) pulse triggers
the sawtooth wave generator transistor (Q202).
Sawtooth Wave Generator Circuit
1. While Q202 is turned off, C204 is charged up
through R210, R209 and RV201 (or R208 and
RV201 for 45 rpm). But when the positive part
of the differentiated wave turns Q202 on,
C204 discharges immediately, resulting in the
collector voltage of Q202 dropping to OV.
2. After the © portion of this differentiated wave
has passed on, Q202 is again turned off, and
C204 is charged up, resulting in the Q203 base
voltage forming a sawtooth wave signal.
Comparator
1. The sawtooth wave signal is applied to the
comparator circuit formed by Q203 and Q204.
The base of Q204 is biased by the reference
voltage produced by voltage division of B+ by.
the output from R213, R214, R215 and the
phase control circuit.
from phase
control circuit
Q207
284677
switching
Q205
2SA677
buffer amp
irlint
amp
(sawtooth
wave
waveform
shaper}
generator)
IC1-2 pPC324C
MGH
rotational speed
detection head
7
Q205
To hall element
Hi, H2
oe eee) ee oe
reference voltage
Fig. 1-1.