Sony PS-X7 - Page 30

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2. Maintaining Forward Rotation (See Fig. 2-3)
1. When a® signal is applied to the input terminal
of the Hall elements, the motor is turned in the
forward direction, but when a © signal is
applied, the motor is turned in the reverse
direction. Therefore, the reverse rotation that
is caused by applying © input serves as a brake
for rapid stopping of the turntable.
(Conventional speed reduction systems use the
phenomenon of hysteresis loss or mechanical
loss.)
2. F-V converter circuit
This. circuit converts the frequency of the MG
head output signal into a voltage signal in
accordance with the curve as shown in Fig. 2-1.
y®
1
1
|
4
0
—— fout vO i
foc (33 rpm: 284 Hz}
Fig. 2-1. Fig. 2-2.
This curve shows ‘frequency against output
voltage’’ for both forward and reverse rotations.
3. DC amplifier (IC1-1) operation
The output level of IC1-1 is determined by the
relationship:
Vout =(V°In ~ V°In) x Gain,
or VouT = V® IN x Gain when Von is OV.
In this equipment, output levels of approx.
-1.5V to +13V are possible by applying ®
voltage to V°yn through Q207 and R223.
When the speed selector is instantly switched
from 45 rpm to 33 rpm, V®qjn becomes OV,
Von becomes positive, and Vout minus,
thereby activating the brake operation.
4. If the frequency of the MG head exceeds foc
during forward rotation, Vout becomes 0 or
minus, thereby slowing down rotational speed.
But during reverse rotation, the result is an
acceleration of the reverse rotational action.
5. In order to prevent such “run-away” accelera-
tion, a switching. circuit consisting of Q210,
Q209, and Q208 is employed, thus maintaining
forward rotation.
Normal Operation
1. For normal operation, the emitter of Q210 is
fixed to a level of -O.5V by D203.
Q205
Since the base potential of Q210 is normally
kept at about 1.5V, Q210 turns on.
Since the base of Q209 is -0.5V, Q209 turns
off. Q208 also turns off, and then Q207 turns
on. |
|
A ® signal is thereby applied to the © input
terminal of IC1-1 through R223.
ON + (OFF) ON — (OFF) OFF + (ON) ON-OFF)
Q207 Q208 Q209 Q210
tow
Conditions in brakes (
to reverse rotation case.
Fig. 2-3.
Reverse Rotation
1.
When operating too fast during reverse rotation
(when the frequency of the MG head output
signal exceeds fg,), Q205 is turned off and no
signal is obtained on the @ input terminal of
ICi-1. The output level of IC1-1 then drops
from the normal 1.5V (relative to ground) to
less than +0.5 V, resulting in Q210 being turned
off.
Consequently, the base voltage of Q209 rises in
accordance with the R231/C208 time constant,
finally turning Q209 on once the base potential
exceeds +0.5V.
Q208 then turns on, and Q207 turns off.
Consequently, no voltage is applied to the }
input terminal of IC1-1. ;
That is, the operation of IC1-1 will be stopped,
resulting in the output level reducing to OV,
and the slowing down of rotational speed.
The MG head output frequency consequently
drops below fog, resulting in a positive output
signal from the F-V converter, and forward
motor rotation. Once the ICl-1 output level
exceeds +0.5V, the reverse-rotation prevention
circuit is put into standby status.
This reverse rotation prevention circuit,can be
switched off again extremely rapidly. When
Q210 is turned on, C208 discharges through
R233 to the B- bus. i
1 and H2
Hall elements
) refer
Stroboscope
The stroboscope system employed in this equip-
ment consists of stroboscope stripes marked on the
outer rim of the turntable platter, plus a neon lamp
directed onto the outer rim stripes: The number of
stroboscope stripes for 331/;rpm and 45 rpm are
determined by the following equation:
. 60 (m + 2f)
where n= number of stapes
m-=the number of stripes to pass any one
point during 1 second,
f = power supply frequency (Hz),
and N = turntable speed (rpm).
Table 1
Number of stroboscope stripes
50 Hz 60 Hz
As can be seen from this table, the above
equation does not give an integral number of stripes
for the 45 rpm speed at 50 Hz.
Therefore, the strips of the stroboscope are
difficult to judge at 50Hz, 45 rpm: However, this
speed error can be ignored while a record is being
played. To eliminate this error, a 120Hz signal
corresponding to a 60 Hz power line is generated to
light the neon lamp.
Stroboscope Operation (See Fig. 2-5 and Fig. 2-6)
1. A signal of approximately 7.9 MHz is generated
by IC2 and the crystal oscillator, and then
divided within IC2. The output signal appearing
at terminal No. 1 of IC2 has a frequency of
3840 Hz.
7864320 Hz (2
Fig. 2-6.
| PS-X6/X7
This frequency is then fed into IC3 where it is
further divided. A 480Hz signal appears at
terminal No. 8, and a 240 Hz signal at terminal
No. 11.
The 240 Hz signal is divided again into a 120 Hz
signal by the flip-flop of IC4.
Although . stroboscope stripes designed with
smaller t/T ratios appear to be sharper to the
eye, they do require stronger illumination than
is normally available. The stripes marked on
the turntable platter of this equipment have
a t/T ratio corresponding to 13%.
Lee