Akai VS-2EGN - Page 49

3-3-2 SPEED CONTROL
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The drum motor FG frequency is 625 Hz (NTSC: 750
Hz). The drum motor FG signal is shaped by the FG
AMP (IC4 1/2), and is fed to ICI pin (4.
The FG signal fed to IC1 pin (4 is amplified by the FG
AMP, and divided by 2 at the next flip-flop, XFF3.
Mono-Multi 5 is triggered at the trailing edge of the FF3
output pulse, and the delay time is determined by C17
and R22 connected to the control terminal pin (6) and
by the output voltage from the phase control circuit.
If the output voltage of the phase control circuit is con-
stant, the delay time of MMS is constant.
The output of this MMS and the output of FF3 inverted
at the inverter, and fed to GATE 3. Since the delay time
of MMS is constant, the output signal of GATE 3 hasa
pulse width corresponding to the speed error, GATE 3
output pulse is fed to TRAPEZOID 2, and the rise time
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Fig. 7-50 Drum Motor Speed Control Circuit
of the trapezoidal wave is controlled there. That is,
the peak value of the trapezoidal wave changes accord-
ing to the GATE 3 output pulse width.
The wider the pulse width, the longer the charging time
becomes, and as a result the peak value of the
trapezoidal es becomes high. Conversely, the narrow-
er the pulse width, the lower the peak value of the
trapezoidal wave becomes.
TRAPEZOID 2 is triggered at the trailing edge of FF4
output pulse.
The output of TRAPEZOID 2 is fed to S & H 2.
The sampling pulse is an output pulse of GATE 4 which
is generated from the MMS output pulse and the GATE
3 output pulse.
The output of S & H 2 is inverted at DC AMP (IC4),
and fed to the next drum motor drive circuit.
Service Manual VS-2EGN