Akai VS-2EGN - Page 47

In the recording mode, the reference signal used for the
servo differs according to whether or not an input video
signal is present.
When the input video signal is present, the vertical SYNC
signal resulting from SYNC separation of the input video
signal by the SYNC separation circuit (SYNC SEP), be-
comes the servo reference signal.
When the input video signal is not present, 50.0002 Hz
(PAL/SECAM) or 59.9468 Hz (NTSC) is used as the
reference signal.
50.0002 Hz is obtained by dividing down the 4.433618
MHz oscillation of the APC local oscillator circuit by a
factor of 88672.
59.9468 Hz is obtained by dividing down the 3.57954
MHz oscillation by a factor of 59712 in the case of
NTSC.
The reference signal fed to IC1 (AN6350) pins () and
@ is amplified by the VSS (Vertical SYNC signal)
AMP, and is then fed to Mono-Multi 4 and flip-flop 2.
The delay time of Mono-Multi 4 is determined by C3
and R2 connected to the control terminal (pin @).
The output of this Mono-Multi 4 is amplified by the
CTL REC AMP. The amplified CTL signal is output
to pin © through the control REC/PB switch, and
supplied to the CTL head.
The contro] REC/PB switch is transferred by REC 12V
to be supplied to pin 8.
On the other hand, the reference signal fed to flip-
flop 2. is divided by 2 by flip-flop 2 (25 Hz for PAL/
SECAM system, 30 Hz for NTSC system), and fed to
TRAPEZOID, where it triggers the TRAPEZOID at the
leading edge. A trapezoidal wave is formed at TRAPE-
ZOID. The slope time of the trapezoidal wave is deter-
mined by C14 connected to pin (1), while the height
of the trapezoidal wave is determined by R14 connect-
ed to pin 5
This trapezoidal wave is supplied to the S & H 1 cir-
cuit as a reference signal.
Service Manual VS-2EGN
44
Now, the comparison signal circuit will be described.
The PG pulse from the drum pick-up head is amplified
at each amplifier + PG AMP and —PG AMP, which have
positive and negative clamp effects respectively, and is
picked up as alternating pulse trains of positive direc-
tion negative direction pulses. The alternating pulse
trains are fed to the trigger Schmitt circuit, which
separates the positive and negative alternating pulses.
The separated pulse signals is fed to the switching point
adjustment Mono-Multi circuit (MM1, MM2), which
delays each signal.
The delay time of this Mono-Multi is determined by the
capacitor and resistor (including the switching point
volume control) connected to control terminal pins
and @ :
MM1 and MM2 are designed so as to discharge at con-
stant current. Each signal delayed by MM1 (MM2) trig-
gers flip-flop 1 (FF 1), and generates the switching pulse.
The MMI output pulse is fed to the GATE | circuit, and
the trigger pulse is generated.
This pulse is fed to MM3 which follows. The delay time
of MM3 is determined by VR4, R13, and C13 connected
to the control terminal (pin © ). MM3 output and the
pulse of control terminal (pin (9) ) are fed to GATE 2,
and the sampling pulse is generated.
S & H 1 circuit is ON during the positive period of this
pulse, and C16 is charged with the voltage of trapezoidal
wave during this time. The S & H output voltage con-
nected output to IC] pin ® is fed to the speed cont-
rol circuit via the analog switch (IC2). The drum motor
is phase-controlled by this voltage.