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Akai VS-2EGN - Page 40

Akai VS-2EGN
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3-2-2
SPEED
CONTROL
SERVO
12v
fo)
-OCUE
or
REVIEW:H
(CUE
or
REVIH
io}
SAMPLE
HOLD
1
REC
12V
TO
CAPSTAN
MOTOR
DRIVE
CIRCUIT
GAPSTAN
MOTER
SPEED
CONTROL
1)
Recording
and
playback
modes
The
capstan
motor
FG
frequency
is
400
Hz
(570
Hz
in
NTSC
system).
The
capstan
motor
FG
signal
is
shaped
to
a
square
wave
at
the
FG
AMP
(Schmitt
circuit)
of
TR3
and
TR4.
The
400
Hz
(570
Hz
for
NTSC
system)
pulse
shaped
at
FG
AMP
is
fed
to
the
analog
switch
IC10
pin
@Q.
Since
12V
is
supplied
to
the
analog
switch
control
ter-
minal
IC10
pin
12
in
REC
and
PB
modes,
this
analog
switch
is
ON.
Accordingly,
the
signal
fed
to
IC10
pin
0
is
supplied
to
IC11
pin
(6)
via
IC10
pin
().
The
FG
signal
fed
to
IC11
pin
is
amplified
at
FG
AMP,
and
its
frequency
is
divided
by
2
at
flip-flop
1
making
it
200
Hz
(285
Hz
in
NTSC
system).
Since
IC11
pin
@
is
at
the
high
level
except
in
the
ele-
ctronic
editing
mode,
flip
flop
2
does
not
operate,
and
200
Hz
is
fed
to
Mono-Multi
2.
Mono-Multi
2
is
triggered
at
the
leading
edge
of
this
pulse,
and
the
delay
time
is
determined
by
the
time
Fig.
7-38
Capstan
motor
speed
control
circuit
constant
of
C49,
R81,
and
VR8
connected
to
the
con-
trol
terminal
pin
@
and
by
the
output
voltage
of
the
phase
control
circuit.
If
the
output
voltage
of
the
phase
control
circuit
is
constant,
the
delay
time
of
Mono-Multi
will
be
constant.
The
output
of
this
Mono-Multi
2
and
the
inverted
out-
put
of
flip-flop
2
are
fed
to
GATE.
Since
the
delay
time
of:
this
Mono-Multi
2
is
constant,
a
signal
(GATE
out-
put)
whose
pulse
width
corresponds
to
the
speed
error,
is
output.
The
GATE
output
pulse
fed
to
TRAPEZOID
controls
the
rise
time
of
the
trapezoidal
wave.
That
is,
the
peak
value
of
the
trapezoidal
wave
varies
accord-
ing
to
the
pulse
width
of
the
GATE
output,
and
the
wider
the
pulse
width,
the
longer
the
charge
time
be-
comes.
This
raises
the
peak
value
of
the
trapezoidal
wave.
Conversely,
as
the
pulse
width
becomes
narrower,
the
peak
value
of
the
trapezoidal
wave
becomes
lower.
Charge
time
constant
of
the
trapezoidal
wave
is
deter-
mined
by
R86,
R87,
and
C50
connected
to
pin(4).
Service
Manual
VS-2EGN
TRAPEZOID
2
is
triggered
at
the
trailing
edge
of
the
output
of
flip-flop
3.
The
output
of
TRAPEZOID
2
is
supplied
to
S &
H
2.
During
the
period
while
the
sampling
pulse
is
‘H’,
ie.,
throughout
the
top
part
of
the
trapezoidal
wave,
the
pulse
is
sampled,
and
an
error
voltage
is
produced.
In
contrast
to
the
method
when
the
sampling
pulse
is
generated
using
a
capacitor,
in
a
differential
circuit,
for
example
the
width
of
the
sampling
pulse
does
not
change.
Let’s
assume
the
case
when
the
rotation
of
the
capstan
motor
has
become
fast.
As
the
speed
of
the
capstan
motor
increases,
the
frequency
of
the
capstan
motor
FG
becomes
high,
the
frequency
divided
at
flip-flop
1
be-
comes
high,
and
the
period
of
the
output
pulse
becomes
short.
37

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