Servo Circuits; Sync Separator And Pulse Amplifier; Waveshaper; Integrator And Gate - Sony AV-3600 Service Manual

output of the frequency doubler.
R329
and
C315
reduce high-frequency gain to
provide de-emphasis.
Video
Amplifiers-Q307 to
Q310
Video output from
the low-pass
filter is
amplified
by
Q307 and Q308 and then supplied to the video
output stages (to
Q309
for the
rf
adaptor
and
TV
jack
and
to
Q310
for
VIDEO
OUT.) R335
adjusts
the
video
output
level
and
is
adjusted to provide
1.0
V
(p-p)
at
the
output
terminals.
2-2.
SERVO
CIRCUITS
Basic principles
of the servo
and
pulse
system of
the
AV-3600
are
shown
in Fig. 2-1.
The
rotational speed and
angular position of the
heads are
controlled
by means
of a
magnetic-brake
servo
system. In
this
system the
head
table
is
belt-
driven at
a speed greater
than
30
rps
by
a
syn-
chronous
motor.
The
servo
system controls a
magnetic brake
that
slows the
head
table to precisely
30
rps.
Each
video head
begins to scan the tape about
10
H
(horizontal
lines)
before the
vertical
sync
interval (rf
signal)
supplied to the video
head and
ends
its
scan about 10
H
after
the next
vertical
sync
interval.
During
TV
record operations,
separated
60-Hz
sync signals
serve as
the timing reference for
the
servo.
The
arrival
of every other
60-Hz sync
pulse
is
compared
with
a
30-Hz
pulse
generated
by
30
PG
coil
B
in
the
Rotary-Head
Drum
Assembly. See
Fig.2-1.
The
comparator system of the servo controls
brake current to
maintain the correct
time
reference
between
the sync pulse
and
the
30
PG
pulses.
In
playback, the recorded control track pulses
serve as
the
reference for the servo system.
Sync Separator
and
Pulse Amplifier-Q402,
Q403
A
sample of composite video signal
from
Q104
on
the
V2
Board
is
fed to
Q402. Sync
is
separated
from the
composite video signal
by Q402, and
horizontal
sync pulses are
removed by
the following
integrator
circuit
(R424 through
R426, C412
through
C414).
The
separated vertical
sync pulse
is
amplified
by
Q403 and then
triggers
monostable
multivibrator
MM402.
During playback,
Q401
amplifies control
pulses
from the recorded
tape.
Frequency Divider-MM402,
R433
A
negative
sync
pulse
from Q403, coupled through
D404,
flips
the
monostable
multivibrator
MM402
(hybrid IC) into the
unstable
state.
Pulse duration,
determined by R433,
is
about
24
milliseconds,
long
enough
for the circuit
to ignore the next 60-Hz
sync
pulse.
Thus
the multivibrator runs
at
30-Hz and
is
triggered
by
every other pulse.
The
60-Hz
sync
pulse
is
fed to Pin 2 of
MM402,
and
the divided
30-Hz
pulse
is
obtained
at
the
same Pin
2.
Since the
sync
signal
is
referenced to the
30-Hz
signal
from
30
PG
coil
B,
60-Hz sync
pulses
must
be
halved.
Waveshaper-MM404,
R441
During playback, the output
from
MM402
is
supplied
to
monostable
multivibrator
MM404
via
SW7-4 and D406, and
triggers
MM404
(hybrid
IC).
This
multivibrator
shapes the 30-Hz
pulse
from
MM402
to
form 30-Hz
pulses
with
a
50%
duly
cycle
so that
a
reference
dc
level
may
be
obtained
for
the
following gate
circuit.
R441
sets
the
duty
cycle (pulse
duration).
When
the
TRACKING
control
is
ON
(in
playback)
the
output
of
MM403
triggers
MM404.
Integrator
and Gate-R446,
C426, C427,
Q405
The
output
from
MM404
is
integrated
by R446
and
C426
(or
C427
in
playback) to
form
the wave-
shape
shown
at
TP-410
of Fig. 2-1.
The
integrated
pulse
is
fed to the gate
circuit,
Q405. The
gate
is
turned
on by
the
30
PG
pulse
(from Q404)
applied
to
its
base.
For
the duration of
30
PG
pulse,
the
integrated pulse applied to the emitter of
Q405
is
sampled and
appears
at
the
collector,
and
charges
C432. Thus, the output of the
gate
is
a function
of
the
arrival
time of
the
30
PG
pulse.
The
integrated
pulse
is
gated
on
its
leading
edge
by
the
30
PG
pulse as
shown
in
TP-410
in Fig. 2-1.
If
the 30
PG
pulse
is
early (servo too
fast)
the output of
tlie
gate
is
less.
A
late
30
PG
pulse (servo too slow)
results in greater output.
During record
the in-
tegrated
30-Hz
pulse
rises
with
a short
time
con-
stant
(R446
x
C426)
as
shown by
the
solid
line
in
TP-410
in Fig. 2-1. Therefore,
a slight deviation
of
the
30
PG
pulse
from
its
stable position
on
the 30-Hz
integrated pulse
ramp
causes a
large
variation
in
ga/te
output
for the servo. This
results in
quick
servo
response,
and
also
minimizes
the variation of control
track
signals
recorded
on
the
tape.
In playback, the
integrated
30-Hz
pulse
rises
nth
a
faster
time
constant
(R446
x
C427)
as
shown
fc>y
the dotted
line in
TP-410
in Fig. 2-1.
A
deviation
of
30
PG
pulses (from
control track signals
on
the
taptf)
on
the
ramp
causes
less
of
a
variation
in
the ga*e
output
resulting in a
slower
servo response compared
23