Circuit Description; Video Circuits; Video Amplifiers; Modulator - Sony AV-3600 Service Manual

SECTION
2
CIRCUIT
DESCRIPTION
Refer to the
block diagram
on
page
2-7 and the
schematic diagram
on
page 9-15.
2-1.
VIDEO CIRCUITS
Video
information
is
recorded
on
the tape
in
the
form
of a
frequency-modulated
carrier.
The
video
circuits
on
the
VI
and
V2
Boards process the video
signal
during record
and playback
operations.
In the record
mode
the video input
is
supplied to the
V2 Board, where
it
is
gain-controlled automatically,
clamped, pre-emphasized, white-clipped, dark-clipped,
fm modulated, and
supplied to
the record amplifier.
The video output
signal
from the record amplifier
is
supplied to the
video heads through the
slip
rings
and
is
recorded
on
the tape.
In
the playback
mode
the recorded
signals
from
the
rotary
heads
are
fed to the
playback preamplifier
on
the
V2
Board.
The
amplified video
outputs
are
mixed
together
and these
continuous
fm
signals are
fed
to
the
VI
Board.
The
circuits
located
on
the
VI
Board include a
limiter, differentiator, rectifier
and demodulator.
The
video
output from the
VI
Board
is
a
reproduction
of the original
video
signal.
Video
Input
PIN 4
of the
TV
(8-pin)
jack for
TV
recording.
PIN
1
of
the
CAMERA
(6-pin) jack for
camera
recording.
UHF
connector
for
LINE
(auxiliary)
video input.
Video Amplifiers
andAGC-QlOl
to
Q104
The incoming
video signal
is
supplied to the
first
video amplifier
Q102
through
a
pre-emphasis
circuit
consisting
of
R
101 and
CI
01.
The
input
signal level
for
Q102
is
controlled
by
AGC
amplifier
Q101
(FET), without
manual
video
level
control.
Q102
and
Q103 amplify the video
signal
and feed
it
to
Q104.
A
sample of the video signal at
the emitter
of
Q104
is
fed to the
peak
detector
consisting of
CI
08,
D101, and
D102. The output of the
rectifier
is a positive
dc voltage that
is
proportional to the
peak white
component of the video
signal.
This
dc
voltage
controls the
conduction of
AGC
amplifier
Q101
and
this
controls the
amount
"of
input
signal
to
Q
102.
R
101 and
C101 accomplish pre-emphasis
so
that the
AGC
circuit
is
effective for high
frequency
signals.
Low
Pass Filter-L102,
L103,
CI 13
The
low-pass
filter
rejects
the video
signals at
the
high
end of the band, such
as
the 3.58
MHz
color
subcarrier
and
the
4.5
MHz
audio
i-f
signal.
Deviation Setting
and Clamp-Q105, R121, D103,
R126
The
output of the low pass
filter is
supplied to
Q105 through R121.
Rl
2
1
sets
the video
signal level to
Q
1
05 so that
the
following
fm
modulator produces an
fm
signal
of 1.4
MHz
deviation,
from
3.2
MHz
to 4.6
MHz. D103
clamps the sync
tip
of the composite video
signal
to the reference voltage picked off
at
the
arm
of
R126.
This reference voltage
sets
the
sync
tip (or
no-
signal)
frequency
of the
fm
modulator.
R
126
is set
to
establish the
sync
tip carrier
frequency of 3.2
MHz.
Pre-emphasis
and
amplifiers-Ql
06,
Q107,
Rl
30,
R131,
C118
The dc clamped
video signal
is
applied
through
Q106
to the pre-emphasis
circuit consisting
of R130,
R131, and CI
18 to improve the signal-to-noise ratio
of the video information,
and then
to
grounded-base
amplifier
Q107.
Base-bias voltage for
Q107
is
sup-
plied
from
the
arm
of
R126.
White
Clipper,
Dark
Clipper,
and Mod. Driver-
Q1Q8,
D104
y
R137, D107,
R155
The pre-emphasized
video signal
contains
over-
shoot
in
both the
positive
and negative directions.
D104
limits
the
maximum
positive
signal
(wkite
peaks) of
Q
107 to the voltage picked
off at
the
arm
of
R137.
D107
clips negative spikes.
Video from
D104
is
fed to the
modulator through modulator
driver
Q108.
Modulator
~Q1
09,
QUO
The modulator
is
a
free-running
symmetrical
multivibrator.
Frequency
control
is
achieved
"fcy
returning
both
bases to the
low-impedance
source
of
modulating voltage-the
emitter of
Ql OS-
through
R140. Frequency
varies
from the
tip-
of-sync value of
3.2
MHz
to a
maximum
4.6
MHz
for
peak white
signals.
R148
and CI 22 are adjusted
to
obtain
a
symmetrical output waveform (equal
pulse
durations and
slopes for
each
half cycle).
Waveform symmetry
is
important
as
it
determines
2-1