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EBI-3641
CHANNEL AMPLIFIER
aperture corrected video signal from both the plate and cathode
of
V3A
is
mixed through
the time constant
of
either
R33 and C25, or
R33 and
C26.
White overshoots
are
eliminated from the resulting phase corrected video
signal.
Relay
Kl
determines the polarity
of
the input
signal
to V4,
providing
either
a
black negative
or
black positive video signal.
This video reversal
feature
is
necessary when negative film
is
used.
This relay
is
deenergized
when black negative video
is
used.
V3B
is a
cathode follower.
V4
is a
shunt peaked amplifier.
The cathode resistor
is
shunted by
different values
of
capacity depending
on the
position
of
the CABLE COMPen-
sation
switch,
S2.
This alters the
frequency response
in
such
a
way
as
to
compensate for the loss
of
high frequencies
due to the common cable attenuation.
V5
is a
shunt
peaked amplifier whose output
drives cathode
follower
V6A through
C39.
V6B
is a
voltage regulator used
to
furnish
a
regulator voltage for
V6A.
The grid of
V6A
is
clamped by diodes
CRIA
and
CR6,
which are keyed
with pulses from
the clamp keyer, V13.
CLIPPER
The
first half
of
clipper stage V7
is the
white clipper with WHITE
CLIPPER ADJ R65 provided for level adjustment.
CR10
is
used
as a
transient
suppressor.
The
second half
of V7
is
the
black clipper with CR11 in series
to
lower
the
capacity across the clipper.
CR16
is
used
to
protect CR11 by
not allowing the voltage
to
rise above ground.
Blanking pulses
are added to the clipper stage
by
V12B.
GAMMA CORRECTION
CR12 and CR14 are
used for gamma correction.
When GAMMA ON-OFF switch
S3 is
placed
in the
ON position and relay Kl
is
deenergized (black negative
video output), both diodes turn
on,
but
at
different levels.
Diode CR14
is
biased with
a
higher positive voltage than
CR12 and re-
quires
a
higher signal level
to
turn
it on.
Until the first diode turns
on,
the clipper load, R67,
is
1000 ohms.
As the level increases above the black
level, diode CR12 conducts,
placing
510 -ohm resistor R68 in parallel with R67
(neglecting the forward resistance
of
the diode).
As
a
higher voltage
is
approached, diode CR14 will conduct placing another 510 -ohm resistor,
R74,
in
parallel with R67 and
R68.
The resulting signal waveshape
is
shown
in Fig. 10.
With
the
GAMMA
ON
-Oki/
switch,
S3, set to
the OFF position, diode CR12
will conduct and CR14 will be
cut off.
This makes the clipper load equal
to
the effective load
at
the
white level; therefore, the gain for GAMMA ON
and GAMMA OFF
is
equal.
It
is
possible, therefore,
to change
from
one
position
to
the other
with negligible change
in level.
8
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