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FOCUS
COIL
GLASS FACEPLATE
CONDUCTIVE
COATING
(SIGNAL
ELECTRODE)
BSS
BSS
nd
WLLL
LIL
haga
RL
APPROX. 50VDC
CATHODE
12.6V (M-3016)
6.3V (7262A)
= —=—-|NDEX PIN
HEATER
Gi (BEAM) 0~ —100V
FIG. 2-2.
VIDICON
Light reflected
from the scene
to be televised
is
focused optically on the target area by the lens sys-
tem. As the moving election beam strikes a point of
light on the target area, a larger current
can flow
through the cathode-target circuit.
In this way,
vary-
ing light values cause varying currents to flow in the
target circuit. The cathode, electron beam, target and
load resistor constitute a series
circuit in which the
light-controlled signal current flows. The voltage de-
veloped
across
the
load resistor
is amplified
and
combined with blanking and synchronizing signals to
form the composite video output signal of the camera.
2-3. VIDEO CIRCUITS
Refer.to the Block Diagram, Fig. 2-3, and the fold-out
schematic
diagram.
Output
from the vidicon
is de-
veloped
across
the
load
resistor
R3
and
coupled
through Cl and LI] to the first video amplifier. The
L1/R4 circuit combines
the total output capacitance
of the vidicon and the input capacitance of X1 to form
a frequency compensation
circuit that partially com-
pensates
for the loss of high video -frequencies that
is inherent in coupling the high impedance vidicon to
the low input impedance
of the transistor amplifier.
Transistors
X1 and X2 form a feedback pair with dc
feedback
to ensure
circuit stability
and
wide
fre-
quency response: Partial emitter peaking (C7) is used
in the emitter circuit of X2. The output of X2 feeds
through C8 to the base of X3. This transistor employs
shunt peaking (L2) in its collector circuit to provide
high frequency boost and emitter peaking (C10/VR1)
to boost
low-to-mid
video
frequencies.
Transistors
X4,
X5,
and
X6
form
a direct-coupled
group in a NPN/PNP/NPN
configuration. Transistor
X10, in the base circuit of X6 performs the ''cleaner''
operation.
It is driven
into saturation
by blanking
pulses to eliminate noise in the set-up level that may
be generated in the early video stages.
The coupling circuit between X6 and X7 contains the
keyed clamp X12. This clamp is keyed into conduc-
tion by syne pulses and clamps the tips of blanking
2-2
pulses
to the voltage picked
off at the junction of
R30, R29 and VR-5. The latter is adjustable to con-
trol the set-up level of the composite
video output
signal.
Transistor
X7 is direct-coupled
to X8, the emitter
follower that drives the video output stages. The out-
put circuit for X8 contains a diode clipper circuit that
conducts and clips off positive signal excursions that
exceed the voltage picked off at the arm of VR-3: This
is the white-clip adjustment. It is set to limit white
peaks so that the r-f modulator will not be overdriven
during r-f operation.
The output of X8 drives the bases of X9 and X11 in
parallel. Transistor X9 is the video output stage for
the camera; X11 is the video output stage that sup-
plies signal to the CVF-4 Viewfinder.
Sync is added to the video output at the emitter of X9
through
the SYNC-LEVEL
control
VR-4.
The
com-
posite video is then routed to the VTR connector, the
UHF
connector or the RF Modulator through the se-
lector switch $2. The output of X11 is routed directly
to pin 5 of the Viewfinder connector. Note that sync
is not added (not needed) to this feed.
Automatic
sensitivity control is achieved
by making
the d-c target voltage a function of the average-white
video
signal. A sample
of the video signal at the
emitter of X6 is fed to X13 and thence to a voltage
doubler
consisting
of C35,
D3
and D4. Transistor
X13 prevents the rectifier from loading the video cir-
cuits.
The output of the rectifier is a positive dc
voltage that is proportional to the average white com-
ponent of the video signal. This de voltage controls
the conduction of X14. The collector voltage for X14
is also the target voltage for the vidicon, since there
is little de voltage drop across resistors R2 and R3.
These
resistors,
in conjunction
with
C39
and C2
decouple
the target voltage supply and prolong the
time constant
of
the feedback
system
to
prevent
oscillation.
An increase
in light input results in greater output
from the D3/D4 rectifier, increased conduction of X14
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