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EBI-6351
COLOR CAMERA
plification is accomplished concurrently with
signal amplification.
The blanking signals are obtained by apply-
ing the vertical signal to the base of Q19
and the horizontal signal to the base of Q22.
Both
signals are amplified and mixed in
stages Q20 and Q21, providing a composite
blanking output signal. The amplitude of the
viewfinder sweep blanking signal is at least
55 volts peak -to -peak of mixed horizontal
and vertical blanking.
The circuit is so arranged that if one or
both sweeps are missing, the viewfinder
picture tube
is
cut off to prevent damage
to the viewfinder tube.
Viewfinder High -Voltage Power Supply
Board, PL -7781684G2
Refer to the Component Board Diagram,
Fig. 58, and the Schematic Diagram, Fig. 59.
The power supply is designed to provide
regulated 16 kilovolts for the Camera view-
finder from a 15,750 -hertz sine wave input.
The sine -wave input is applied to the base
of Q3, which, in conjunction with Q4 and Q5,
amplifies the signal. The primary of T2,
the collector load for Q4 and Q5, is tuned
to resonate at 15,750 -hertz per second for
maximum efficiency. T2 is a step-up trans-
former, stepping the 15,750 -hertz signal to
approximately two kilovolts peak -to -peak.
The rectifier voltage multiplier arrangement
provides
a multiplying factor of eight to
increase the two kilovolts to 16 kilovolts.
The emitters of Q4 and Q5 are returned
to ground through Q2. By controlling the
current through Q2, the gain and output
voltage of sine -wave amplifiers Q3, Q4, and
Q5 can be controlled. A portion of the output
signal is
rectified by CR2 and CR10, then
applied to the base of Q1 to control the
drive voltage on the base of Q2. Any change
in the output voltage provides compensation
of the drive voltage on Q4 and Q5 to cor-
rect for the change in the output voltage.
Iris Control Servo Amplifier Board,
PL -7782027G2
Refer to the Printed Circuit. Diagram, Fig.
60, and the Schematic Diagram, Fig. 61.
24
The
iris
control
servo amplifier is
a
high -gain d -c amplifier used in a closed
loop
circuit to position the Camera lens
iris. Position sensing is accomplished by
a
differential amplifier, Q1 and Q2, used in
conjunction with two external potentiometers.
These potentiometers are the EXPosure con-
trol, which is the Camera operator's control,
and the IRIS follow-up. Adjustment of the
EXPosure control causes the base voltage
of Q1 to change, and this change appears
as a voltage of the same polarity at the
collector of Q2. After buffering in an emitter
follower, Q3, this voltage is amplified and
inverted by Q4 and passes to another emit-
ter -follower buffer, Q5, and then to the out-
put stages, Q6 and Q7. The polarity of the
voltage appearing at the emitter of Q5 de-
termines which output transistor turns on
and, consequently, what polarity voltage ap-
pears at the point feeding the servo motor.
Potentiometers R2 and R5 are used to set
the limits of the two external potentiometers.
R7 is the gain control.
The two EXPosure control potentiometers
used in this system are
con-
venient operation at two points: one on the
local Camera control panel and the other
at the remote control desk. The selection
of the operating point at each location is
accomplished
with
illuminated push -push
switches which operate an external relay
to switch the desired control into the cir-
cuit.
Iris Control Servo Power Supply Board,
PL -7672613
Refer
to the Component and Schematic
Diagram, Fig. 62.
This dual power supply is designed to pro-
vide regulated positive and negative 20 volts
d -c to the servo amplifier.
The negative section, through diodes CR1
and CR2, develops -31.5 volts d -c. This
voltage is applied to the collector of the
series regulator, Ql. The collector current
for Q2 and the base voltage for Q1 are
furnished by R1 and R2. Since the base cur-
rent of Q1 is of low level, the base voltage
of Q1 is determined primarily by the col-
lector current of Q2, which in turn, is de-
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