GE 4PE350A3 Instructions Manual page 37

COLOR CAMERA
EBI-6351
impedance. The pulses are fed from the emit-
ter to J30, located under the registration
control panel, where they are mixed with
the video from the viewfinder switcher board
and fed into the 75 -ohm load of the monitor.
These monitor trigger pulses have a typical
amplitude of 0.5 to 1.0 volt peak -to -peak.
Viewfinder Video Amplifier Board,
PL -767268103
Refer
to
the Waveforms, Fig. 49, the
Printed Circuit Diagram, Fig. 50, and the
Schematic Diagram Fig. 51.
This video amplifier has been adjusted
for an overall voltage gain of 100. Should
greater gain be required for specific ap-
plications, it can be obtained by reducing
the value of the 01 emitter resistor, R3,
The input noncomposite video signal
is
a -c coupled to the base of Q1, which pro-
vides a voltage gain of approximately five
times.
The collector input of Q1 is d -c
coupled to the base of Q2, which provides
a high -impedance coupling to the video output
of Q1 and a low -impedance drive to the base
of Q3.
The low -impedance drive from the emitter
of Q3 is
a -c coupled to a feedback group
consisting of Q3, Q4, and Q5, which pro-
vides an overall voltage gain of approxi-
mately 20. Diode CR1 in the base circuit
of Q3 restores the d -c level to the video sig-
nal, thereby preventing changes in the back-
ground level
for
variations in the video
duty cycle. Also located in the base circuit
of Q3 is the BIAS ADJ control, R10, which
sets the bias of Q3 to avoid clipping the
video signal. Zener diode CR2 provides a
low -impedance emitter bias supply for Q3,
which maintains a relatively constant base -
to -emitter (base) potential without the use
of a bypass capacitor.
The video output signal at the collector of
Q5 is applied to the cathode of the picture
tube. A portion of the output signal is also
coupled through R13 and C10 and applied
to the emitter of Q3 as a feedback signal to
adjust gain and bandwidth of the output am-
plifier
group. C12 and C18 are used to
facilitate viewfinder peaking.
Viewfinder Sweep Board,
PL -767379762
Refer to the Waveforms, Figs. 52 through
55, the Component Diagram, Fig. 56, and the
Schematic Diagram, Fig. 57.
The 15,750 -hertz horizontal driving signal
from the drive converter board, PL -7674581,
is applied to the base of emitter follower Ql.
Phase adjustment of the viewfinder sweep
is performed externally to the board.
Q1 shapes
the
input sine wave into a
square wave, with the output being applied
to Q2, which amplifies the square wave and
drives Q3 and Q4 between saturation and
cutoff. CR1 is a diode clipper restricting
the peak of the horizontal input signal to a
specific reference level.
During the time interval when Q3 and Q4
are cut off, a damped sine wave is generated
by the resonant circuit, L2, C12, and C13,
with damper diode CR2 restricting the sig-
nal to positive excursions only.
This pulse is capacitively coupled to the
viewfinder yokes to produce a sawtooth cur-
rent. Because of the resistance inherent in
an inductor, the sawtooth has an exponential
characteristic which produces a nonlinear
display. The pulse voltage is sampled and
reformed by C14 and L4, which drives
Q5.
A sample of the output current from T3
is also applied to the base of Q5, further
shaping the driving pulse. The driving pulse
is amplified by Q5, Q6, and Q7, with the
correction waveform being added to the yoke
current through the secondary of T3.
The vertical drive signal is applied to the
base of Q12, which phase -inverts the signal
and biases Q13 on. When Q13 conducts, C24
and C25 discharge through the transistor,
developing a sawtooth voltage. Optimum lin-
earity is provided by the feedback stage, Q14.
The time constant of R41, C24, and C25 is
such that a linear charging slope, up to the
point of discharge, is obtained. Q15, Q16,
Q17, and Q18 provide additional amplification.
The output from the collector of Q17 and the
emitter of Q18 is applied to the vertical
yoke coil.
Because Q15, Q16, Q17, and Q18 are all
d -c coupled, centering current can be added
to the base of Q15. Centering current am -
23