Theory of O peration-60a
Z PR EAM PLl F I E R
Input signals are applied to J501 ( +Z INPUT).
Provisions are made on the input line to permit
installation of an attenuating resistor and to change
the input impedance (see Z-Axis Input Attenuation
Selection in Section 3, Installation).
The Z Preamplifier employs a matched pair of FETs to
provide a high input impedance and temperature
stability. This stage consists of two identical and
inverting feedback amplifiers, 0520A-0530-0534 and
05208-0630-Q634, which operate as a paraphase
amplifier. A push-pull signal is produced at the
collectors of 0534 and Q634. The FET gates are
diode-clamped on negative-going overdrive signals to
protect the field-effect transistors from excessive input
voltages. Potentiometer R525 (Z Gain) allows an
adjustable amplification factor to provide maximum crt
grid drive when a signal of at least +1 volt to +5 volts
is applied to the +Z INPUT connector, and R547
(INTENSITY) is set to about midrange. Under this
condition, a zero-volt input cuts off the display
intensity to below the visible level.
I NTE N S ITY AN D
LlMITER
Display intensity is varied by front-panel control R547
( INTENSITY) through the current sources of 0542 and
0642. The push-pull signals from the Z Preamplifier
are applied to the Z Output Amplifier after being offset
by this stage, Diodes CR565 and CR540 prevent
overdriving the Z Output Amplifier by limiting the Z
Preamplifier signals to within 3.4 volts of each other.
Z OUTPUT
AMPLIFIER
T h e push-pull signals from the Z Preamplifier stage
are applied to the bases of 0570 and 0670. These
two transistors are voltage-to-current converters
which, with 0576, produce a single-ended current
signal.
Transistors 0580-0680-0590-0690-Q596-0696 are
connected as an operational amplifier, with the
feedback path provided through R692. H igh-current,
low-impedance drive is provided by the complementary
configuration of emitter followers 0580-Q680. The dc
and low-frequency signals from 0580-0680 are
coupled through R584 to the base of 0690; the high
frequency signals are capacitively coupled through
C 5 85 and C586 to the base of Q590. Complementary
amplifiers 0590-0690 provide the final gain for the Z
output signals, with emitter followers 0596-0696
supplying the high current necessary to drive the
capacitive load. High-frequency compensation is
provided by C693-R693 ( H F Comp).
DYNAM I C FOCUS
�
The Dynamic Focus circuit provides focus correction as
the crt beam is deflected to the edges of the display
area in both the vertical (Y) and horizontal
(X)
axis. A
schematic diagram of the Dynam ic Focus circuit is
shown in diagram 4. A detailed block diagram,
showing the m ajor stages of this circuit, is
superimposed on the schematic diagram with wide
shaded lines. The stage names (given as sub-headings
in the following discussion) can be found in the
shaded boxes on diagram 4.
Geometric defocusing, a contributing factor to overall
cft defocusing, occurs when the beam is deflected
from the crt center. The electron beam, at center
screen, is focused for a particular beam length. When
the beam is deflected, either vertically or horizontally,
the beam length changes. H owever, the focusing
voltage remains the same. As a result, the display is
defocused, appearing larger at the edges of the screen
then at crt center (see Fig. 4-2).
The Dynamic Focus circuit varies the voltage to the
focus element of the crt depending upon the vertical
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2025-34
Figure
4- 2.
Simplified illustration of geometric defocusing.
4-4
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