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Theory of Operation— 2440 Service
DISPLAY OUTPUT
The Display Output circuits (diagram 18) convert the
current outputs from the Horizontal and Vertical digital-to-
analog converters (DACs) to the voltage levels used to
drive the crt deflection plates. The Display Output circuit
includes a vector-generation function that allows the indivi
dual dots of a waveform display to be translated into
smooth lines connecting the waveform points (vectors on).
A Display Mode switching circuit under control of the Sys
tem
nP
selects which type of signal is applied to the out
put amplifiers for the various display types (envelope, dots,
vectors, or readout).
V ertical and Horizontal Input Buffers
Operation of the Vertical and Horizontal Input Buffers is
identical; so for brevity, only the Vertical Input Buffer cir
cuit operation is described.
The Vertical Input Buffer, JFET operational amplifier
U170 and its associated components, translates the com
plementary output currents from the Vertical DAC (U142,
diagram 16) to an output voltage. Complementary, in this
case, means that the sum of the currents is a fixed value;
if one current increases, the other decreases by the same
amount.
Current from the Vertical DAC output connected to
pin 3 of U170 develops a voltage across R163. This volt
age causes the output of U 170 to move in the same direc
tion until the feedback current through R164 applies an
equal voltage to pin 2 of U170. The output voltage of the
Input Buffer at pin 6 is the (signed) sum of voltages across
R163 ( + ) and R164 ( - ) . The gain of the stage is 1 V per
mA (differential).
V ertical and Horizontal V e cto r G en erato rs
Operation of the Vertical and Horizontal Vector Genera
tors is similar. For brevity, only the Vertical Vector Genera
tor is described in detail, and the differences in the two
Vector Generators pointed out. Each Vector Generator
consists of a High-Current Difference Amplifier, a Sample-
and-Hold circuit, and an Integrator circuit that transforms
the step voltages output from the Sample-and-Hold circuit
to smooth transitions (vectors). See Figure 3-9 for a
simplified diagram.
The step transitions from Vertical Input Buffer U170 are
applied to the High-Current Difference Amplifier, made up
of U281, Q182, Q181, and associated components,
through R172. Initially (before the first integration occurs),
input pin 3 of U281 is referenced to ground through R161;
deviation from this ground reference seen at the other
OLD POSITION VOLTAGE
3 -7 3
Figure 3-9. Vertical Vector Generator.
Summary of Contents for 2440
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