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Theory of O peration—2440 Service
To produce a vector display of a waveform, the System
Processor selects the X I a n d Y l inputs of U290. This routes
the outputs from the Vertical and Horizontal Vector Genera
tors (previously described) to the Horizontal and Vertical
Output amplifiers.
For non-vector waveform displays, the X2 and Y2 inputs
are routed to the outputs of U290. These signal lines, V
DOTS and H DOTS, come directly from the output of the
Vertical and Horizontal Input Buffers (U170 and U370B),
bypassing the Vertical and Horizontal Vector Generators.
Since the data applied to the Horizontal DAC in YT mode is
from the incrementing Display Counter, the Y-Axis vertical
deflections are displayed versus a linear X-Axis ramp
(horizontal time axis). If XY mode is in effect, the data
applied to the Horizontal DAC is the digitized waveform data
used to provide the X-Axis deflection signal. In either YT
mode with vectors off or XY mode, a dot waveform display is
seen on the CRT.
To display readout, the H READO UT and V READOUT
signals at the Y3 and X3 inputs are switched to the outputs of
U290. The resistive divider formed by R171 and R282
slightly decreases the amplitude of the signal from the
Vertical DAC to ensure that all the Readout vertical data
points are limited to eight vertical graticule divisions and
will appear on screen. Operational amplifier U392B and its
associated resistors perform the opposite function on the H
READOUT signal from the Horizontal DAC, increasing the
gain of that signal. This horizontal expansion causes the
center 40 characters of a displayed readout line (out of a
possible 64) to horizontally fill the screen. (See the Readout
State Machine description for further details.)
H o rizo n ta l and V e rtica l O u tp ut
Operation and circuitry of the Horizontal and Vertical
Output are nearly identical. Therefore, only the Horizontal
Output circuit operation is described.
The selected horizontal signal from U290A is applied to
operational amplifier U392A configured with a variable gain
set by R586. (The corresponding buffer in the Vertical
Output has a slightly different variable gain range.) Opera
tional amplifier U392D is an inverting amplifier having a
gain of about two. Horizontal offset is adjusted with R587.
The output of U392D drives the negative horizontal-de
flection plate (H -) of the CRT and operational amplifier
U392C. Operational amplifier U392C is configured as an
inverting buffer with unity gain, and its output drives the
positive horizontal-deflection plate (H + ).
S p o t-W o b b le C o rre c tio n
The Spot-Wobble Correction circuit provides a dynamic
correction of spot-shift on the CRT caused by signal
intensity changes (CRT electron-beam current changes).
Correction is accomplished by injecting offsetting currents
that vary linearly with beam-current changes into the
Vertical and Horizontal Output.
The beam-current control voltage is inverted by U460A
and applied to one end of R583 and R584 while the other
end of both potentiometers is connected to the noninverted
control signal. Each potentiometer is adjusted over this
“differential” range to minimize the associated spot wobble
while viewing a special calibration display provided with the
Extended Calibration function.
HIGH-VOLTAGE SUPPLY AND CRT
The High-Voltage Supply and CRT circuit (diagram 19)
provides the voltage levels and control circuitry for opera
tion of the cathode-ray tube (CRT). The circuitry consists of
the HV Oscillator, the HV Regulator, t h e + 6 0 V Supply
( + 61 V
Supply
for
instruments
with
serial
num
bers B012402 and below), the Cathode Supply, the Anode
Multiplier, the DC Restorer, Focus and Z-Axis Amplifiers, the
Auto Focus buffer, the CRT, and the various CRT Control
Voltage Circuitry.
H V O s c illa to r
The HV Oscillator transforms power obtained from the
-1 5 V unregulated supply into the various AC levels
necessary for the operation of the CRT circuitry. The circuit
consists primarily of transformer T525 and switching
transistor
Q628
connected
in
a
power
oscillator
configuration. Sinusoidal low-voltage oscillations set up in
the pri mary winding of T525 are raised by transformer action
to high-voltage levels in the secondary windings. These AC
secondary voltages are applied to the + 6 0 V Supply
( + 61 V
Supply
for
instruments
with
serial
numbers B012402 and below), the DC Restorer, the
Cathode Supply, and the Anode Multiplier circuits that
provide the necessary CRT operating potentials.
Oscillation occurs due to the positive feedback from the
primary winding (pin 4 to pin 5) to the smaller base-drive
winding (pin 3 to pin 6) used to provide base drive to
switching transistor Q628. The frequency of oscillation is
approximately 50 kHz and is determined primarily by the
parallel resonance frequency of the transformer.
O S C ILLA TIO N START UP. Initially, when power is
applied, the HV Regulator circuit detects that the CRT
cathode voltage is too positive and pulls pin 3 of
transformer T525 negative. The negative level is applied to
the base of switching transistor Q628 through the
transformer winding and forward biases it. Charge begins to
flow in the primary winding through the transistor collector
circuit and produces a magnetic field around the
3-75
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