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Theory of Operation— 2440 Service
Selecting an input to pass through multiplexer U811 is
done by two active input signals, BRIGHTZ and RO. (The
third select input is a permanent LO, so one of the first
four inputs only can be selected.) For normal-intensity
waveform displays, all select bits will be LO to select
input 0 to switch through U811. If the waveform display
should be intensified at any time, the BRIGHTZ input will
go HI, selecting input 1. When readout is to be displayed,
the RO input will go HI, selecting either input 3 or input 4,
depending on the setting of the BRIGHTZ bit. Since inputs
3 and 4 are both connected to the INT-RO (readout inten
sity) control voltage level, the readout displays are not
intensified.
The selected intensity control voltage is applied to
U810B, configured as an inverting buffer with a gain of
- 1 . The output voltage is offset - 4 . 0 6 V by the voltage
divider at pins 3 and 5 of U810 (R814 and R815) and
resistor R816 at pin 6. The resulting inverted and shifted
output is converted to a current by R812 and applied to
the emitter of Q 810.
The circuitry of operational amplifier U810A and transis
tor Q810 is arranged so that the transistor is on with its
emitter held at - 2 . 7 V. The - 2 . 7 V level at the emitter is
set by the bias on input pin 3 of operational amplifier
U810A. The voltage developed at the output of U810B
causes a current to flow in R812 and sets the current
drive level for the Z-Axis circuit (diagram 19). This Z-INT
drive current supplied via U812E from pin 14 may vary
from 0 mA to 4 mA ( - 1 . 3 6 V to + 1 .3 6 V respectively at
the output pin of multiplexer U811).
When the intensity of the selected display is at
minimum, the output control voltage from multiplexer U811
will be below - 1 . 3 6 V. This causes the output of U810B
to go to approximately —2.7 V, reducing the emitter
current to Q810 to approximately zero. Diode CR810 limits
the reverse-bias voltage across the base-emitter junction
of Q810 to about 0.6 volts and protects the base-emitter
junction from excessive voltage.
Automatic compensation of the Z-Axis Amplifier gain is
carried out in five-transistor array U812. Transistors
U812B and U812C form the bias network for U812D,
one-half of the Z-Drive compensation amplifier. Biasing for
the other transistor of the differential pair is supplied by
U812A, R817, and a resistor internal to the Z-Axis
Amplifier that is tied to the + 5 V D supply. The differential
amplifier pair is biased so that the total current is divided
between the two sides. The resistance value of the internal
resistor in the Z-Axis Amplifier is an indication of the gain
of that device. Changes in that value that occur between
different Z-Axis Amplifiers shift the biasing level of U812E
to either increase or decrease the share of the total
current through that transistor by a small amount. The
change in current is in the appropriate direction to make
the display intensity of different instruments comparable
with exactly the same Intensity control settings. Capacitor
C817 bypasses high-frequency noise present on the
ZGAIN signal line.
The SPOTW OB (spot wobble) signal line, at the output
of Operational Amplifier U810B, picks off the various inten
sity levels. Those levels are used in the Horizontal and
Vertical Output Amplifiers (diagram 18) to dynamically
correct
intensity-related
position
shifts
on
the
crt
(described in the Display Output circuitry discussion).
G raticule Illum ination
The Graticule Illumination circuit, composed of U820A,
U520G, and associated components, sets the brightness
of the three lamps used to light up the graticule lines
etched on the crt faceplate.
Operational amplifier U820A is configured as an invert
ing integrator. Inverting buffer U520G may be thought of
simply as an open-collector transistor following operational
amplifier U820. The circuit appears this way because the
negative feedback around the loop via U820 and voltage
divider R 824-R825 keeps U520G in its linear operating
range. Gain around the loop (11) is set by the ratio of
R822 to R823 plus 1. The DAC control voltage applied to
pin 2 of U820A causes the integrator output to slowly
ramp in the opposite direction. This output is inverted by
U520G, and it sets the current in the graticule lamps.
Between DAC-updates no integration takes place, and the
charge held on C822 holds the output of the inverting
buffer, and thereby the graticule lighting, constant.
A uxiliary Front Panel
The Auxiliary Front Panel circuitry provides a means of
reading the front-panel bezel push buttons, located directly
below the crt, as well as several analog voltages associ
ated with the front-panel BNC input connectors. The cir
cuit consist of analog multiplexer U600 (used to route the
various analog voltages to the A/D converter), parallel
loading shift register U700 (used to relay switch-closure
data to the Front Panel
m
P. shown in diagram 3), and
associated components.
Analog multiplexer U600 routes one of the eight input
levels to the A/D converter internal to Front Panel
m
P
U700 (diagram 3), depending on the three-bit code applied
to its select inputs. The selected signal may be one of the
four probe-coding voltages (developed by the voltage
divider formed by the encoding resistance of the probe
attached to the input connectors and the associated pull-
up resistor within R601), the C H I OVL (overload) or CH2
3 -3 4
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