Analog multiplexer U600 routes one of the eight input
levels to the
AID
converter internal to Front Panel J.LP
U700 (diagram3), dependingonthe three-bit code applied
to its select inputs. The selectedsignal may be one of the
four probe-coding voltages (developed by the voltage
divider formed by the encoding resistance of the probe
attached to theinput connectors and the associated pull-
up resistor within R601),the CH1 OVL (overload)or CH2
Auxiliary Front Panel
The Auxiliary Front Panelcircuitry provides a means of
readingthe 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
AID
converter), parallel-
loading shift register U700 (used to relay switch-closure
data to the Front Panel J.LP,shown in diagram 3), and
associatedcomponents.
Operationalamplifier U820A is configured as an invert-
ing integrator. Inverting buffer U520G may be thought of
simply as an open-collectortransistor following operational
amplifier U820. The circuit appears this way because the
negative feedback around the loop via U820 and voltage
divider R824-R825 keeps U520Gin 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-updatesno integration takes place, and the
charge held on C822 holds the output of the inverting
buffer,and thereby the graticulelighting,constant.
The Graticule Illuminationcircuit, composed of U820A,
U520G, and associated components, sets the brightness
of the three lamps used to light up the graticule lines
etchedon the crt faceplate.
Graticule Illumination
The SPOTWOB(spot wobble) signal line, at the output
of OperationalAmplifier U81OB,picks off the various inten-
sity levels. Those levels are used in the Horizontal and
Vertical Output Amplifiers (diagram18) to dynamically
correct intensity-related position shifts on the crt
(describedin the DisplayOutput circuitry discussion).
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 signalline.
3-32
Automatic compensationof 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 compensationamplifier. Biasing for
the other transistor of the differential pair is supplied by
U812A, R817, and a resistor internal to the Z-Axis
Amplifierthat is tied to the+5 V
D
supply. The differential
amplifier pair is biased so that the total current is divided
betweenthe two sides.The resistancevalue of theinternal
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
When the intensity of the selected display is at
minimum,the output control voltage from multiplexerU811
will be below-1.36 V. This causes the output of U810B
to go to approximately -2.7 V, reducing the emitter
current to 0810 to approximatelyzero. DiodeCR810limits
the reverse-biasvoltage across the base-emitter junction
of 0810 to about 0.6 volts and protects the base-emitter
junction from excessivevoltage.
The circuitry of operationalamplifier U810A and transis-
tor 0810 is arranged so that the transistoris on with its
emitter held at -2.7 V. The -2.7 V levelat 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 flowin R812 and setsthe current
drive level for the Z-Axis circuit (diagram19). This Z-INT
drive current supplied via U812E from pin 14 may vary
from 0 mA to 4 mA (- 1.36V to
+
1.36 V respectively at
the output pin of multiplexerU811).
The selected intensity control voltageis applied to
U810B, configured as an inverting buffer with a gain of
-1. The output voltage is offset - 4.06 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 0810.
Selecting an inputto pass through multiplexer U811 is
done by two active input signals, BRIGHTZ and RO. (The
third selectinput 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 BRIGHTZinput will
go HI, selectinginput 1. When readout is to be displayed,
the RO input will go HI, selectingeither input 3 or input 4,
dependingon the setting of the BRIGHTZbit. Sinceinputs
3 and 4 are both connectedto the INT-RO(readout inten-
sity) control voltage level, the readout displays are not
intensified.
Theory of Operation-2430 Service
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