Theory of Operation—2465B/2467B Service
succeeding capacitors to values 2.33 kV higher than the
preceding capacitor until all six capacitors are fully
charged.
This places the output of the last capacitor in the
multiplier at +14 kV above ground potential. Once the
multiplier reaches operating potential, succeeding cycles
replenish current drawn from the Anode Multiplier by the
crt beam. The 1-Mfi resistor in series with the output pro-
tects the multiplier by limiting the anode current to a safe
value.
Focus Amplifier
The Focus Amplifier, in conjunction with the auto-focus
circuitry of Z-Axis hybrid U950 (diagram 6), provides
optimum focus of the crt beam for all settings of the front-
panel INTENSITY control. The Focus Amplifier itself
con-
sists of two shunt-feedback amplifiers composed of
Q1851,
Q1852, and associated components. The outputs
of the amplifiers set the operating points of a horizontally
converging quadrapole lens and a vertically converging
quadrapole lens within the crt. The convergence strength
of each lens is dependent on the electric field set up
between the lens elements.
Since the bases of Q1851 and Q1852 are held at
con-
stant voltages (set by their emitter potentials), changing
the position of the wiper arms of the ASTIG and FOCUS
pots changes the amount of current sourced to the base
junctions through R1856 and R1857 respectively. This
changes the base-drive currents and produces different
output levels from the Focus Amplifiers; that, in
turn,
changes the convergence characteristics of the quadrapole
lenses.
Initially, at the time of adjustment, the FOCUS and
ASTIG potentiometers are set for optimum focus of the crt
beam at low intensity. After that initial adjustment, the
ASTIG pot normally remains as set, and the FOCUS
con-
trol is positioned by the user as required when viewing the
displays. When using the FOCUS control, transistor
Q1852 is controlled as described above; however, an
addi-
tional current is also supplied to the base node of Q1851
from the FOCUS pot through R1855. This additional
current varies the base-drive current to Q1851 and pro-
vides tracking between the two lenses as the FOCUS
con-
trol is adjusted during use of the instrument.
The convergence strengths of the quadrapole lenses
also dynamically track changes in the display intensity.
The VQ OUT signal, applied to the crt at pins 5 and 6, is
exponentially related to the VZ OUT (intensity) signal driv-
ing the crt control grid and increases the strength of the
lenses more at higher crt beam currents. (A higher beam
current requires a stronger lens to cause an equal conver-
gence of the beam.)
DC Restorer
The DC Restorer provides crt control-grid bias and
cou-
ples both the dc and the low-frequency components of the
Z-Axis drive signal to the crt control
grid.
This circuit
allows the Z-Axis Amplifier to control the display intensity
by coupling the low-voltage Z-Axis drive signal (VZ OUT)
to the elevated crt control-grid potential (about —1.9 kV).
The DC Restorer circuit (Figure 3-9) operates by
impressing the crt grid bias setting and the Z-Axis drive
signal on an ac voltage waveform. The shaped ac
waveform is then coupled to the crt control grid through a
coupling capacitor that restores the dc components of the
signal.
GRID BIAS LEVEL. An ac drive voltage of
approximately 300 V peak-to-peak is applied to the DC
Restorer circuit from pin 7 of transformer T1970. The
negative half cycle of the sinusoidal waveform is clipped by
CR1953, and the positive half cycle (150 V peak) is applied
to the junction of CR1930, CR1950, and R1941 via R1950
and R1953. Transistor Q1980, operational amplifier
U1890A, and associated components form a voltage
clamp circuit that limits the positive swing of the ac
waveform at the junction.
Transistor Q1980 is configured as a shunt-feedback
amplifier, with C1991 and R1994 as the feedback ele-
ments. The feedback current through R1994 develops a
voltage across the resistor that is positive with respect to
the +42.6 V on the base of the transistor. The value of
this additive voltage plus the diode drop across CR1950
sets the upper clamping threshold. Grid Bias potentiome-
ter R1878 sinks varying amounts of current away from the
base node of the transistor and thus sets the feedback
current through R1994. The adjustment range of the pot
can set the nominal clamping level between +71 V and
+
133
V.
When the amplitude of the ac waveform is below the
clamping threshold, series diode CR1950 will be reverse
biased and the ac waveform is not clamped. During the
time the diode is reverse biased, transistor Q1980 is kept
biased in the active region by the charge retained on
C1971 from the previous cycle. As the amplitude of the ac
waveform at the junction of CR1930 and CR1950 exceeds
the voltage at the collector of Q1980, diode CR1950
becomes forward biased, and the ac waveform is clamped
at that level. Any current greater than that required to
maintain the clamp voltage will be shunted to the +42 V
supply by transistor Q1980.
3a-37
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