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Theory of O peration—2440 Service
A n o d e M u ltip lier
The Anode
Multiplier circuit
(also contained
in
High-Voltage Module CR565) uses voltage multiplication to
produce the + 1 4 kV CRT anode potential. Circuit operation
is similar to that of the voltage-doubler circuit of the
Cathode Supply.
The first negative half cycle charges the 0.001 pF input
capacitor (connected to pin 8 of the High Voltage Module)
to a positive peak value of + 2.33 kV. The following positive
half cycle adds its positive peak amplitude to the voltage
stored on the input capacitor and boosts the charge on the
second capacitor of the multiplier (and those following) to
+ 4.66 kV. Following cycles continue to boost up succeed
ing capacitors to values + 2 .3 3 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 + 1 4 k V above ground potential. Once the
multiplier reaches operating potential, succeeding cycles
replenish charge drawn from the Anode Multiplier by the
C RT beam. The 1 M O resistor in series with the output
protects the multiplier by limiting the anode current to a
safe value.
F o c u s A m p lifie r
The Focus Amplifier, in conjunction with the auto-focus
circuitry, provides optimum focus of the CRT beam for all
settings of the front-panel INTENSITY control. The Focus
Amplifier itself consists of two shunt-feedback amplifiers
composed of Q145, Q152, and their associated compo
nents. The outputs of these 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 Q145 and Q 152 are held at constant
voltages set by their emitter potentials, changing the
position of the wiper arms of the ASTIG and FOCUS pots
changes the current in the base resistors, R261 and R145.
This changes the feedback currents in R245 and R246 and
produces different output levels from the Focus Amplifiers;
that in turn, changes the convergence characteristic 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 control is
positioned by the user as required when viewing the
displays. When using the FO C US control, transistor Q 152 is
controlled as described above; however, an additional
current is also supplied to the base node of Q145 from the
FO C US pot through R262. This additional current varies the
base-drive current to Q145 and provides tracking between
the two lenses as the FOCUS control is adjusted during use
of the instrument.
A u to FOCUS
The convergence strengths of the quadrapole lenses
also dynamically track changes in the display intensity. The
VQ signal, applied to the CRT at pins 5 and 6, is linearly
related to the VZ (intensity) signal driving the CRT control
grid, and increases the strength of the lenses at higher CRT
beam currents. (A higher beam current requires a stronger
lens to cause an equal convergence of the beam.) The
emitter follower Q500 buffers the VZ signal (offset 15 V by
VR316) to the first and second quadrapole lenses. A linear
relationship (as opposed to the “ideal” exponential relation
ship) between the Z-Axis drive (VZ) and quadrapole voltage
(VQ) provides adequate dynam ic focusing for low to
medium Z-Axis drive. The High-Drive Focus adjustment
R400 sets the attenuation factor at the output of buffer Q500.
Capacitors C409 and C295 compensate for the capacitive
loading of the quadrapole elements.
Z -A x is A m p lifie r
The high-voltage, high-speed transresistance amplifier
U227 produces VZ, the Z-Axis drive signal. The amplifier
has two signal inputs: Z IN T - a current input that determines
the output voltage VZ, and Z O N - a TTL gating signal that
causes VZto g oto its lowest value (approximately 8 V) when
HI. Capacitor C l 39 supplies current to U227 during VZ
transitions, R137 is a current limiter, and C234 is a bootstrap
capacitor to speed up VZ edges.
D C R e s to re r
The DC Restorer provides CRT control-grid bias and
couples 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) to the
elevated CRT control grid potential (about -1 .9 kV). Refer to
Figure 3 -1 0 for the following description.
The DC Restorer circuit operates by clipping an AC
voltage waveform at the grid bias and the Z-Axis drive levels.
The shaped AC waveform is then coupled to the CRT control
grid through a coupling capacitor that restores the DC
components of the signal.
G R ID 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 T525. The negative half-cycle of
the sinusoidal waveform is clipped by CR541, and the
positive half-cycle (150 V peak) is applied to the junction of
CR442, CR644, and R546 via R543. Transistors Q640 and
Q641 (Q641 for instruments with serial numbers B012403
and above), and associated components form a voltage
clam p circuit that limits the positive swing of the AC
waveform at the junction.
3-77
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