Tektronix 2445, Инструкция по эксплуатации

Страница: 79 / 351

Theory o f Operation—2445 Service
to +4.66 kV. Following cycles continue to boost up
succeeding capacitors to values 2.33 kV higher than the
preceding capacitor until all six capacitors are fu lly charged.
This places the output of the last capacitor in the m ultiplier
at +14 kV above ground potential. Once the m ultiplier
reaches operating potential, succeeding cycles replenish
current drawn from the Anode M ultiplier by the crt beam.
The 1-M£2 resistor in series w ith the output protects the
m ultiplier by limiting the anode current to a safe value.
Focus Amplifier
The Focus Am plifier, in conjunction w ith the auto-focus
circuitry of Z-Axis hybrid U950 (diagram 6), provides
optimum focus o f the crt beam fo r all settings of the
front-panel INTENSITY control. The Focus Am plifier
itself consists of two shunt-feedback amplifiers composed
o f Q1851, Q1852, and associated components. The outputs
of these amplifiers set the operating points of a horizontally
converging quadrapole lens and a vertically converging
quadrapole lens w ithin the crt. The convergence strength
o f each lens is dependent on the electric field set up
between the lens elements.
Since the bases o f Q1851 and Q1852 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 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 fo r optimum focus o f the
crt beam at low intensity. A fter 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 FOCUS control, transistor
Q1852 is controlled as described above; however, an
additional 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 provides tracking between the tw o lenses as the FOCUS
control is adjusted during use of the instrument.
The convergence strengths of the quadrapole lenses
also dynamically track changes in the display intensity.
The VQOUT signal, applied to the crt at pins 5 and 6, is
exponentially related to the VZOUT (intensity) signal
driving 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
convergence of the beam.)
DC Restorer
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 A m plifier to control the display intensity
by coupling the low-voltage Z-Axis drive signal (VZOUT) to
the elevated crt control-grid potential (about —1.9 kV).
The DC Restorer circuit (Figure 3-10) 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 o f the signal.
GRID BIAS LEVEL. An ac drive voltage o f approxi­
mately 300 V peak-to-peak is applied to the DC Restorer
circuit from pin 7 of transformer T1970. The negative
half cycle o f the sinusoidal waveform is clipped by
CR1953, and the positive half cycle (150 V peak) is applied
to the junction of CR1930, CR1951, and R1941 via
R1950 and R1953. Transistor Q1980, operational amplifier
U1890A, and associated components form a voltage clamp
circuit that limits the positive swing o f the ac waveform
at the junction.
Transistor Q1980 is configured as a shunt-feedback
amplifier, w ith C l991 and R1994 as the feedback elements.
The feedback current through R1994 develops a voltage
across the resistor that is positive w ith respect to the
+42.6 V on the base of the transistor. The value of this
additive voltage plus the tw o diode drops across CR1950
and CR1951 sets the upper clamping threshold. Grid Bias
potentiometer 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 Vand +133 V.
When the amplitude of the ac waveform is below the
clamping threshold, series diodes CR1950 and CR1951
w ill be reverse biased and the ac waveform is not clamped.
During the time the diodes are reverse biased, transistor
Q1980 is kept biased in the active region by the charge
retained on C l971 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 Q198Q,
the tw o series diodes (CR1950 and CR1951) become
forward biased, and the ac waveform is clamped at that
level. Any current greater than that required to maintain
the clamp voltage w ill be shunted to the +42-V supply
by transistor Q1980.
3-37