Tektronix 2445A - Page 67

The oscillating magnetic field in the primary winding
couples power into the secondary windings of the
transformer.
The amplitude of the voltages induced in the
secondary windings
is a function of the turns ratios of the
transformer windings.
High-Voltage Regulator
The High-Voltage Regulator consists of U1956A and B
and associated components. It monitors the crt Cathode
Supply voltage and varies the bias point of the switching
transistor
in the High Voltage Oscillator to hold the
Cathode Supply voltage at the nominal
level. Since the
output voltages at the other secondary winding taps are
related
by turns ratios to the Cathode Supply voltage, all
voltages are held in regulation.
When the Cathode Supply voltage is at the proper level
( -1900 V), the current through R 1945 and the þÿ19 M©
resistor internal to High Voltage Module U1830 holds the
voltage developed across C1932 at zero volts. This
Is the
balanced condition
and sets base drive in 01981 via
integrator U1956A
and voltage-follower U1956B. Varying
base drive to 01981 holds the secondary voltages
in regu-
lation.
If the Cathode Supply voltage
level tends too positive, a
slightly positive voltage will develop across C1932. This
voltage causes the outputs
of integrator U1956A and
voltage-follower U1956B to move negative. The negative
shift charges capacitor
C1951 to a different level, around
which the induced feedback voltage at the base-drive
winding will swing. The
added negative bias causes 01981
to turn
on earlier in the oscillation cycle, and a stronger
current pulse
in induced in the secondary windings. The
increased power
in the secondary windings increases the
secondary voltages until the Cathode Supply voltage
returns to the balanced condition (zero volts across
C1932). Opposite action occurs should the Cathode Sup-
ply voltage tend too negative.
Cathode Supply
The Cathode Supply circuit is composed of a voltage-
doubler
and an RC filter network contained within High-
Voltage Module U1830. This supply produces the
-1900
V accelerating potential applied to the CRT cathode and
the -900 V slot lens voltage. The -1900 V supply is
monitored by the High Voltage Regulator to maintain the
regulation of
all voltages from the High Voltage Oscillator.
The alternating voltage
(950 V peak) from pin 10 of
transformer T1970
is applied to a conventional voltage-
doubler circuit at
pin 7 of the High Voltage Module. On the
positive half cycle, the input capacitor of the voltage dou-
bler
(0 .006 µf) is charged to -950 V through the forward-
Theory of Operatlon-2445A/2455A Service
biased diode connected to ground at pin 9 of the module
(charging path
is through the diode, so stored charge is
negative). The following negative half cycle adds its ac
component
(-950 V peak) to this stored de value and pro-
duces a total
peak voltage of -1900 V across the capaci-
tor. This charges the 0.006-µf storage capacitor (con-
nected across the two doubler diodes) through the second
diode (now the forward-biased diode) to
-1900 V. Two
RC filters follow the voltage doubler to smooth out the ac
ripple. A resistive voltage divider across the output of the
filter network provides the
-900 -V slot lens potential.
Anode Multiplier
The Anode Multiplier circuit (also contained in High Volt-
age Module U 1830) uses voltage multiplication to produce
the +
14 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-µf Input
capacitor (connected to pin 8 of the
High Voltage Module)
to a positive peak value of
+2 .33 kV. The following posi-
tive half cycle adds its positive peak amplitude to the volt-
age stored
on the input capacitor and boosts the charge
on the second capacitor of the multiplier (and those follow-
ing) 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 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 þÿ1 M© 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
consists of two shunt-feedback amplifiers composed of
01851, 01852,
and associated components. The outputs
of the amplifies 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 01851 and 01852 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
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