Tektronix 2445, Инструкция по эксплуатации

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Theory o f Operation—2445 Service
Other adjustable level-setting circuits include Y-Axis
Alignment pot R1848, used to rotate the beam alignment
after vertical deflection. This adjustment controls the
amount of current through the Y-Axis Alignment coil
around the neck of the crt and is set to produce precise
perpendicular alignment between x- and y-axis deflections.
The TRACE ROTATION adjustment R975 is a front-
panel screwdriver-adjustable control. The effect o f the
adjustment is similar to the Y-Axis Alignment pot, but
when adjusted, it rotates both the x-axis and the y-axis
deflections o f the trace on the face of the crt. A final
adjustable level-setting control is the Geometry pot R1870,
adjusted to optimize display geometry. The potential at
pin 8 fo r the vertical shield internal to the crt is produced
by zener diode VR1891 and associated components.
SIGNAL-HANDLING CIRCUITRY. The crt termination
adjustment R1301 is set to match the loading character­
istics o f the crt's vertical deflection structure to the Vertical
Output Am plifier.
LOW VOLTAGE POWER SUPPLY
The low voltages required by the 2445 are produced
by a high-efficiency, switching power supply. This type of
supply directly rectifies and stores charge from the ac
line supply; then the stored charge is switched through a
special transformer at a high rate, generating the various
supply voltages.
Line Rectifier
Ac line voltages of either 115 V or 230 V may provide
the primary power for the instrument, depending on the
setting of the LINE VOLTAGE SELECTOR switch S90
(located on the instrument rear panel). Power Switch S350
applies the selected line voltage to the power supply
rectifier (CR1011).
With the selector switch in the 115-V position, the
rectifier and storage capacitors Cl 021 and C l022 operate
as a full-wave voltage doubler. When operating in this
configuration, each capacitor is charged on opposite half
cycles of the ac input, and the voltages across the two
capacitors in series w ill approximate the peak-to-peak value
of the source voltage. For 230-V operation, switch S90
connects the rectifier as a conventional bridge rectifier.
Both capacitors charge on both input half cycles, and the
voltage across C1021 and C1022 in series w ill approximate
the peak value of the rectified source voltage. For either
configuration, the dc voltage supplied to the power supply
inverter is the same.
Thermistors RT1010 and RT1016 lim it the surge
current when the power supply is first turned on. As
current flow warms the thermistors, their resistances
decrease and have little effect on circuit operation. Spark-
gap electrodes E l001 and E l002 are surge-voltage pro­
tectors. If excessive source voltage is applied to the
instrument, the spark-gaps conduct, and the extra current
flow quickly exceeds the rating of F90. The fuse then
opens to protect the instrument's power supply. The EMI
(electromagnetic interference) filter, inductors L1011 and
L1012, capacitors C1014 and C1016, and resistors R1011,
R1014, and R1016 form a line-filter circuit. This filte r,
along w ith common-mode rejection transformer T1020,
prevents
power-line interference from entering the
instrument and prevents power supply switching signals
from entering the supply line.
Preregulator Control
The Preregulator Control circuit monitors the drive
voltage applied to the Inverter output transformer T1060
and holds it at the level that produces proper supply
voltages at each of the secondary windings.
The Preregulator Control circuit consists prim arily of
control 1C U1030, its switching buffers, and its power
supply components. The control 1C senses voltage on the
primary winding of T2060 and varies the "o n tim e " of a
series-switching transistor, depending on whether the sensed
voltage is too high or too low. The switching transistor
Q1050, rectifier CR1050, choke T1050, and capacitor
C l050 form a buck-switching regulator circuit. The output
voltage at W1060 is proportional to the product o f the
rectified line voltage on C1020-C1022 and the duty cycle
of Q1050. In normal operation, Q1050 is on about one-half
of the time. When Q1050 is o ff, current flows to W1060
and T1060 through CR1050.
PREREGULATOR CONTROL POWER SUPPLY. Since
the Preregulator control network controls supply startup
and preregulates the secondary supplies, an independent
power source must be established fo r it before any of the
other power supplies w ill operate. The independent power
supply for the control circuitry is composed of Q1021,
Q1022, and the associated components.
Initially, when instrument power is applied, the positive
plate o f capacitor C l025 is charged toward the positive
rectified line voltage through R1020. The voltage at the
base o f Q1022 follows at a level determined by the voltage
divider composed o f R1022, R1024, CR1023, and the load
w ithin U1030. When the voltage across C1025 reaches
about +21 V, the base voltage of Q1022 r6.8 V and
Q1022 turns on, saturating Q1021. The +21 V on the
emitter o f Q1021 appears at its collector and establishes the
positive voltage supply for the Preregulator !C. With 0.1021
on, R1024 is placed in parallel w ith R1022, and both
Q1022 and Q1021 remain saturated,