Tektronix 422 - AC-DC Power Supply

Circuit Desctiption—Type 422 AC-DC
trol R855, which is used in conjunction with the FOCUS
control to provide a well-defined display, varies the level on
the astigmatism grid. Geometry adjustment R854 varies the
level on the horizontal deflection plate shield to control the
overall geometry of the display. Since the source voltage for
both the ASTIGMATISM control and the Geometry adjust
ment is obtained from the low-voltage supply, the voltage
on these elements of the CRT is about zero volts.
Two adjustments control the trace alignment by varying
the magnetic field around the CRT. The Y Axis Align
adjustment R856 controls the current through L856, which
affects the CRT beam after vertical deflection but before
horizontal deflection. Trace Rotation adjustment R851
controls the current through L859 and affects both vertical
and horizontal rotation of the CRT beam.
Unblanking Amplifier
The CRT in the Type 422 uses deflection blanking to
deflect the CRT beam off the viewing area during retrace
time and when the sweep is not operating. The deflection
blanking plate connected to pin 7 is fixed at about +17
volts by the voltage drop of zener diode D841 from the
+55-volt supply. Since the +55-volt supply is not closely
regulated the exact voltage on this deflection-blanking
plate varies somewhat with changes in input voltage
without affecting operation of the instrument. The level on
the deflection-blanking plate connected to pin 12 is
determined by the unblanking gate from the Sweep
Generator circuit. Quiescently, before the sweep is
triggered, the unblanking gate level is about zero volts and
Q864 is biased near cutoff. The base level of Q864 is fixed
at about +0.6 volt by R865 and D865 connected between
the +12-volt supply and ground. Very little current is
flowing through Q864 as determined by R868-R869, and
the collector of Q864 rises positive toward the +55-volt
supply. The potential at pin 12 of the CRT rises positive to
about +50 volts and, since the difference in potential
between the two deflection blanking plates is significant,
the electron beam is deflected off the viewing area of the
CRT to blank the display. Notice that the +55-volt supply
provides the operating potential for the circuits connected
to both deflection-blanking plates. Therefore, the ripple
and voltage variations from the +55-volt supply will appear
equally on both deflection-blanking plates. Since this
appears as a common-mode signal at both plates, however,
it does not deflect the electron beam passing between the
plates.
When the instrument is triggered, the negative-going
unblanking gate supplies current to Q864 through R866
and C477-R477 (see Sweep Generator diagram). Q864 is
forward biased to conduct more, and the deflection
blanking plate connected to pin 12 drops to about the same
potential as on the other plate (pin 7). Now the electron
beam can reach the phosphor to produce a display. The
Unblanking Center adjustment R869 is adjusted to provide
maximum beam current (brightest display). The circuit
remains in this condition until the unblanking gate ends.
The unblanking gate can be interrupted during a trace to
blank portions of the display. In the CFIOPPED position of
the vertical Mode switch, a positive-going signal is con
nected to R866 during the between-trace switching time.
This signal interrupts the current supplied to Q864 and the
trace is momentarily blanked so the transition between the
Channel 1 and the Channel 2 trace cannot be seen. Provi
sions are also made in the Sweep Generator circuit for trace
blanking from an external signal connected to the EXT
BLANKING connector (see Unblanking Circuit discussion).
When the unblanking gate ends, Q864 returns to its quies
cent conduction level and Q863 pulls pin 12 of the CRT to
its quiescent level of about +50 volts to blank the CRT.
When this instrument is operated in the External Hori
zontal mode, the CRT must be unblanked to produce a
display, since the Sweep Generator is inoperative in this
mode. When the TIME/DIV switch is set to the EXT
HORIZ position, a negative level is connected to the
emitter of Q864 through R867 (see diagram 6 for sche
matic). Q864 is biased on to unblank the CRT so the exter
nal horizontal display can be presented. The CRT remains
unblanked until the TIM E/DIV switch is changed from the
EXT HORIZ position (external blanking signals connected
to the EXT BLANKING connector can be used to blank
portions of the display).
AC-DC Power Supply
General
The AC-DC Power Supply provides the operating power
for this instrument from a variety of power sources. DC-DC
regulation is used in this supply to provide stable, low-
ripple output voltage and to attain high operating efficien
cy. This circuit also provides the drive signal for the High-
Voltage Multiplier stages in the CRT circuit.
AC-DC Power Selector
General The AC-DC Power Selector circuit provides
rectified, unregulated power to the DC-DC Regulator cir
cuit from an AC line voltage source, external DC source or
from the internal battery pack. This circuit also contains
battery charging and low-voltage indicating stages. Fig. 3-12
shows a detailed block diagram of the AC-DC Power Selec
tor circuit. A schematic of this circuit is shown on diagram
11 at the back of this manual.
AC Power Input. AC power is applied to the primary of
T1001 through the AC line fuse F1000, filter
C1000-T1000, thermal cutout TK1000, POWER switch
SW1001 and Power Mode switch SW1030. The Power Mode
switch SW1030 connects the split primary windings of
T1001 in parallel fo r 115-volt nominal operation
(CHARGE BATT 115V AC and OPERATE 115V AC posi
tions) or in series for 230-volt nominal operation
(CHARGE BATT 230V AC and OPERATE 230V AC posi
tions).
Filter C1000-T1000 reduces the EMI radiation con
ducted out of this instrument through the AC power cord.
Thermal cutout TK1000 provides thermal protection when
3-20