Tektronix T921 User Manual

Any change in
the level at the base of Q446 produces an error
signal at the collector of 0446, which
is
amplified
by Q454 and
applied to the base of Q458 through the feedback winding of
T460 . Regulation occurs as follows :
If the cathode voltage at the
-2000
V point
starts to go positive
(less negative), this positive-going change
is
applied
to the base
of Q446 . Q446 conducts harder, which in turn causes Q454
to
conduct harder . This results in greater bias current
to
the
base of
0458 through the feedback winding of T460 . Now, Q458 is biased
closer to its conduction level so that it comes into conduction
sooner to produce a larger induced voltage in the secondary of
T460 . This increased voltage appears as a more negative voltage
at the crt cathode
to correct the original positive-going change .
By sampling the
output from the crt cathode supply in this
manner, the total output of the high-voltage
supply is held
relatively constant .
Components R445 and C445 damp the response of the
regulator
against fast changes in the load such as when the crt is
unblanked at the beginning of the sweep .
Resistors R446, R453, and R457 help prevent instabilities in the
high-voltage oscillator .
HIGH VOLTAGE RECTIFIERS
AND
OUTPUT
The high-voltage transformer, T460, has 3 output windings .
One winding provides about 6 .3 V for the crt filament . The crt
filament is referenced to the cathode voltage (= -2 kV), preven-
ting cathode-to-filament breakdown . A second winding provides
high ac voltage
to
the
multiplier, U460, to produce a 10 kV crt
anode accelerating
voltage . Thesame winding is tapped and
rectified by CR465
to produce the do voltage for the crt cathode .
Components C465, R465, and
C466
filter the
do voltage .
11 . 2
The third winding is used to control the crt intensity . Com-
ponents CR463, C462, C463, C464, R462, and R463 rectify and
filter the
secondary voltage to provide approximately -2100 V
which is applied to thecrtgrid . The entire winding is referenced to
the output of the Z axis amplifier whose-output voltage variations
are used to control the crt intensity by varying the grid to cathode
voltage . The do path for the Z axis signal to the grid is through
R462, CR463, R463 and the transformer winding . Resistor R462
isolates the transformer
capacitance from the
Z
axis Amplifier .
Capacitors
C463
and
C464
provide a path for fast changes
in the Z
axis output
to
the
crt grid . Resistor
R464
provides a discharge
path for C462, C463, and C464 . Glow lamps DS463 and DS 465
prevent the grid-to-cathode voltage from rising high enough to
cause breakdown within the crt during turn-on or when the
cathode
or grid is shorted to ground .
CRT CONTROL CIRCUITS
Crt display focus is controlled by FOCUS control R468 . ASTIG
adjustment R477, which is used in conjunction with the FOCUS
control to provide awell-defined display, varies the voltage on the
astigmatism grid . Geometry adjustment, R473, varies the voltage
on the horizontal deflection plate shields to control the overall
geometry of the display .
Two adjustments control the
trace
alignment
by
varying the
magnetic field around the crt
.
Y axis adjustment, R474, controls
the current through L470, which affects the crt beam aftervertical
deflection but before horizontal deflection . Therefore, it affects
only the vertical (Y) components of the display . Trace Rotation
(TR ROT)
adjustment, R472, controls the current through L472
and
affects
both vertical and horizontal rotation of the beam
.
R475 and R476 provide the proper voltage forthevertical plate
shield and R478 and C478 decouple the first accelerator electrode
from
the +33
V
supply .

Tektronix T921 Specifications

General

Brand	Tektronix
Model	T921
Category	Test Equipment
Language	English