Tektronix 2235A Instruction Manual page 65

Theory of Operation- 2235A Instruction
stage. This transistor provides a low-impedance
termination for the input signals and isolates the signal
sources from following stages of the Z-Axis Amplifier.
Common-base transistor Q829 establishes a constant
current through R832. This current is divided between
Q825 and Q829 with the portion through Q829 driving the
shunt-feedback output amplifier consisting of (2835,
Q840, and (2845. The bias level of Q825 therefore
determines the amount of emitter current available to
(2829. Feedback-resistor R841 establishes the trans-
resistance gain which converts the input current to output
voltage. Emitter-follower Q835 is dc coupled to (2840,
and for low-speed signals Q845 acts as a current
source. Fast transitions couple through C845, providing
additional current gain through Q845 for fast voltage
swings at the output of the amplifier.
increases to the blanking level. When blanked, the out-
put of the Z-Axis Amplifier drops to a level that reduces
the crt beam current below viewing intensity during the
chop-switching transition.
For an X-Y display, CR818, CR817, and CR816 are
reverse biased. The XY2(L) signal is LO to reverse bias
CR551 and allow current in R820 to flow through CR820.
The crt intensity is then controlled by the A INTENSITY
potentiometer which sets the current in R820 through
Q804.
BEAM FIND switch S390 controls the base biasvoltages
of Q825 and (2829. When the BEAM FIND button is out,
-8.6 V is supplied to a base biasing network. When the
button is pushed in, the -8.6V supply is removed and the
voltage at the anode of VR828 rises to about -5.6 V . This
turns off Q829 so that the amplifier output voltage is
External Z-Axis input voltages establish proportional
determined by R835 and the vo'ltage at the BEAM FIND
input currents through ~ 8 2 2 and ~ 8 2 3 , and amplifier
switch, as set by other parts of the Beam Find circuitry.
sensitivity is determined by the transresistance gain of
The output voltage of Q835 will then be at a fixed level so
the shunt-feedback amplifier. Diode CR823 protects the
that the lNTENSlTYcontrOls and the Z-Drive signal have
Z-Axis Amplifier if excessive signal levels are applied to
no control over the crt intensity. A bright trace or dot will
the EXT Z AXIS INPUT connector.
then be displayed.
The intensity of the crt display in the A, B, and Alt Hori-
zontal modes is determined by the INTENSITY controls
and associated circuitry. The A INTENSITY potenti-
ometer controls the base voltage of Q804 to determine
the amount of emitter current that will flow through the
transistor and therefore the level of the Z-Axis signal.
Likewise, the B INTENSITY potentiometerwill control the
base voltage of Q814 and the intensity of the B and Alt
Sweep displays.
when only the A Sweep is displayed, Q586 and Q583
are biased off. The current through R818, as set by the A
INTENSITY potentiometer, will flow through CR818 and
Q825 to fix the voltage level at the Z-Axis Amplifier out-
put. For a B-only display, Q586 is biased on to reverse
bias CR818 and prevent A-intensity current from
reaching Q825. Current determined by the base voltage
of Q814 will flowthrough CR817 (Diagram 7) to Q825 and
determine the B Sweep intensity. For an alternating A and
I3
display, Q586 will be biased off when the A Sweep is
displayed. During the portion of the A Sweep in which the
B Sweep runs, current from R816 (Diagram 7) is allowed
to flow through CR816 by the B Z-Axis Logic circuit to
provide an intensified zone.
When CHOP Vertical MODE is selected, the Chop Blank
signal is applied to the collector of Q825 through CR824
during the display-switching time. Signal current is
shunted away from CR825, and the forward bias of Q829
The Dc Restorer circuit produces the crt control-grid
bias and couples both dc and ac components of the
Z-Axis Amplifier output to the crt control grid. Direct
coupling of the Z-Axis Amplifier output to the crt control
grid is not employed due to the high potential differences
involved. Refer to Figure 3-5 during the following
discussion.
Ac drive to the Dc Restorer circuit is obtained from pin 16
of T948. The drive voltage has a peak amplitude of about
k1 O O V , a frequency of about 20 kHz, and is coupled into
the Dc Restorer circuit through C853 and R853. The
cathode of CR851 is biased by the voltage applied from
the wiper of Grid Bias potentiometer R851, and the ac-
drive voltage will be clamped whenever the positive
peaks reach a level that forward biases CR851.
The Z-Axis Amplifier output voltage, which varies
between
+
10 V and
+
75 V , is applied to the Dc Restorer
at the anode of CR853. The ac-drive voltage will hold
CR853 reverse biased until the voltage falls below the
Z-Axis Amplifier output voltage level. At that point,
CR853 becomes forward biased and clamps the junc-
tion of CR851, CR853, and R854 to the Z-Axis output
level. Thus, the ac-drive voltage is clamped at two
levels to produce a square-wave signal with a positive
dc-offset level.
Scans
by ARTEK MEDL4 =>