Tektronix 5441 Instruction Manual page 41

Theory of Operation-5441/~5441
Z-AXIS AMPLIFIER AND CRT CIRCUIT
The crt circuit produces the high voltage potentials and of Darlington transistor Q390 goes negative from its quies-
provides the control circuits necessary for operation of the
cent 0 V. The output of Q390 goes more positive, reducing
cathode-ray tube (crt). The Z-Axis amplifier circuit is includ- the conduction of Q400 and (2410. This reduces the peak-
ed with the crt circuit discussion, since it sets the intensity of
to-peak sinusoidal signal amplitude, resulting in a reduced
the crt display. voltage in the secondary of T410. Conversely, if the output
decreases below
-
1.5 kV (goes more positive), Q410 will
conduct more, i.e., have a larger sinusoidal signal amplitude.
2-Axis Amplifier CR395 and C395 form a delay turn-on circuit to prevent the
The Z-Axis amplifier is a current-driven, shunt-feedback
operational amplifier with a voltage output. The amplifier
consists of Q345, (2352, and Q356. The feedback path is
from the Q352-Q356 collectors through C350-R349-R350
to the summing point at the base of Q345. Q352 and Q356
are connected as a collector-coupled complementary ampli-
fier that provides a fast linear output signal while consuming
minimum quiescent power. Q356 acts as the pull-up transis-
tor and Q352 acts as the pull-down transistor for the amplifi-
er. The output voltage from the amplifier provides the drive
signal to control the crt intensity level through the control-
grid supply.
The output voltage level of the Z-Axis amplifier is deter-
mined by the voltage drop across R349 and R350 in refer-
ence to the voltage level at the summing point for the
amplifier (base of Q345). The current through R349-R350 is
determined by the input current from any combination of
several sources, such as the INTENSITY control, plug-in in-
terface (unblanking, readout unblanking) and from Q320 and
Q335. Q320 is an operational amplifier that sets the EXT
INTENSITY INPUT connector signal to a level suitable for
proper Z-Axis amplifier response. Q335 acts as an electron-
ic switch to cause the crt display intensity to increase when
the BEAM FINDER switch is pushed. Q340 acts as an
impedance-matching and bias-setting transistor for the Z-
Axis amplifier. CR352 and current limiting resistor R352 act
as a protection circuit for the Z-Axis amplifier in case of a
high-voltage short.
High-Voltage Regulator
High-Voltage Primary. A repetitive, sinusoidal signal is
produced by a regenerative feedback oscillator in the prima-
ry of T410 and induced into the secondary. Current drive for
the primary winding is furnished by Q410.
crt beam from coming on immediately at instrument turn-on.
The delay time is controlled by the time it takes the (+) end
of C395 to charge to +30.6 V through R392 from the
+200 V power supply. At the moment the top of C395
reaches +30.6 V, diode CR395 will turn on and clamp the
CR395-C395-R397-R395A junction at +30.6 V. R402 and
C402 limit the bandwidth of the regulator to prevent
oscillations.
High-Voltage Outputs
The secondary winding of T410 provides the negative
and positive accelerating potentials for the crt and the bias
voltage for the control grid.
Positive accelerating voltage for the crt screen is sup-
plied by voltage doubler U410. The applied voltage to the
input of U410 from the T410 secondary winding is approxi-
mately
+
1.5 kV peak-to-peak. The output voltage of U410
is approximately
+7
kV at the crt anode. The negative ac-
celerating voltage for the crt cathode is also obtained from
the T410 secondary winding. CR412 half-wave rectifies the
transformer output and supplies the -1.5 kV to the crt
cathode. R419 connects the crt cathode voltage to the crt
filament to prevent cathode-to-filament breakdown.
Diodes CR420 and CR422 provide the rectified negative
control voltage for the crt control grid. The output level of
this supply is set by lntens Range adjustment R435. Diodes
CR428 and CR430 clip the crt grid bias voltage from the
T410 secondary, to determine the operating level at the con-
trol grid. CR428 limits the negative excursion of the bias
voltage, depending upon the output voltage of the Z-Axis
amplifier. The positive clipping level at the cathode of
CR430 is set by the lntens Range adjustment. CR420 acts
as a dc restorer and CR422 as a rectifier. This results in a
dc level across R422 equal to the peak-to-peak excursion at
the anode of CR430.
The conduction of Q410 is controlled by the collector
voltage of (2400.
CRT Control Circuits
High-Voltage Regulation. Regulation is accomplished
The focus of the display is determined by the Focus con-
by sampling the
-
1.5 kV across voltage divider R395A-
trol R332B. This control and the Auto Focus amplifier main-
R395B. If the output level of the cathode supply goes above tain a well-defined display for fast changes in intensity. The
the nominal -1.5 kV (goes more negative), the input base network consisting of CR457, CR456, CR454, CR452, and
REV MAY 1982
3-5