Tektronix 543B, Instruction Manual

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Circuit Description — Type 543B/RM543B
cathode follower stage to charge and discharge the stray
capacitance in the plate circuit of the output amplifier.
The positive feedback network of C368, R360, R367, R368,
R369 and R370 gives the output amplifier its extremely
high gain. This network is connected between the cathode
of V374B-V384B and the input grid of V374A-V384A. C368
is adjustable so that the feedback network will provide
the same impedance ot each plate of the driver stage.
At the faster sweep rates the current through the output
cf tubes is too small to discharge the capacitance of the
horizontal deflection plates and their associated wiring at
the required rate. Additional current for this purpose is
provided by the gated pentode V394 connected to the
cathode-return circuit of V374B. This permits the cathode
of V374B, the negative-sawtooth cathode follower, to run
down at the required rate. A similar current boost is not
required for V384B since this tube is the positive-sawtooth
cathode follower and the cathode runs down during the
retrace.
Because the plate current of a pentode is fairly constant
over a large range of plate voltage, the cathode current
of V374B will remain nearly constant even though its cath­
ode falls about 100 volts during the trace portion of the
negative sweep waveform.
The additional current required for faster sweep rates is
obtained by applying a positive flat-topped pulse to the
grid of the pentode V394 during the period of the sweep.
This pulse is derived by differentiating the positive-going
sawtooth, available at the cathode of V384B, in C394 and
the resistance in the grid circuit of V394. The amplitude of
the pulse is proportional to the slope of the sawtooth, and
thus proportional to the sweep rate.
Beam-Position Indicators.
The beam-position indicators
B380 and B390 located on the front panel above the crt
indicate the relative horizontal position of the spot or center
of the trace with respect to the center of the graticule.
When the spot or trace is centered horizontally, the po­
tential across either neon is insufficient to light it. As the
beam is positioned left or right on the crt, the voltage
across the neons will change. The voltage across one neon
will increase, causing it to light, and the voltage across
the other neon will decrease, causing it to remain extin­
guished. The neon that lights will indicate the direction
in which the spot or trace has been moved.
External Horizontal Amplifier. The setting of the EXTER­
NAL HORIZONTAL VOLTS/CM switch determines whether
the signal is directly coupled to the grid circuit of V324A,
or whether one or two frequency-compensated attenuators
is connected to the signal path. For all positions of this
switch the input impedance is 1 megohm shunted by approx­
imately 43 pf.
The External Horizontal Amplifier V324 operates as a
cathode-coupled amplifier. V324A is the cathode follower
and V324B is the grounded-grid stage. VARIABLE control
R325 provides a means for adjusting the gain over a 10
to 1 range. EXT HORIZ DC BAL control R334 adjusts the
de level of V324B so that its cathode will be at the same
voltage as the cathode of V324A when no signal is applied
to the grid of V324A. With the cathode at the same volt­
age there will be no current through VARIABLE control
R325. By this arrangement an adjustment of the VARIABLE
gain control will not change the dc level at the plate of
V324B and therefore will not affect the positioning of the
beam.
The gain of the Horizontal Amplifier, when connected to
the External Horizontal Amplifier, is calibrated by means
of EXT HORIZ AMP CAL control R361M, shown on the
switch layout section of the Horizontal Amplifier diagram.
R361M is adjusted so the horizontal deflection will agree
with the setting of the EXTERNAL HORIZONTAL VOLTS/CM
switch when the VARIABLE control is turned fully clockwise
to the CALIBRATED position.
Amplitude Calibrator
The amplitude calibrator is a square-wave generator
with approximately a 1-kc output available at the front­
panel CAL OUT connector. The amplitude calibrator con­
sists of multivibrator V875 and V885A connected to switch
cathode follower V885B between two operating states: cut­
off and conduction.
During the negative portion of the multivibrator wave­
form, the grid of V885B is driven well below cutoff and
its cathode rests at ground potential. During the positive
portion of the waveform, V875 is cut off and its plate
rests slightly below +100 volts. The cutoff voltage at the
plate of V875 is determined by the setting of CAL ADJ
control R879; part of the divider connected b100
volts and ground.
Cathode-follower V885B has a precision tapped divider
for its cathode resistor. When the CAL ADJ control is
properly adjusted, the cathode of V885B is at +100 volts
when V875 is cut off. 18 output voltages from 0.2 mvolts
to 100 volts are available through tapped divider, R885-
R893, and 1000/1 divider R896-R897. C885, connected be­
tween the cathode of V885B and ground, connects the out­
put waveform for overshoot.
The amplitude calibrator provides a 0.1-volt output when
the AMPLITUDE CALIBRATOR switch is set to 0.5 volt and
a 50-Ω load is connected to the CAL OUT connector.
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