Tektronix 502A, Instruction Manual

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Circuit Description— Type 502A
The negative going trigger pulse when it arrives will
cause V135A to go into cutoff and V145A to come into
conduction. The action of the Sweep Generator circuit as
explained still applies.
As the sweep ends, the waveform through V145B turns
V135A on and turns V145A off. When V135A comes into
conduction, its plate will go slightly negative and forward
bias D126. The current flow through D126, R126 and R127
will drop the voltage at the base of Q124 far enough
negative, in relation to ground, to turn the transistor on. The
voltage on the collector will now go positive towards ground
and the READY light will go out. As the collector voltage of
Q124 goes positive, D24 becomes forward biased and will
control the voltage level on the grid of V135A. This voltage
level will be held positive enough to prevent further
triggers from switching the Sweep Gating Multivibrator. The
circuit will remain in this condition until the MODE switch is
either thrown to the RESET position again or put in the
NORMAL position.
The positive rectangular pulse appearing at the cathode of
V135B is coupled to the grid circuits of the crt. This pulse,
whose start and duration are coincident with the trace
portion of the sweep sawtooth, unblanks the crt and
permits the trace to be observed.
The Time-Base Generator is inoperative when the UPPER
BEAM Vertical Amplifier is connected to the horizontal­
deflection plates. One section of the HORIZ DEF PLATE
SELECTOR switch SW489 is located in the cathode circuit
of the Multivibrator, and immobilizes this circuit for the
application just described. The same circuit is
immobilized,
and in the same manner, when the HORIZONTAL DISPLAY
switch is set to any of the EXT ranges. With this arrange­
ment the horizontal sweep voltage is obtained through a
front-panel EXTERNAL connector rather than from the
Time-Base circuits.
HORIZONTAL AMPLIFIER
The Horizontal Amplifier consists of an Input Cathode-
Follower, a cathode-coupled Input-Amplifier stage, and a
plate-loaded Output Amplifier to drive the horizontal de­
flection plates in the crt.
The HORIZONTAL DISPLAY switch SW331 determines
whether the input waveform is received from the Time-Base
Generator or from an external source.
When this switch is
in either the NORMAL or the SWEEP MAGNIFIED position,
the waveform is received from the Time-Base Generator.
With this configuration the sweep sawtooth is coupled to
the Input C.F. via the frequency-compensated voltage
divider R311-R312. The Horizontal POSITION control R398
supplies a
manually adjustable dc voltage to the grid of
the Input C.F. V183B for horizontal positioning of the crt
beam. The Input C.F. isolates the Miller circuit from the
Horizontal Amplifier and provides a low-impedance source
to drive the switch capacitances and the Input Amplifier.
The Horizontal Amplifier is controlled by feedback net­
works much the same as the Vertical Amplifiers. This is
illustrated in Fig. 3-4. A negative feedback loop extends
from the plate circuit of the Output Amplifier to the cathode
circuit of the Input Amplifier. The Output Amplifier, on the
other hand, has
a positive feedback loop between the plate
circuit on one side and the grid circuit on the other.
The Input Amplifier is a cathode-coupled paraphase
amplifier, and converts the positive-going sawtooth voltage,
obtained from the Time-Base Generator, to a push-pull
output sawtooth voltage. The gain of the Input Amplifier
is determined by the amount of resistance connected
between the two cathodes, which in turn determines the
degree of negative feedback applied to the stage. In the
NORMAL position of the HORIZONTAL DISPLAY switch,
R331 is not in the circuit and the cathode resistance is com­
posed of R334 in parallel with the series combination of
R336, R337, and R338. In any of the SWEEP MAGNIFIED
positions, one of the R331 resistors is switched into the
cathode circuit and shunts the total cathode resistance to
a lower value. This decreases the amount of negative feed­
back applied to the stage and consequently increases the
gain of the stage.
The function of the MAG REGIS Control R337 is to insure
that
the waveform will be expanded symmetrically about
the center of the crt when the HORIZONTAL DISPLAY
switch is changed from NORMAL to any of the SWEEP
MAGNIFIED positions. This control dc-balances the ampli­
fier so that the horizontal positioning of the beam will not
be affected when the beam is positioned in the center of
the crt, as the value of R331 is changed. The X1 CAL Con­
trol R357 adjusts the plate-to-plate gain of the stage slightly
to compensate for any circuit nonsymmetry. This control is
most effective when the HORIZONTAL DISPLAY switch is
in the NORMAL position.
The Output Amplifier, by virtue of its positive feedback
network, is an extremely high-gain stage. The gain of the
stage can be varied over a limited range with the X20 CAL
Control R368 which varies the amount of cathode degenera­
tion. This control is most effective when the HORIZONTAL
DISPLAY switch is in the X20 SWEEP MAGNIFIED position.
When the HORIZONTAL DISPLAY switch is in any of the
EXT VOLTS/CM positions, the horizontal signal is coupled
directly from the front-panel EXTERNAL connector to the
grid circuit of the Input Amplifier. This action changes the
configuration of the amplifier slightly. The Input C.F. is
disconnected
from the circuit, which changes the dc level at
the cathodes of the Input Amplifier slightly. To compensate
for
this change an EXT HORIZ DC BAL control R341 is
adjusted to equalize the cathode voltages.
In the external positions of the HORIZONTAL DISPLAY
switch, the Horizontal POSITION control is disconnected from
the input circuit and connected between the plates of the
Output Amplifier.
POWER SUPPLY
Plate and filament power for the tubes in the 502A
Oscilloscope is furnished by a single power transformer
T602. The primary has two equal tapped windings; these
may be connected in parallel for 117 volt operation, or in
series for 234 volt operation. The primary of T602 also
has two more windings which may be used as voltage buck­
ing or aiding windings. These windings along with the two
main primary windings may be used to allow the instrument
to run on
line voltages of 110, 117, 124, 220, 234 and 248,
depending upon how all the windings are connected. The
three main full-wave power supplies furnish regulated
voltages of —150, 4-100 and 4-350 volts. The 4-350 volt
supply also has an unregulated output of about 4-485 volts
for the high-voltage power supply for the crt. It is unnec­
essary to regulate this supply as the high-voltage power
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