Tektronix 503, Instruction Manual

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Circuit Description— Type 503
CALIBRATOR
The calibrator provides a 500-millivolt square wave and
a 5-millivolt square wave for use in calibrating the gain of
the Horizontal and Vertical Amplifiers. The two amplitudes
are obtained by tapping off at different points in a voltage
divider network.
The square wave is produced by the turning off and on
of B886. This is accomplished by the combined action of
6886, 8883, and C883. 8886 and 8883 are neon tubes which
nominally drop about 60 volts when they are conducting.
However, if they are not conducting, they require about 80
volts across them to start conduction.
During the time that B886 is turned off, B883 is conducting.
This causes C883 to discharge which allows both plates of
8883 to move in a positive direction (60 volts apart). When
the common connection at the top of the two neon tubes
reaches a potential of about + 8 0 volts with respect to
ground, B886 conducts. The current through R886 and R887
produces a 0.5-volt drop across them and the voltage at the
upper end of B886 is, therefore, about +60.5 volts with
respect to ground. Since the upper plate of C883 is now
at about + 2 0 volts with respect to ground, the potential
across B883 in only about 40 volts, and B883 stops conduct­
ing. W ith no current through B883 to maintain the charge
on C883, the upper plate of C883 starts to move in a nega­
tive direction. The upper end of B883 is held steady at
+60.5 volts by the drop across B886, R886, and R887, so
when the upper plate of C883 becomes —20 volts with
respect to ground, B883 conducts. This drops the voltage at
the upper end of the two neon tubes to + 4 0 volts, and
B886 cuts off, completing one cycle of the square wave.
It should be noted that the potentials mentioned in the
foregoing discussion (except the drop across R886 and R887)
are typical nominals only, and may vary considerably
among different units. The only effect will be a slight vari­
ation in the frequency and symmetry of the output waveform.
The CAL. ADJ. control, R880, provides a means of adjusting
the voltage drop across R886 and R887 by controlling the
current through them.
POWER SUPPLY
T601 provides filament power for the graticule lights,
all of the tubes, except the first stage of the Input Amplifier,
and B + voltage (about 500 volts) for the power supply
oscillator tube, V620. The rest of the voltages used in the
oscilloscope are provided by the secondary of T620.
V620, the primary of T620, and part of the secondary of
T620 form an Armstrong oscillator circuit to drive T620 at
about 30 kHz. Each of the outputs of the secondary of T620
bears a fixed turns ratio to the others such that a change
in one effects a proportional change in each of the others.
Adjustment and regulation of all of the output voltages are
accomplished through adjustment and regulation of just one
output, the — 100 volt output. This, in turn, is referenced to
the 85-volt drop across the voltage regulator tube, V659.
Adjustment of the output voltages is accomplished by
means of the — 100 V ADJ. control as follows.- moving the
wiper arm of the — 100 V ADJ. control in a positive direction
reduces the bias on V634B. This, in turn, lowers the voltage
at the plate of V634B and, therefore, at the grid of V634A.
This causes an increase in voltage at the plate of V634A
which, in turns increases the screen voltage of V620. In­
creasing the screen voltage of V620 increases the G m, and
therefore the gain, of the tube, and thereby increases the
amplitude of oscillations in the secondary of T620. This
results in greater output from all of the supplies.
Regulation is accomplished in virtually the same manner.
A lowering of any o f the output voltages due to loading,
causes the volts per turn in the secondary of T620 to
decrease. This causes the —100-volt supply to drop (move
positively) with the resulting rise in the grid voltage of
V634B. This results, as before, in a rise in the screen voltage
of V620 and an increase in the amplitude of oscillations,
bringing the power supply outputs back to their nominal
values.
An increase in any of the output voltages, whatever
the reason, has the opposite effect on the screen voltage
of V620 and decreases the amplitude of oscillations in T620.
Regulation of the power supply outputs w ill be accom­
plished as long as the source voltage remains between 105
and 125 for 117-volt operation, or between 210 volts and
250 volts for 234-volt operation.
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