Tektronix 502A, Instruction Manual

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Circuit Description —Type 502A
R424 provides additional differential balance for the low-
sensitivity ranges, and is most effective when the SENSI­
TIVITY switch is in the .2 VOLTS PER CM position.
The Second Amplifier and Driver C.F. Stages
The push-pull output from the Input Amplifier stage is
amplified in the Second Amplifier and coupled through the
Driver C.F. stage to the Output Amplifier stage. As men­
tioned previously, the positive feedback from the plate
circuit on one side of the amplifier to the grid circuit on
the other makes this stage an infinite gain amplifier. How­
ever, the amplifier does not oscillate due to the overall
negative feedback.
The .2 MV/CM GAIN Control R456, located between the
two cathodes, sets the small-signal gain of the Vertical
Amplifier. This control is most effective when the SENSI­
TIVITY switch is in the .2 mVOLTS PER CM position. The
FEEDBACK BAL control R455 balances the negative feedback
from both sides of the Driver C.F. stage; this control is
also most effective when the SENSITIVITY switch is in the
.2 mVOLTS PER CM position.
The Driver C.F. stage is the load for the Second Amplifier
stage. Its grid circuit provides the necessary high impedance
with low capacitance to maintain the gain of the Second
Amplifier stage, its cathode circuit provides a very low
impedance to drive the input capacitance of the Output
Amplifier stage.'The low output impedance of the cathode
circuit also provides an excellent point from which to
obtain most of the compensated negative feedback for
the Input Amplifier stage.
The Output Amplifier
The Output Amplifier stage is the stage that drives the
vertical-deflection plates in the crt. The gain of this
stage can be adjusted by means of the .2 V/CM GAIN
Control R488. This control is most effective when the
SENSITIVITY switch is in the .2 VOLTS PER CM position. The
variable capacitor C478 adjusts the high-frequency response
of the amplifier, and is used principally to provide a
constant output capacitance.
The FINDER
switch reduces the swing of the Output Am­
plifier, enough to bring the trace onto the crt.
By means of the HORIZ. DEF. PLATE SELECTOR switch
SW489 the UPPER BEAM Vertical Amplifier can be con­
nected to the horizontal-deflection plates in the crt. With
this configuration the instrument can be used as a single­
beam X-Y oscilloscope. The GAIN ADJ FOR HORIZ MODE
Control R489 is used to increase the gain of the Amplifier
slightly for this application.
Trigger Pickoff
When internal triggering of the Time-Base Generator is
desired (TRIGGER SELECTOR in either of the UPPER or
LOWER positions) a sample of the vertical-output signal is
used to develop the triggering pulse. This sample, obtained
from the plate circuit of V484B, is coupled through a
frequency-compensated voltage divider to V493, a cathode­
follower which drives the Time-Base Trigger circuitry. The
DC TRIG Control R495 is adjusted to set the cathode volt­
age to zero when triggering with DC coupling and with the
beam positioned at its respective zero-center graticule line.
TIME-BASE TRIGGER
The Time-Base Trigger circuit consists of a triggering­
signal amplifier V24 and a multivibrator (Schmitt Trigger)
circuit V45. The function of the trigger circuitry is to pro­
duce a negative-going rectangular pulse at the plate of
V45A
whose repetition rate is the same as that of the trig­
gering signal. This negative step is then differentiated to
produce a very sharp negative spike (trigger) to trigger the
Time-Base Generator in the proper time sequence. A posi­
tive spike is also produced by the differentiation process, but
this spike is not used.
The signal from which the rectangular output is produced
may emanate from one of four sources. When the TRIGGER
SELECTOR switch is in the LINE position, a 6.3-volt signal
at the power line frequency is used for this application.
When the switch is in the UPPER or LOWER position (AC or
DC), the signal is obtained from the UPPER or LOWER Beam
Vertical Amplifier, respectively. In the EXT position (AC or
DC), the signal is obtained from an external source through
a
front-panel connector (TRIGGER INPUT). In any of the
DC positions of the TRIGGER SELECTOR switch the signal is
coupled directly from its source to the Slope switch SW20.
In any of the AC positions, the signal is coupled through
C10 and then to SW20.
Although the output of the Trigger Multivibrator is always
a negative rectangular pulse, the start of the pulse may
be initiated by either the rising (positive-going) or falling
(negative-going) portion of
the triggering signal. To see
how this is accomplished the operation of the Trigger Multi­
vibrator will be described first.
In the quiescent state, that is, ready to receive a sig­
nal, V45B is conducting and its plate voltage is down. This
holds the grid of V45A below cutoff, since the two circuits
are dc-coupled. With
V45A in a state of cutoff its plate
voltage is up, hence no
output is being produced.
A negative-going signal is required at the grid of V45B
to force the Trigger Multivibrator into its other state in
which a trigger pulse can be produced. However, since the
signal at the grid of V45B is an amplification of the trig­
gering signal, it contains both negative- and positive-going
portions.
The negative-going portion of the signal will drive the
grid of V45B in the negative direction, and the cathodes of
both tubes will follow the grid down. At the same time the
plate voltage of V45B starts to rise, which causes the grid
voltage of V45A to rise. With the grid of V45A going up
and its cathode going down, V45A starts to conduct.
As V45A starts conducting its cathode starts going up; hence
the cathode of V45B starts going up. With the grid of V45B
down and its cathode up, V45B cuts off. And since V45A is
conducting its plate voltage drops, creating a negative step
in the output. This transition occurs very rapidly, regard­
less of how slowly the grid signal of V45B falls.
When the signal at the grid of V45B starts in the posi­
tive direction, just the opposite chain of events will occur.
V45B will start conducting again, which
in turn will drive
the grid of V45A below cutoff. This will cause the voltage
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