Tektronix 502A Instruction Manual page 21

(or C440 for Input B) plus the tube and stray capacitance.
C410 is then adjusted so that the total input capacitance
is 47 pf.
When the X 1 00 attenuator is connected into the circuit,
C406L (or C407L for Input B) shunts the 47 pf capacitance.
This value of capacitance is then reduced to a very small
value by series capacitor C406J (or C407J).
at the input to the attenuator then shunts this small capa ­
citance, and is adjusted to make the total capacitance 47 pf.
The X10 attenuator is designed in the same manner, except
that stray capacity forms the lower branch of the divider.
Since the attenuator networks are frequency-compensated
voltage dividers, a constant attenuation ratio is maintained
from DC to the upper-frequency limits of the Vertical
fiers.
The nput Amplifier
When the Input Selector switch is in the
C- or DC-coupled) the grid of V414 is connected to the
input circuit and the grid of V444 is returned
either through the switch or through one of the series at ­
tenuator networks. When in position B ( C or DC), V444
is the input tube and V414 is the grounded-grid tube. With
either of these configurations, the
cathode coupled, paraphase amplifier; it converts a single-
ended input signal to a push-pull output signal.
The cathode resistor R408 plays an important role in
determining the amount of negative feedback applied to
the Input mplifier stage.
feedback voltage comes from the cathodes of the Driver
C.F. stage and the Output
from the Driver C.F. stage is applied through a divider
consisting of R470 on one side and R408, and R480 on the
other side of R408. To this compensated feedback network
is added a small amount of feedback from the uncompen ­
sated network of R465 and R467.
Output mplifier stage helps in positioning the trace.
The smaller the value of R408 the greater the drop across
the series resistor (R470, R480, R465, and R467) and the less
the negative feedback applied to the Input
Conversely, the greater the value of R408 the greater the
drop across it and the greater the negative feedback. Thus,
for very small input voltages, when the SENSITIVITY switch is
set so that the resistance of R408 is quite small, there is
very little negative feedback and the Input
operates with high gain.
is set to accommodate larger input voltages the resistance
of R408 is increased. This means that there is a greater
amount of negative feedback and the gain of the Input
mplifier stage is decreased.
The switch diagram shows the makeup of R408. R408
in the circuit for all positions of the SENSITIVITY switch
In the .2, 2, and 20 VOLTS/CM positions of the switch, R408
alone makes up the resistance of R408. In all other positions
of the switch R408 is shunted by at least one other resistor.
In the .1 mVOLTS/CM position R408
R408L and C409L; in the .5 mVOLTS/CM through .1 mVOLTS/
CM positions, and in the .5, 1, 5 and 10 VOLTS/CM positions,
it is shunted by both a resistor and an R-C network. The total
value of R408 is determined by the degree to which R408
is shunted. The greater the shunting of R408
the value of R408 and the greater the gain of the Input
The capacitor
position (either
to ground
Input mplifier is a
s mentioned previously, this
mplifier stage. The feedback
This feedback from the
mplifier stage.
mplifier stage
When the SENSITIVITY switch
is shunted by both
the smaller
mplifier stage. Conversely, the less the shunting of R408
the greater the values of R408 and the smaller the gain of
the stage.
The cathodes of the Input
to the — 150 volt supply through 82. 5K resistors and R429
(V R DC B L).
With the grids of the Input
the cathodes will operate very close to ground (actually,
a couple of volts positive to bias the stage). The approx ­
imately 150 volt drop across the 82. 5K cathode resistors
(R427 and R428) and R429 provides a constant supply of
cathode current to stabilize the performance of the 6 U6
Input mplifier tubes.
mpli ­
In order for the Input
state of DC balance, there must be no d c voltage drop
across R408 when there is no input signal. This means that
the difference in potential between the two cathodes must
always be zero, regardless of the value of R408. In order
to provide for equal cathode voltages under this condi ­
tion, the screen voltage of the two tubes can be varied
with respect to each other with the DC B L Control.
change in the voltage at the screens will be reflected to the
cathode by a factor of 1/ μ . (the screen grid μ ) and thus the
cathode voltages can be equalized.
long with the DC B L Control, the DC unbalance in
the Input mplifier tubes is also removed by R429.
ment of this control will remove any vdltage change which
might be seen as a
mplifier.
Vertical positioning
through the action of the POSITION control. This is a dual
control, connected between — 150 volts and +100 volts.
It is wired so that as the voltage between — 150 volts and
the movable arm in one side increases, the voltage between
— 150 volts and the movable arm in the other decreases.
change in the setting of the POSITION Control will
produce a large change in the cathode voltage of the
Output mplifier stage and at the same time produce a
small voltage change at the cathodes of the Input
stage. However, the compensated negative feedback circuit
of the Driver C.F. stage reacts quickly to prevent any
change in the
stage, and as a result a change in the voltage at the
cathodes of the Driver C.F. stage is produced. This change
in voltage at the cathodes of the Driver C.F. stage along
with the voltage change at the Output
is amplified by the Output
change in the positioning voltage at the vertical-deflection
plates.
When the Input Selector switch is set to the
position both grids of the Input
is
ed to the Input circuit. With this configuration the Input
mplifier stage
Two input voltages are required, and the push-pull output
voltage is proportional to the difference between the two
input voltages.
The .2 MV/CM DIFF B L Control R435 adjusts the plates
of the Input
common mode signals are applied to the grids. This control
is equally effective in
switch, but is
position (.2 mVOLTS/CM).
Circuit Description — Type 502A
mplifier stage are "long-tailed"
mplifier at ground potential,
mplifier stage to remain in a
positioning change by the Output
of the crt beam
cathode voltage of the
mplifier cathode,
mplifier and appears as a
mplifier stage are connect ­
is connected for differential
mplifier stage for equal
all positions of the SENSITIVITY
primarily adjusted in the
The .2V/CM DIFF B L control
ny
djust ­
is accomplished
mplifier
Input mplifier
-B DIFF
operation.
voltages when
high-sensitivity
3-3