Tektronix 502 series, Instruction Manual

Page: 33 / 114

This decreases the amount of negative feedback
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 amplifier
so that the horizontal positioning of the beam
will not be affected, when the beam is posi­
tioned in the center of the crt, as the value of
R331 is changed. The IX CAL. control R357 ad­
justs 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, however, with the 20X CAL. con­
trol R368 which varies the amount of cathode
degeneration. 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 sweep
voltage is coupled directly from the front-panel
EXTERNAL connector to the grid circuit of the
Input Amplifier. This action changes the config­
uration of the amplifier slightly. The Input C.F.
is disconnected from the circuit which changes
the dc level at the cathodes of the Input Ampli­
fier slightly. To compensate for this change an
EXT. HORIZ. DC BAL. control R341 is adjusted
to equalize the cathode voltages.
Also, when using external sweep, the Hori­
zontal POSITIONING 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
502 Oscilloscope is furnished by a single power
transformer T602. The primary has two equal
tapped windings; these may be connected in
parallel for 105- to 125-volt operation, or in
series for 210- to 250-volt operation. The three
main full-wave power supplies furnish regulated
voltages of —150, +100 and +350 volts. The
+350-volt supply also has an unregulated out­
put of about +485 volts for the high-voltage
power supply for the crt. It is unnecessary to
regulate this supply as the high-voltage power
supplies have their own regulation circuits. In
addition to the three main power supplies, a
transistorized full-wave supply furnishes a regu­
lated —6.2-volt output for the heaters in the In­
put Amplifier tubes in the Upper and Lower
Beam Vertical Amplifiers.
Reference voltage for the —150-volt supply is
furnished by a gas-diode voltage-reference tube
V639. This tube, which has a constant voltage
drop, establishes a fixed potential of about —70
volts at the grid of V636B, one-half of a differ­
ence amplifier. The grid potential for the other
half of the difference amplifier, V636A, is ob­
tained from a divider consisting of R621, R622
and R623. R622, the —150 ADJ. determines the
percentage of total voltage that appears at the
grid of V636A and thus determines the total volt­
age across the divider. When this control is pro­
perly adjusted the output is exactly —150 volts.
Should the loading on the supply tend to
change the output voltage, the voltage at the
grid of V636A will change in proportion, and an
error voltage will exist between the two grids of
the difference amplifier. The error signal is am­
plified by V636B, whose plate is dc-coupled to
the grid of the series tube V637. The error volt­
age appearing at the grid of V637 will change
the drop across the tube and hence change the
the voltage at the plate of the tube. The change
in voltage at the plate of V637, which will be in
a direction to compensate for the change in the
output voltage, is coupled by the impedance of
the rectifier V602 back to the output and thus
pulls the output voltage back to its established
—150 volts. C623 improves the gain of the feed­
back loop and thus increases the response of the
circuit to sudden changes in the output voltage.
The —150-volt supply serves as a reference
for the + 100-volt supply. The divider R671-R673
establishes a voltage of essentially zero at the
grid of the amplifier V666B. (The actual voltage
at this grid will be equal to the bias voltage re­
quired by the stage.) If the loading should tend
to change the output voltage an error voltage
will appear at the grid of V666B. This error volt­
age will be amplified and will appear at the
grid of the series tube V677. The cathode of
V677 will follow the grid and hence the output
©
CIRCUIT DESCRIPTION — TYPE 502
4-9