Tektronix 535A, Instruction Manual

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Circuit Description— Type 535A
LOW-VOLTAGE POWER SUPPLY
Plate and filament power for the tubes in the Type 535A is
furnished by a single power transformer T601, The primary
has two equal windings which may be connected in parallel
for 117-volt operation, or in series for 234-volt operation.
The power supply will maintain regulation over line voltage
ranges of 105 to 125 volts, or 210 to 250 volts, rms, 50-60-
cycles. Bridge rectifiers are employed for the five separate,
full-wave, power supplies. The five supplies furnish regulated
output voltages of —150, +100, +225, +350 and +500
volts.
— 150-Volt Supply
Reference voltage for the —150-volt supply is fur­
nished by a gas diode voltage-reference tube V609. This
tube,
which has a constant voltage drop, establishes a
fixed potential of about —87 volts at the grid of V624A,
one-half of a difference amplifier. The grid voltage for
the other half of the difference amplifier, V624B, is obtained
from a divider consisting of R615, R616 and R617. The
—150 Adj. control R616 determines the percentage of total
voltage that appears at the grid of V624B and thus deter­
mines the total voltage across the divider. This control is
adjusted so that the output voltage is exactly —150 volts.
If line-voltage or load fluctuations tend to change the
output voltage, an error signal exists between the two grids
of the difference amplifier. The error signal is amplified
in V624 and V634 and applied to the grids of the series
tubes, V627, V637 and V647. The resulting change in voltage
at the plates of the series tubes which will be in a direction
to compensate for any change in output voltage, is coupled
through the rectifiers to the output to keep this voltage
constant. Capacitors, C617 and C628 improve the ac gain
of the feedback loop to increase the response of the regu­
lator circuit to sudden changes in output voltage.
A small amount of unregulated bus ripple is coupled to
the screen of V634 through R637. The phase of the ampli­
fied ripple voltage at the plate of V634 is such as to cancel
most of the ripple on the —150-volt bus.
4-225-Volt Supply
Rectified voltage from terminals 7 and 10 of the power
transformer is added to the voltage supplying the +100-
volt regulator to furnish power for the +225-volt regulator.
This supply is regulated by comparing to ground (the grid
of V684A) the voltage of a point near ground obtained from
the divider R680-R681, connected between the +225-volt
bus and the regulated —150-volt supply. Any error voltage
that exists between the grids of the difference amplifier
(V684) is amplified in both V684 and V694, and coupled
through the cathode follower V677B to the +225-volt bus.
The change in the voltage at the cathode of V677B, due
to the regulator action, will be opposite in polarity to the
original error signal and will thus tend to keep the out­
put constant. This supply also furnishes an unregulated
output of about +325 volts for the oscillator in the crt high-
voltage supply. It is unnecessary to regulate this voltage as
the crt supply has its own regulator circuits.
+ 350-Volt Supply
Rectified voltage from terminals 5 and 14 of T601 is
added to voltage supplying the +100-volt regulator to
furnish power for the +350-volt regulator. This supply is
regulated by comparing to ground the voltage of a point
near ground obtained from the divider R710-R711 connected
between the +350-volt bus and the regulated —150-volt
supply. The operation of the regulator circuit is the same
as that described for the +100-volt supply.
+ 500-Volt Supply
Rectified voltage from terminals 20 and 21 of T601 is
added to the regulated side of the +350-volt supply to
furnish power for the + 500-volt regulator. This supply is
regulated by comparing to the reg350-volts the
voltage of a point near +350 obtained from the divider
R740-R741 connected between the +500-volt bus and the
regulated —150-volt supply. The regulator action of this cir­
cuit is the same as that described for the +100-volt supply.
+ 100-Volt Supply
The +100-volt supply is regulated by comparing to
ground (the cathode of V664) the voltage of a point near
ground potential obtained from the divider R650-R651 con­
nected between the + 100-volt bus and the regulated —150-
volt supply. Any error voltage that exists is amplified and
inverted in polarity by V664 and coupled through the
cathode follower V667A to the output to prevent the output
voltage from changing. Capacitor C650 improves the ac
gain of this circuit.
A small sample of the unregulated bus ripple appears at
the screen on V664 through R667. This produces a ripple
component at the grid of the cathode follower V677A that
is opposite in polarity to the ripple at the plate; this tends
to cancel the ripple at the cathode and hence on the + 100-
volt bus. This same circuit also improves the regulation in
the presence of line-voltage variations.
Time-Delay
A Time-Delay relay K600 delays the application of dc
voltages to the amplifier tubes in the instrument for about
25 seconds. This delay is to allow the tube heaters time to
bring the cathodes up to emission temperature before opera­
ting potentials are applied.
CALIBRATOR
The Calibrator is a square-wave generator whose appro xi-
mately 1-kc output is available at a front-panel connector
labeled CAL. OUT. It consists of a multivibrator V875 and
V885A connected so as to switch the cathode follower V885B
between two operating states ... cutoff and conduction.
During the negative portion of the Multivibrator waveform
the grid of V885B is driven well below cutoff and its cath-
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