Tektronix 516 series, Instruction Manual

Page: 29 / 132

Circuit Description — Type 5 1 6
for the high-voltage supply for the crt. It is unnecessary to
regulate this supply as the high-voltage supply has its own
regulating circuit.
Reference voltage for the —150-volt supply is established
by a gas diode, VR tube V609. This tube, which has a con­
stant voltage drop, establishes a fixed potential of about
—84 volts at one grid of the Difference Amplifier, V634. The
grid potential for the other half of the Difference Amplifier
is obtained from a voltage divider consisting of R615, R616
and R617. The setting of the —150 ADJ., R616, determines
the percentage of the total output voltage that appears at
the grid of V634B and thus determines the total voltage across
the divider. When this adjustment is properly set, the output
voltage of the —150-volt supply will lie somewhere between
—147 volts and —153 volts.
Should the loading on the supply tend to change the out­
put voltage, the potential at the grid of V634B will change
in proportion, and an error voltage will exist between the
two grids of the Difference Amplifier. The error signal is
amplified by V634A, whose plate is dc-coupled to the grid
of the Series Tube, V627. The error voltage appearing at
the grid of V627 will change the bias and, hence, the effec­
tive resistance of V627. This will allow more or less current,
as required, to flow through the load to bring the output
voltage back toward its original level. C617 improves the
ac gain of the feedback loop, and thus increases the response
of the circuit to sudden changes in output voltage.
The —150-volt supply serves as a reference for both the
+ 100-volt and +300-volt supplies. In the -f 100-volt supply,
the voltage divider R650-R651 establishes a voltage of es­
sentially zero at the grid of the Amplifier, V654. (The actual
voltage at this grid will be equal to the bias voltage required
by the tube.) If the loading should tend to change the out­
put voltage, an error voltage will appear at the grid of the
amplifier. The error voltage will be amplified and will ap­
pear at the grid of the Series Tube, V667A. The cathode
of V667A will follow the grid, and thus the output voltage
will be returned to its established value of +100 volts. C650
improves the response of the regulator circuit to sudden
changes in output voltage.
A small sample of the unregulated-bus ripple will appear
at the screen of V654 through R657. This ripple signal ap­
pearing at the screen (which acts as an injector grid) will
produce a ripple component at the grid of V667A which is
opposite in polarity to the ripple appearing at the plate of
V667A. This tends to cancel the ripple at the cathode of
V667A, and hence reduces the ripple on the +100-volt bus.
This same circuit also improves the regulation of the circuit
in the presence of line voltage variation.
The +300-volt supply functions in the same manner as
the +100-volt supply. Rectified voltage from terminals 8
and 9 of the power transformer is added to the voltage
supplying the +100 volt regulator to supply power for the
+300-volt regulator. As mentioned previously, the +300-volt
supply also provides an unregulated output of about 400
volts for the crt high-voltage supply.
CRT CIRCUIT
A 60-kHz Hartley oscillator circuit furnishes energy for
the three power supplies that provide accelerating voltages
for the crt. The main components of the oscillator are V800
and the primary of T801 tuned by C808.
The rectifier circuits are the half-wave type, with capaci­
tor-input filters. Separate supplies are required for the grid
and cathode circuits in order to provide dc-coupled un­
blanking to the grid supply.
V822 supplies —1675 volts for the cathode of the crt.
V842 supplies +2325 volts for the post-anode acceleration.
This provides an accelerating voltage of 4000 volts for the
crt beam. V832 supplies about —1750 volts for the grid of
the crt (the actual voltage depends upon the setting of the
INTENSITY control, R826).
In order to maintain a constant deflection sensitivity in the
crt, and thereby maintain the calibration of the oscilloscope,
it is necessary that the accelerating potentials in the crt re­
main constant. This is accomplished by regulating the three
supplies by comparing a "sample'' of the high voltage to the
regulated —150-volt supply. The "sample" voltage, obtained
from the arm of the HV ADJ., R841, is applied to the grid of
V814B; the cathode of this tube is connected to the regulated
-150-volt supply. The error signal is amplified by V814B
and V814A; the output of V814A varies the screen voltage
of the oscillator tube, thereby controlling the amplitude of
its output.
Unblanking
As mentioned previously, dc-coupled unblanking is ac­
complished by employing separate high-voltage supplies for
the grid and cathode. The cathode supply is tied to the
+ 100-volt supply via the decoupling network, R801-C801A.
The grid supply, on the other hand, is not tied to any other
supply, and is therefore "floating". The unblanking pulses
from the Time-Base Generator are transmitted to the grid of
the crt via the floating grid supply.
The stray capacitance in the circuit makes it difficult to
move the floating supply fast enough to unblank the crt in
the required time. To overcome this, an isolation network
composed of C827, R827, and R828 is employed. By this
arrangement, the fast leading edge of the unblanking pulse
is coupled directly to the grid of the crt via C827. For short-
duration unblanking pulses (at the faster sweep rates), the
power supply itself is not appreciably moved. For longer
unblanking pulses (at the slower sweep rates), however, the
stray capacitance of the circuit is charged through R827.
This holds the grid at the unblanked potential for the dura­
tion of the unblanking pulse.
AMPLITUDE CALIBRATOR
The Amplitude Calibrator is a square-wave generator
whose output is available at the CAL. OUT connector on
the front panel. It consists of an astable multivibrator, V885A
and V875, connected so as to switch the Cal. Out Cathode
Follower, V885B, between two operating states, cutoff and
conduction, at approximately 1 kHz.
During the negative portion of the multivibrator waveform,
the grid of V885B is driven well below cutoff and the cath­
ode rests at ground potential. The CAL. ADJ. potentiometer,
R879, is adjusted so that the voltage at the cathode of V885B
(CAL. TEST PT.) rises to exactly +100 volts during the posi­
tive portion of the mutivibrator waveform. The tapped cath­
ode resistor provides eleven calibrated outputs, from 0.05
volt to 100 volts, which are used for calibrating the gain
of the Vertical Amplifier
a n d a d ju stin g the com pensation o f
attenuator probes.