Tektronix 502A - Page 31

Circuit Description — Type 502A
supplies have their own regulation circuits. In addition to
the three main power supplies, a transistorized full-wave
supply furnishes a regulated — 6.2 volt output for the
heaters in the Input 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 difference amplifier. The grid potential for the other
half of the difference amplifier, V636A, is obtained from a
divider consisting of R621, R622 and R623. R622, the — 150
Control determines the percentage of total voltage that ap 
pears at the grid of V636A and thus determines the total
voltage across the divider. When this control is properly
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 on error voltoge will exist between the
two grids of the difference amplifier. The error signal is
amplified by V636B, whose plate is dc-coupled to the grid
of the series tube V637. The error voltage appearing at the
grid of V637 will change the drop across the tube and hence
change 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 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 feedback loop
and thus increases the response of the circuit to sudden
change 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
required 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 voltage 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 voltage
will be returned to its established value of +100 volts.
C671 improves the response of the circuit to sudden changes
in the output voltage.
A small sample of the unregulated bus ripple appears at
the screen of V666B through R662. The ripple signal which
appears at the screen, which acts as an injector grid, will
produce a ripple component at the grid of V677 which will
be opposite in polarity to the ripple appearing at the plate
of V677. This tends to cancel the ripple at the cathode of
the tube, and hence reduces the ripple on the +100 volt
bus. This same circuit also improves the regulation of the
supply in the presence of line voltage variations.
The +350 volt supply functions in the same manner as
the +100 volt supply. Rectified voltage from the cathode of
V722 is added to the voltage supplying the +100 volt
regulator, to supply voltage for the +350 volt regulator.
As mentioned previously, the +350 volt supply also furnishes
an unregulated output of about +485 volts for the crt
high-voltage supply.
The — 6.2 volt supply works in essentially the same manner
as the vacuum-tube supplies. The divider R648-R649 estab 
lishes a reference voltage at the base of Q644. If we now
assume that the output tends to go more negative, the
emitter of Q644 will also go more negative since it is
strapped directly to the output. The collector of Q644
will then go more negative, carrying with it the base of
Q634. The collector of Q634 will then go up, carrying with
it the base of Q647 . The series-regulator transistor Q647
is essentially an emitter-follower, so the emitter will follow
the base. Hence, the emitter of Q647 also goes up. This
increase in the voltage at the emitter of Q647 will be
coupled through the rectifiers, D642A and B, back to the
output and will thus pull the output back up to its specified
value.
CRT CIRCUIT
A single 40-Kc Hartley oscillator circuit furnishes energy
for the three power supplies that provide voltages for
the crt. The main components of the Oscillator circuit are
V800 and a portion of the primary T801 tuned by C807.
The three half-wave rectifier circuits employ capacitor 
input filters. Separate supplies are required for the grid and
each of the cathode circuits of the crt in order to provide
dc-coupled unblanking to the crt grids.
V822 and V832 supply about — 2900 volts for the cathodes
of the crt. V862 supplies about — 3100 for the grids (the
actual voltage depends on the setting of the INTENSITY
control.)
In order to maintain a constant deflection sensitivity in
the crt, and thereby maintain the calibration of the instru 
ment, it is necessary that the accelerating potentials in
the crt remain constant. This is accomplished by regulating
the three supplies by comparing a sample of the cathode
voltage to the regulated — 150 volt supply. This sample
voltage, obtained from the arm of the HV Control R826,
is applied to the grid of V814A; the cathode of this tube is
connected to the — 150 volt regulated supply. The error
voltage is amplified by V814A and V814B; the output of
V814B varies the screen voltage of the Oscillator tube and
thus controls its output.
The HORIZ SENS BAL Control R833 allows a more perfect
balance of the sensitivity of the two horizontal beams.
Varying this control can change the crt cathode potentials by
approximately 100 volts with respect to each other. This
allows for a corresponding change in horizontal sensitivities
of approximately + or — 3%.
The HORIZ BEAM REGIS will allow the beams to be
brought into register; i.e., the sweep for each vertical ampli 
fier will start at the same place on the crt screen.
Unblanking
As mentioned previously, dc-coupled unblanking is ac 
complished by employing separate high-voltage supplies
for the grids and cathodes. The cathode supplies are tied
to the LV power supply. 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 grids 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
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