Circuit Description—Type 422 AC-DC
duces a 200-millivolt internal signal to the vertical deflec
tion system.
A t the time Q775 turned off, C760 was charged nega
tively on the Q775 side and it begins to discharge toward
+12 volts through R770. When it reaches a level about 1.2
volts more positive than the base of Q775 (as determined
by divider R763-R775) Q775 is forward biased and it turns
Q765 off through R773. The collector of Q775 rises posi
tive and forward biases diode D780 to shunt the
R780-R781 current away from the output divider. Diode
D782 is reverse biased and there is no voltage across the
output divider. Therefore, the voltage level of both outputs
rises to zero volts. The cycle has been completed and now
C760 begins to discharge through R760 to start the next
cycle.
—81 -Volt Regulator
SN 28000 and up. The regulating circuit for the -81 -
volt power supply is basically a feedback amplifier. Zener
diode D730 provides a 6.2 volt reference voltage that is
applied to the amplifier input resistance comprised of R731
and R732. Variable resistor R732 adjusts the amplifier gain,
thereby adjusting the output voltage level. D734 sets the
level at the base of Q734 at or near zero volts. Any devia
tions in the —81 volt output level are coupled back to the
base of amplifier Q734 by C733-R733. The amplified and
inverted error signal at the collector of Q734 controls the
output level of the power supply through emitter follower
Q735 and zener diode D736. Emitter follower Q737 is a
series regulator for the power supply output voltage. Field
effect transistor Q736 acts as a relatively high-impedance
current source for the Q734-Q735 circuitry.
SN 20000 thru 27999. Unregulated —110 volts from the
power supply provides the negative voltage source for the
-81-V olt Regulator stage. This voltage is connected to the
emitter of Q734 through R733 and to the base of Q734
through zener diode D739. D739 holds the forward base-
emitter voltage drop of Q734 constant so Q734 operates as
a constant current source for voltage-regulator tube V739.
The voltage drop of V739 is 81 volts and along with the
forward drop of diode D735, it sets a level of about—81.6
volts at the base of Q737. The base-emitter drop of Q737
produces an output voltage of —81 volts at the emitter of
Q737. Due to the constant-current action of transistor
Q734 and zener diode D739 and the constant-voltage
action of voltage regulator V739, the output voltage level is
held very stable.
+10.5-Volt Regulator
Reference voltage for the +10.5-Volt Regulator stage is
provided by the -81-V olt Regulator. Transistors Q714and
Q717 operate as a feedback-stabilized voltage regulator
circuit to maintain a constant +10.5 volt output level. At
time of turn on, circuit operation is as follows: Positive
voltage from the +55-volt supply provides current to the
base of Q717 through R716 and R715. Q717 is forward
biased and its emitter rises positive, following the voltage at
its base. The voltage at the base of Q714 also rises positive
until it reaches about +0.6 volt; then Q717 is forward
biased and the resulting collector current through
R714-R715-R716 offsets the base current of Q717 from
the +55-volt supply to stabilize the circuit voltages at this
point.
The output of this supply is stabilized a t+10.5 volts by
the feedback through Q714 to oppose changes due to the
load or due to ripple from the unregulated +55-volt supply.
For example, if the output voltage at the emitter of Q717
attempts to rise further positive, more current flows
through Q714 (forward bias of Q714 increases through
R712-R718). This increase in collector current from Q714
reduces the base current of Q717 so that it conducts less.
Thus Q717 provides less current to the load and the output
voltage returns to +10.5 volts. Likewise, if the output volt
age at the emitter of Q717 begins to fall below+10.5 volts,
the feedback through Q714 allows more current to flow
through Q717 to raise its emitter voltage to +10.5 volts.
Zener diode D714 holds the junction of R715-R716 at a
level about 22 volts more positive than the output level of
this supply. This action provides a stable source voltage for
Q714 and Q717 to reduce the ripple due to the +55-volt
source. Diode D718 provides temperature conpensation for
this stage.
Other Functions
The circuitry shown on the Calibrator and Regulators
diagram also includes the voltage interconnections between
the power supply unit and the indicator. Neon bulb B728
receives voltage from the power supply unit to indicate
when the POWER switch is on. SCALE ILLUM control
R725 controls the current through B725 and B726 to con
trol the illumination of the graticule lines on the CRT.
Neon bulbs B741, B743 and B745 provide indication of
uncalibrated conditions in the Channel 1 Input Amplifier,
Channel 2 Input Amplifier and Sweep Generator circuits
respectively. SW741, SW743 and SW745 apply voltage to
the respective UNCAL lights when the associated VARI
ABLE control is not in the CAL position.
CRT Circuit
General
The CRT Circuit provides the high-voltage and control
circuits necessary for operation of the cathode-ray tube
(CRT). Fig. 3-11 shows a detailed block diagram of the
CRT circuit. A schematic of this circuit is shown on dia
gram 10 at the back of this manual.
High-Voltage Multipliers and High-Voltage
Regulator
The drive signal for the primary of high-voltage trans
former T801 is obtained from the associated power supply.
The frequency of this drive signal is about 20 kilohertz.
When connected to the AC-DC Power Supply, the drive
signal is connected to the two halves of the T801 primary
3-18
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