Tektronix 422 Instruction Manual Download Page 56

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Circuit Description—Type 422 AC-DC

duces a 200-m illivolt internal signal to the vertical deflec
tion system.

A t the time Q775 turned o ff, 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  o f  Q775  (as  determined
by  divider  R763-R775)  Q775  is forward  biased  and  it turns
Q765  o ff  through  R773.  The  collector  of  Q775  rises  posi
tiv e   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  o f 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 fo r the

-8 1 -

vo lt power supply is basically a feedback amplifier. Zener

diode D730 provides a 6.2 vo lt 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 o f Q734 at or near zero volts. Any devia

tions in the —81 vo lt 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. Em itter follow er Q737 is a
series regulator fo r the power supply output voltage. Field

effect transistor Q736 acts as a relatively high-impedance

current source fo r the Q734-Q735 circuitry.

SN 20000 thru 27999.

Unregulated —110 volts from the

power  supply  provides  the  negative  voltage  source  fo r  the
-8 1 -V o lt  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  fo r  voltage-regulator  tube  V739.

The  voltage  drop  of  V739  is  81  volts  and  along  w ith  the
forward  drop  of  diode  D735,  it  sets  a  level  of a b o u t—81.6
volts  at  the  base  o f  Q737.  The  base-emitter  drop  of  Q737

produces  an  output  voltage  o f  —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 fo r the +10.5-Volt Regulator stage is

provided by the -8 1 -V o lt Regulator. Transistors Q714and
Q717 operate as a feedback-stabilized voltage regulator

circuit to maintain a constant +10.5 vo lt output level. A t

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 em itter rises  positive, following the voltage at
its  base.  The  voltage  at  the  base  o f  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 o f 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 unreg55-volt supply.
For example, if the output voltage at the emitter o f 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 o f 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 outp ut vo lt
age  at  the  emitter  o f Q717  begins to fall  b elow +10.5 volts,
the  feedback  through  Q714  allows  more  current  to   flo w

through Q717 to raise its emitter voltage to +10.5 volts.

Zener diode D714 holds the junction o f R715-R716 at a
level about 22 volts more positive than the outp ut level o f

this supply. This action provides a stable source voltage fo r
Q714 and Q717 to reduce the ripple due to the +55-volt

source. Diode D718 provides temperature conpensation fo r
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 ILLU M control

R725 controls the current through B725 and B726 to con

trol the illum ination o f the graticule lines on the CRT.

Neon  bulbs  B741,  B743  and  B745  provide  indication  o f
uncalibrated  conditions  in  the  Channel  1  Input  A m plifier,
Channel  2  Input  A m p lifie r  and  Sweep  Generator  circuits
respectively.  SW741,  SW743  and  SW745  apply  voltage  to
the  respective  UNCAL  lights  when  the  associated  V A R I
ABLE  control  is  not  in  the CAL position.

CRT Circuit

General

The CRT Circuit provides the high-voltage and control

circuits necessary fo r operation o f the cathode-ray tube

(CRT). Fig. 3-11 shows a detailed block diagram o f the

CRT circuit. A schematic o f this circuit is shown on dia
gram 10 at the back o f this manual.

High-Voltage

Multipliers

and

High-Voltage

Regulator

The drive signal fo r the primary o f high-voltage trans