Tektronix 502 series Instruction Manual Download Page 31

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charging the capacitor can be selected to cover
a wide range of sweep speeds. Thus, the tinning

circuit determines the speed at which the spot
moves across the crt.

The length of the sweep, that is, the distance

the  spot  moves  across  the  crt,  is  determined  by
the  setting  of  the  SWP.  LENGTH  control  R176.
As  the  sweep  voltage  rises  linearly  at  the  cath
ode  of  V I61B,  there  will  be  a  linear  rise  in  the
voltage  at  the  arm  of  the  SWP.  LENGTH  control.

This will increase the voltage at the grid and

cathode of V183A and at the grid and cathode

of V145B. As the voltage at the cathode of

V145B  rises  the  voltage  at  the  grid  of  V135A  will
rise.  When  the  voltage  at  this  point  rises  to  a
point  that  V135A  comes  out  of  cutoff,  the  Sweep-

Gating  Multi  will  rapidly  revert  to  its  original
state  with  V135A  conducting  and  V145A  cutoff.
The  voltage  at  the  plate  of  V145A  will  then  rise,
carrying  with  it  the  voltage  at  the  plates  of  the

On-Off  Diode  V I52.  The  diodes  then  conduct
and  the  lower-half  (V152B)  provides  a  discharge
path  for  the  Timing  Capacitor  through  R147  and

R148, and through the resistance in the cathode

circuit of V I61B. The plate voltage of the Miller

Tube now falls linearly, under feedback condi

tions  essentially  the  same  as  when  it  generated
the  sweep  portion  of  the  waveform,  except  for  a
reversal  of  direction.

The resistance through C l60 discharges is

much less than that through which it charges

(the Timing Resistor). The capacitor current for

this period will therefore be much larger than

during the sweep portion, and the plate of the

Miller Tube will return rapidly to its quiescent

voltage. This produces the retrace portion of the
sweep sawtooth, during which time the crt beam

returns rapidly to its starting point.

The Hold-Off Circuit prevents the Time-Base

Generator  from  being  triggered  during  the  re
trace  interval.  In  addition,  the  Hold-Off  allows  a
finite  time  for  the  Time-Base  circuits  to  regain  a
state  of  equilibrium  after  the  completion  of  a
sweep.

During the trace portion of the sweep saw

tooth the Hold-Off Capacitor Cl 80 charges

through  V183A  as  a  result  of  the  rise  in  voltage
at  the  cathode  of  V183A.  At  the  same  time  the
grid  of  V135A  is  being  pulled  up,  through  the
cathode-follower  V145B,  until  V135A  comes  out
of  cutoff  and  starts  conducting.  As  mentioned
previously,  this  is  the  action  that  initiates  the  re

trace. At the start of the retrace interval C l80

starts discharging through the Hold-Off Resistor.
The time-constant of this circuit is long enough,

however, so that during the retrace interval,

and for a short period of time after the comple

tion of the retrace, Cl 80 holds the grid of V135A

high enough so that it cannot be triggered.

However,  when  Cl 80  discharges  to  the  point
that  the  cathode-follower  V145B  is  cutoff,  it  loses
control  over  the  grid  of  V135A  and  the  grid  re

turns to the level established by the STABILITY
ADJUST R ill. The amount of hold-off time re

quired is determined by the sweep speed, i.e., by

the size of the Timing Capacitor. For this reason

the  TIME/CM  switch  changes  the  time-constant
of  the  Hold-Off  Circuit  simultaneously  with  that
of the Timing  Circuit.

The STABILITY ADJUST Rll regulates the dc

level at the grid of V135A. This control should

be  adjusted  so  that  the  voltage  at  the  grid  of
V135A  is  just  high  enough  to  prevent  the  circuit
from  free-running.  Adjusted  in  this  manner,  a

sweep  can  only  be  produced  when  a  negative
trigger  pulse,  from  the  Time-Base  Trigger  circuit,
can  drive  the  grid  of  V135A  below  cutoff.  How

ever, should a free-running sweep be desired, the

TRIGGERING LEVEL control can be turned full

right; this closes the RECURRENT switch and con

nects the grid circuit of V135A to the —150-volt

supply. This permits the grid of V135A to fall to
cutoff immediately upon removal of the hold-off

voltage, at which point the next sweep is ini

tiated.

The positive rectangular pulse appearing at

the cathode of V135B is coupled to the grid cir

cuits of the crt. This pulse, whose start and dura

tion are coincident with the trace portion of the
sweep sawtooth, unblanks the crt and permits

the trace to be observed.