Tektronix 503 Page 32 Instruction Manual Download

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Maintenance— Type 503

If the voltage at the cathode of V145B, pin 3, is more

negative  than  —55  volts,  check  the  tubes  in  the  Hold-O ff
Circuit,  the  H old-O ff  capacitor,  and  resistors  in  the  cathode
circuit  of  the  two  tubes.

Low voltage at the plate of the M iller tube indicates

that the tube is conducting quite heavily and is not being
allowed to perform its normal runup operation.

If this

trouble exists on only a few ranges of the SWEEP TIME/CM
switch, the trouble is likely to be an open timing resistor.

If the trouble exists on all ranges of the SWEEP TIME/CM

switch, the trouble is probably due to a defective Sweep-
Gating Multivibrator.

To check the Sweep-Gating M ultivibrator, monitor the

voltage at pin 3 of V145B and adjust the STABILITY ADJUST
control for a reading of

—66

volts on the voltmeter. W ith

this  voltage,  the  Sweep-Gating  M ultivibrator  and  the  sweep
should  free  run.  If  the  multivibrator  does  not  free  run,  check
the  resistances  in  the  stage.

If the voltage at pin 3 of V145B remains relatively con

stant  as  the  STABILITY  ADJUST  control  is  rotated,  a  defect
in  the  Hold-O ff  Circuit  is  indicated.  One  cause  of  this  con
dition  could  be  a  shorted  hold-off  capacitor.  If  the  voltage
does  not  adjust  to  the  proper  level,  check  the  resistors  in  the
cathode  circuit  of  V145B.

Nonlinear Sweep.

A nonlinear sweep voltage w ill be

generated if the circuit charging the Timing Capacitor does

not  remain  constant.  If  the  nonlinearity  exists  at  all  sweep
rates,  a  defective  M iller  tube  (V160A)  is  the  probable  cause
of  the  trouble.  If  the  nonlinearity  occurs  only  at  certain
sweep  rates,  a  faulty  M iller  tube  or  a  leaky  Timing  Capacitor

is the probable cause.

Insufficient Horizontal Deflection.

If the horizontal

trace  starts  at  the  left-hand  side  of  the  screen,  but  does
not  extend  to  the  right-hand  side,  the  Hold-O ff  Circuit  is
resetting  the  Sweep-Gating  M ultivibrator  before  the  sweep
is  complete.  If  the  sweep  cannot  be  adjusted  to  normal
length  with  the  SWP.  LENGTH  control,  R176,  the  resistance
in  the  cathode  circuit  of  V160B  should  be  checked.

Improper Triggering.

If the sweep cannot be triggered

properly,  the  gating  pulse  from  the  M ultivibrator  (assum
ing  the  multivibrator  is  operating  properly),  is  not  turning
the  Disconnect  Diodes  (V152A  and  D152)  off  and  on  proper
ly.  The  start  of  the  gating  pulse,  which  turns  the  diodes
off  and  starts  the  sweep,  is  initiated  by  the  triggering  pulse
at  the  grid  of  V135A.  The  end  of  the  gating  pulse,  which
turns  the  diodes  on  and  initiates  the  retrace,  is  controlled
by  the  hold-off  waveform  at  the  grid  of  V135A.  The  main
component  to  check,  in  addition  to  the  tubes,  is  the  differ
entiating  capacitor  C l31.  Misadjustment  of  the  STABILITY
ADJUST  control  (R ill)   will  also  cause  the  Sweep  Generator
to  trigger  improperly.  Refer  to  the  Calibration  section  for
correct  adjustment.

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