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-Off
Circuit, the Hold-Off capacitor, and resistors in the cathode
circuit of the two tubes.
Low voltage at the plate of the Miller 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 Multivibrator, monitor the
voltage at pin 3 of V145B and adjust the STABILITY ADJUST
control for a reading of —66 volts on the voltmeter. With
this voltage, the Sweep-Gating Multivibrator 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-Off 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 will be
generated if the circuit charging the Timing Capacitor does
not remain constant. If the nonlinearity exists at all sweep
rates, a defective Miller tube (V160A) is the probable cause
of the trouble. If the nonlinearity occurs only at certain
sweep rates, a faulty Miller 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-Off Circuit is
resetting the Sweep-Gating Multivibrator 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 Multivibrator (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.
©
4-9
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