stant network which will affect the low frequency
response of the circuit.
High frequency response can be affected con
siderably by an improperly adjusted probe.
Consequently, in any case of high frequency dis
tortion you should first check the probe adjust
ment. Refer to the Operating Instructions for the
compensation procedure. Other factors which
can affect the high frequency response of the
vertical amplifier are mainly related to the high
frequency compensation networks.
An overshoot waveform shown accentuated in
Fig. 5-6 is the result of excessive high frequency
compensation. This can be caused by a tube
condition known as cathode interface. If this
type of distortion is detected, you should check
the tubes in the amplifier. If tube replacement
does not completely correct the trouble, you
should then check the adjustment and operation
of the high frequency compensation circuits.
For improper triggering
Improper triggering can be caused by a defec
tive trigger pickoff circuit. The trouble may be
caused either by a faulty cathode follower, V493
or by a defective component in the circuit.
Troubleshooting The Time-Base Trigger
If the trouble occurs in some but not all posi
tions of the TRIGGER SELECTOR switch, the
trouble is likely due to a defective TRIGGER
Selector switch or input coupling circuit. If the
trouble occurs in all positions of the TRIGGER
SELECTOR switch, either the trigger amplifier or
the trigger multivibrator will be at fault. You
should check tubes V24 and V45.
NOTE
All voltages in this section are meas
ured with a 20,000 ohms-per-volt
voltmeter.
To determine which stage is defective, rotate
the TRIGGERING LEVEL control fully counter
clockwise to the AUTOMATIC position. With no
triggering signal, the sweep should appear on
the crt. If the sweep does not appear, the trigger
multivibrator is defective. If the sweep does
appear, either the trigger amplifier or the trigger
multivibrator may be at fault.
A check on the trigger amplifier circuit may
be made as follows. With the TRIGGERING
LEVEL control still in the AUTOMATIC position,
measure the voltage at the plate, pin 6, of V24B.
This voltage should be approximately +85 volts.
If the voltage is incorrect, the trigger amplifier
circuit is defective. The trouble will probably
exist in the resistors or switches of the circuit.
If the voltage measured is correct, rotate the
TRIGGERING LEVEL control completely through
its range while monitoring the voltage at pin 6
of V24B. The voltage should vary between ap
proximately 40 and 135 volts. An incorrect volt
age range indicates a defective trigger amplifier
or TRIGGERING LEVEL control. If the voltage
range is correct, the trouble will be in the trigger
multivibrator. A trouble in the multivibrator will
probably be due to defective resistors. The volt
age divider network between the plate of V45A
and the grid of V45B is particularly critical.
Troubleshooting The Time-Base Generator
For free-running operation
If the Time-Base Generator free runs when the
TRIGGERING LEVEL control is not in the RECUR
RENT or AUTOMATIC positions, this is due to
astable operation of the sweep gating multivi
brator. This trouble will probably be due to de
fective resistors in the grid circuit of V135A
(resistors R129, R130, or R ill) or in the grid cir
cuit of V145A (resistors R141 or R143).
For no horizontal sweep
If the Time-Base Generator is not producing a
sawtooth waveform when the TRIGGERING
LEVEL control is in the RECURRENT position,
some defect in the circuit is causing the output
to remain at some fixed voltage. A clue to the
cause of this trouble can be obtained by measur
ing the plate voltage of the Miller tube, V I61 A.
©
MAINTENANCE — TYPE 502 5-11
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