Tektronix 502A, Instruction Manual

Page: 41 / 110

Maintenance —Type 502A
Low frequency distortion is usually caused by a change
in the time constant of the input coupling circuit. If tubes
become gassy, their resultant grid current will establish a
time constant network which will affect the low frequency
response of the circuit.
High frequency response can be affected considerably by
an improperly adjusted probe. Consequently, in any case of
high frequency distortion you should first check the probe
adjustment. 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 freqeuency compensation networks.
An overshoot waveform shown accentuated in Fig. 4-11 is
the result of excessive high frequency compensation. This
can be caused by a tube condition known as cathode inter­
face. 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 ad­
justment and operation of the high frequency compensation
circuits.
For improper triggering
Improper triggering can be caused by a defective 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 positions 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 input amplifier or the
trigger multivitbrator will be at fault. You should check
tubes V24 and V45.
NOTE
All
voltages in this section are measured with a
20,000 ohms-per-volt voltmeter.
To determine which stage is defective, rotate the TRIG­
GERING LEVEL Control fully counter-clockwise to the
AUTOMATIC position. With no triggering signal, the sweep
should apear on the crt. If the sweep does not appear,
the trigger multivibrator is defective. If the sweep does
appear, either the trigger input amplifier or the trigger
multivibrator may be at fault.
A check on the trigger input 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
+ 96 volts. If the voltage is incorrect, the trigger input am­
plifier circuit is defective. The trouble will probably exist
in the resistors or switches of the circuit.
If the voltage measured is correct, rotate the TRIGGER­
ING LEVEL Control completely through its range while
monitoring the voltage at pin 6 of V24B. The voltage should
vary between approximately 34 and 135 volts. An incorrect
voltage range indicates a defective trigger input 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 re­
sistors. The voltage 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 TRIGGER­
ING LEVEL Control is not in the RECURRENT or AUTOMATIC
positions
this is due to astable operation of the sweep gating
multivibrator. This trouble will probably be due to defective
resistors in the grid circuit of V135A (resistors R185, R186,
or R111) or in the grid circuit 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 measuring the plate volt­
age of the Miller tube, V161A. The voltage reading obtained
will probably be either approxi300 volts or approxi­
30 volts. A reading of +300 volts indicates that
the Miller stage has run up
and has not been reset, while a
reading
of +30 volts indicates that the Miller stage is not
being allowed to runup. The condition that actually exists
will depend on the type of trouble occurring in the circuit.
The two conditions of plate voltage will be handled sepa­
rately in the following paragraphs.
Miller Stage is Run-up
If the voltage at the plate of the Miller tube, V161A,
is high, the tube is cutoff. This can result from any one
of the following conditions: (1) Disconnect Diodes do not
conduct, (2) Sweep-Gating Multivibrator does not reset,
(3) Hold-Off Cathode Followers do not reset the Sweep-
Gating Multivibrator and
(4) Runup Cathode Follower does
not drive the Hold-Off Cathode Follower. The defective stage
can be detected by a series of systematic voltage measure­
ments. When an improper voltage reading is obtained, this
will indicate the defective stage.
Check the votage at the grid of the Miller tube. If the
reading is only a few volts negative, the Disconnect Diodes
are probably conducting normally and can be eliminated as
a possible cause of the trouble. If the voltage is more nega­
tive than —50 volts, however, the diodes are not conducting.
Check V152 and resistors R147 and R148.
Measure the voltage at the output of the Time-Base Gener­
ator (pin 3 of V161 B). If this voltage is approxi250
volts, the Runup Cathode Follower stage may be assumed
to be operating correctly. If this voltage is low, however,
the stage is defective and its grid and cathode circuits should
be checked.
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