Tektronix 502A, Instruction Manual

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Maintenance — Type 502A
Fig. 4-6. Soldering to a terminal mounted in plastic. Note the use
of the long-nosed pliers between the iron and the coil form to
absorb the heat.
Fig. 4-7. The ceramic strip and its parts.
NOTE
Considerable force may be necessary to push the
mounting posts into the nylon collars. Be sure
that you apply this force to that area of the
ceramic strip directly above the mounting posts.
TROUBLESHOOTING PROCEDURE
This section of the manual contains information for trouble­
shooting your oscilloscope. Before attempting to trouble­
shoot the instrument, however, make sure that any apparent
trouble is actually due to a malfunction within the instrument
and not to improper control settings. Instructions for the
operation of the oscilloscope are contained in the Operating
Instructions
section of this manual.
Tube failure is the most prevalent cause of circuit failure.
For this reason, the first step in troubleshooting any circuit
in the instrument is to check for defective tubes, preferably
by direct substitution. Do not depend on tube testers to
adequately indicate the suitability of a tube for certain
positions within the instrument. The criterion for usability of
a tube is whether or not it works satisfactorily in the instru­
ment. Be sure to return all good tubes to their original
sockets; if this procedure is followed less recalibration of
the instrument will be required upon completion of the
servicing.
When
replacing any tube in the instrument, check first
to see that components through which the tube draws
current have not been damaged. Shorted tubes will some­
times overload and damage plate-load and cathode resistors.
These damaged components can generally be located by a
visual inspection of the wiring. If no damaged components
are apparent, and if tube replacement does not restore
operation, it will be necessary to make measurements or
other
checks within the circuit to locate the trouble.
The component number of each resistor, inductor, capac­
itor, vacuum tube, control and switch is shown on the
circuit diagrams. The following chart lists the component
numbers associated with each circuit:
All numbers
less than 100...................................... Time-Base Trigger
100 series..................................... Time-Base Generator
300 series....................................... Horizontal Amplifier
400 series................................Upper and Lower Beam
Vertical Amplifiers
700 series...........................................Low-Voltage Power
Supply
800 series........................... CRT Circuits, High Voltage
and Amplitude Calibrator.
Switch wafers shown on the schematic diagrams are coded
to indicate the position of the wafer on the actual switches.
The number position of the code refers to the wafer number
on the switch assembly. Wafers are numbered from the
front of the switch to the rear. The letters F and R indicate
whether
the front or the rear of the wafer is used to perform
the particular switching function.
CIRCUIT TROUBLESHOOTING
Although the Type 502A is a complex instrument, it can
be thought of as consisting of six main circuits, in addition
to the Calibrator circuit. These are the:
1. Low-Voltage Power Supply
2. CRT Circuit and High-Voltage Power Supply.
3. Vertical Amplifiers.
4. Time-Base Trigger Circuit.
5. Time-Base Generator.
6. Horizontal Amplifier.
The first circuit to check, for practically any type of
trouble, is the Low-Voltage Power Supply. Because of the
circuit configuration employed, it is possible for an improper
power supply voltage to affect one circuit more than the
others. For example, if the gain of the Vertical Amplifier
should decrease slightly, while the other circuits appear
to be functioning normally, this could be due to an im­
proper supply
voltage and not to any condition originating
in the Vertical Amplifier. In cases of this type, valuable
can be saved by checking the power supply first.
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