Change In Deflection Sensitivity
A change in the deflection sensitivity of the
instrument is indicated if the deflection, both
vertical and horizontal, is either greater or less
than the value indicated by the front panel set
tings. This can be caused by a change in the
gain of both vertical amplifiers and the Hori
zontal Amplifier, due to improper outputs of the
Low-Voltage Power Supply. Or, it can be caused
by a change in the crt sensitivity, due to im
proper output voltages of the high voltage
power supply. If either the low or high voltage
power supplies require adjustment, these adjust
ments should be made in accordance with the
recalibration procedures given in this manual.
PART i t CIRCUIT TROUBLESHOOTING
This portion of the Troubleshooting Procedure
contains information for locating a defective
stage within a given circuit. Once the stage at
fault is known, the component(s) causing the
trouble can be located by tube and component
substitution, voltage and resistance measure
ments, or by short and continuity checks.
Tube failure is the most prevalent cause of cir
cuit failure. For this reason, the first step in
troubleshooting any circuit is to check for defec
tive tubes, preferably by direct substitution. Do
not depend on tube testers to adequately indi
cate the suitability of a tube for use in the instru
ment. The criterion for usability of a tube is
whether or not it works satisfactorily in the in
strument. Be sure to return any tubes found to
be good to their original socket. If this pro
cedure is followed, less recalibration of the in
strument will be required upon completion of
the servicing.
If replacement of a defective tube does not
correct the trouble, then check that components
which are associated with the tube have not
been damaged. Shorted tubes will often over
load plateload and cathode resistors. These
components can often be located by a visual in
spection of the circuit. If no damaged compo
nents are apparent, however, it will be necessary
to make measurements or other checks within the
circuit to locate the trouble.
Troubleshooting The Low-Voltage Power Supply
Proper operation of every circuit in the Type
502 Oscilloscope depends on proper operation of
the Low-Voltage Power Supply. The regulated
voltages must remain within their specified toler
ances for the instrument to maintain its calibra
tion.
For no output voltage
If the graticule lamps and the fan do not
operate when the power switch is turned on,
check the power switch, the fuse, and the line
voltage. If your instrument is wired for 234-volt
operation, also check the thermal cutout switch.
(If your instrument is wired for 117-volt operation,
the fan will run even though the thermal cutout
switch may be open.) If the fuse is not blown
and the line voltage is correct, next check the
primary windings of the power transformer.
If both the graticule lamps and the fan operate
correctly, the primary circuit of the power trans
former is operating normally and the trouble lies
somewhere in the secondary circuits.
If only one of the outputs of the Low-Voltage
Power Supply is zero, the trouble is probably
due to a defective rectifier, series regulator, or
power transformer secondary winding, although
this trouble can also be caused by a short across
the output. To determine which circuit element is
defective, measure the secondary voltage of the
transformer and the voltage at the output of the
rectifier. The cause of the trouble can be de
termined by the voltage readings obtained.
For failure of the regulated power supplies
to regulate at the correct voltages.
If any or all of the supplies fail to regulate at
the proper voltages, first check the line voltage.
The supplies are designed to regulate between
105 and 125 volts (or 210 and 250 volts), with
the design center at 117 volts (or 234 volts), rms.
MAINTENANCE — TYPE 502
5-7
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