Tektronix 502A, Instruction Manual

Page: 38 / 110

Maintenance — Type 502A
On the other hand, the crt display can often be used to
isolate
trouble to one particular circuit when trouble ob­
viously exists in that circuit. If there is no vertical deflection,
for example, when the intensity and horizontal deflection
appear to be normal, it is apparent that an open condition
exists in the Vertical Amplifier and this circuit should be
investigated first.
TROUBLESHOOTING THE LOW-VOLTAGE
POWER SUPPLY
Proper operation of every circuit in the Type 502A
Oscilloscope depends on proper operation of the Low-Voltage
Power Supply. The regulated voltages must remain within
their specified tolerances for the instrument to maintain its
calibration.
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 220,
234 or 248
volt operation, also check the thermal cutout
switch. (If your instrument is wired for 110, 117 or 124 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 transformer is operating nor­
mally and the trouble lies somewhere in the secondary cir­
cuits.
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 determined by 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. (There are similar line voltage ranges for the
other nominal line voltages). Improper line voltage may
cause one or all of the supply voltages to be off.
All the low voltage power supplies are dependent upon
the — 150 volt supply for regulation, and consequently, a
change in the regulation point of all the supplies is indica­
tive of a defective —150 volt supply. If the output volt­
age of the —150 volt supply is off by only a small amount,
it may be possible to readjust the —150 Control for the pro­
per voltage. In any event it will be necessary to recalibrate
the instrument according to the procedures given in the
Calibration Procedure section of this manual.
Fig. 4-8. Low voltage power supply test points.
In case of the failure of a single power supply to regulate
properly, check the following:
1. Line voltage
2. Transformer voltage
3. Output voltage of the rectifier
4. Tubes or transistors
5. Loading
Important power supply voltages are marked on the power
supply schematic diagram. These voltages may be used to
perform checks on the power supply operation. One cause
of improper regulation by a power supply is incorrect load­
ing
of the supply. To check power supply loading, shut off
the power and check the resistance of the power supply out­
put bus to ground. The —150 volts bus should read approxi­
mately 3,500 ohms, the +100 volt bus approximately 4,000
ohms, and the 4-350 volt bus approximately 10,000 ohms.
If none of the preceding checks determine the cause of
the trouble, the cause of the improper regulation is probably
a change in value of one or more of the resistors or cap­
acitors composing the voltage divider networks. The resis­
tance networks in the grid circuits of V746B, V666B and
V636A and the base circuits of Q644 are particularly critical
since they determine the output voltage of their respective
power supplies. Use resistance checks to isolate the defective
part or parts. The following information may be used as a
quick index to troubleshooting the Low-Voltage Power Supply.
4-4