Tektronix 323, Инструкция по эксплуатации

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Maintenance— Type 323
Direct Replacement. Once a casualty has been isolated
to a specific circuit, the ease of replacing transistors often
makes substitution the fastest means of repair. Adhere to
the following instructions if the replacement method is used:
Determine that the circuit is safe for the substitute
component.
Use only known-good substitutes.
Have only one transistor out of the instrument at a
time to avoid mixing them up.
Insert transistors properly, using Fig. 4-13 as a guide.
Check operation after each component is replaced, and
be sure to return good components to their original
sockets.
If the trouble is not corrected by this procedure, re­
check the semi-conductors under operating conditions.
Check calibration after a bad component has been re­
placed.
WARNING
Voltage, either positive or negative, is often pres­
ent on the cases o f metal-cased transistors when
the oscilloscope is energized.
Transistor Troubleshooting. Transistor defects usually
take the form of the transistor opening, shorting, or de­
veloping excessive leakage. The best means of checking a
transistor for these and other defects is by using a tran­
sistor curve display instrument such as a Tektronix Type
575. If a transistor checker is not readily available, a de­
fective transistor can be found by signal tracing, by mak­
ing in-circuit voltage checks, by measuring the transistor
resistances, or by the substitution method previously de­
scribed.
When troubleshooting with a voltmeter, measure the
emitter-to-base and emitter-to-collector voltages to deter­
mine if the voltages are consistent with normal circuit volt­
ages.
Voltages across a transistor vary with the type
of device and its circuit function. Some of these voltages
are predictable. The base-emitter voltage of a conducting
germanium transistor will normally be 0.3 V and a silicon
transistor's will normally be 0.6 to 0.7 V. The collector-
emitter voltage of saturated transistors will vary between
30 mV and 0.2 V, approximately. Because these values are
small, the best way to check them is by connecting the volt­
meter across the junction and using a sensitive voltmeter
setting, rather than by comparing two voltages taken with
respect to ground.
If values less than these are obtained, either the device
is shorted or no current is flowing in the circuit. If values
are in excess of the base-emitter values given, the junction
is back-biased or the device is defective (open). Values in
excess of those given for emitter-collector could indicate
either a non-saturated device operating normally, or a de­
fective (open) transistor. If the device is conducting, voltage
will be developed across resistances in series with it; where­
as if it is open, no voltage will be developed across re­
sistances in series with it unless current is being supplied by
a parallel path.
An ohmmeter can be used to check a transistor if the
ohmmeter's voltage source and current are kept within safe
limits. 1V2 volts and 2 mA are generally acceptable. Se­
lecting the X 1 k scale on most ohmmeters will automatical­
ly provide safe voltages and currents. If the voltage and
maximum output current of a specific ohmmeter is in doubt,
it should be checked before using it on transistors by con­
necting the test leads to another multimeter.
CAUTION
A transistor’s specifications should be checked to
determine maximum a llo w a b le ratings before sub­
jecting it or associated circuits to voltage or cur­
rent higher than that recommended.
Table 4-1 contains the normal values of resistance to
expect when making an ohmmeter check on an otherwise
unconnected transistor. Fig. 4-13 illustrates transistors and
sockets for pin location purposes.
TABLE 4-1
Transistor Resistance Checks
Ohmmeter
Connections1
Resistance Readings That Can Be Ex­
pected When Using the R X 1 k
Range ( 1 . 5 V ohmmeter operating
voltage)
Emitter-Collector High readings both ways
(100 kQ to 500 kQ, approximately)
Emitter-Base High reading one way (200 kQ or more)
Low reading the other way
(400 O to 3.5 kn, approximately)
Base-Collector High reading one way (200 kQ or more)
Low reading the other way
(400 Q to 3.5 kQ, approximately)
'Reverse the test lead connections to make the second reading.
Reversal o f the a p p lie d volta ge p o la rity causes the junction to
sh ift betw een being reverse and fo rw a rd biased, as indicated by
the diffe ren ce in resistance.
Field Effect Transistor Checks. The voltage and re­
sistance of field effect transistors can be checked in the same
manner as transistors. However, it should be remembered
that normal operation in the Type 323 Oscilloscope has the
gate-to-source junction reverse biased, in a manner similar
to control grid to cathode bias in vacuum tubes. 1 y2 V
and less than 2 mA should be used for ohmmeter checks.
Resistance readings should be:
drain-to-source Less than 500 Q
gate-to-source and 400 Q to lOkQ (approximately)
gate-to-drain in one direction; more than 200 kQ
with leads reversed
Diode Troubleshooting. Checks on diodes (other than
Zeners) can be performed in much the same manner as
on transistor base-emitter junctions.
Germanium diodes
should have approximately 0.3 V and silicon diodes should
have about 0.6 V across the junction when conducting. High­
er readings indicate that they are either back biased or de­
fective, depending on polarity. The ohmmeter precautions
pertaining to transistors should also be observed when
checking diodes.
4-3