Maintenance—Type 324
Transistor Troubleshooting. Transistor defects usually
take the form of the transistor opening, shorting, or devel
oping excessive leakage. The best means of checking a tran
sistor for these and other defects is by using a transistor
curve display instrument such as a Tektronix Type 576. If a
transistor checker is not readily available, a defective tran
sistor can be found by signal tracing, by making in-circuit
voltage checks, by measuring the transistor resistances, or
by the substitution method previously described.
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 ger
manium transistor will normally be 0.3 V and that of a
silicon transistor 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 connec
ting the voltmeter 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
defective (open) transistor. If the device is conducting, volt
age will be developed across resistances in series with it;
whereas if it is open, no voltage will be developed across
resistances 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. 1-1/2 volts and 2 mA are generally acceptable. Selec
ting the X1 k scale on most ohmmeters will automatically
provide safe voltages and currents. If the voltage and maxi
mum output current of a specific ohmmeter is in doubt, it
should be checked before using it on transistors by connec
ting the test leads to another multimeter.
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C A U T I O N
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A transistor's specifications should be checked to
determine maximum allowable ratings before subjec
ting it or associated circuits to voltage or current
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 Expected When Using the
R X 1 k Range (1.5 V ohm
meter operating voltage)
E m itter-Col lector
High readings both ways (100
9
to 500 kft, approximately)
Emitter-Base
High reading one way (200
k ^ or more). Low reading the
other way (400
9
to 3.5 k£3,
approximately)
Base-Collector
High reading one way (200
k ^ or more). Low reading the
other way (400
9
to 3.5
k9,
approximately)
Reverse the test lead connections to make the second reading.
Reversal of the applied voltage polarity causes the junction to shift
between being reverse and forward biased, as indicated by the differ
ence in resistance.
Field Effect Transistor Checks. The voltage and resist
ance of field effect transistors can be checked in the same
manner as transistors. However, it should be remembered
that normal operation in the Type 324 Oscilloscope has the
gate-to-source junction reverse biased, in a manner similar
to control grid to cathode bias in vacuum tubes. 1-1/2 V
and less than 2 mA should be used for ohmmeter checks.
Resistance readings should be:
drain-to-source
Less than 500
gate-to-source and
400
9
to 10
k9
(approximately)
gate-to-drain
in one direction; more than
200 kfi 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. Higher
readings indicate that they are either back biased or defec
tive, depending on polarity. The ohmmeter precautions per
taining to transistors should also be observed when check
ing diodes.
Some diodes used in the Type 324 Oscilloscope are color
coded to identify the diode type. A blue or pink first band
indicates that the next three colors translate to the last
three digits of its part number. Diode polarity can be deter
mined by color code position. See Fig. 4-1.
4-3
