Tektronix 1A5 Instruction Manual page 41

Maintenance — Type 1A5
Check calibration
replaced.
Voltage, either positive or negative, is always
present on the cases of transistors Q 1 37, Q 1 53,
Q 1 74, Q297 and Q393 when the Type 1A5 is
energized.
Transistor Troubleshooting
Transistor defects usually take the
opening, shorting or
best means of checking
defects is by using
such as a Tektronix Type 575.
not readily available, a defective transistor can be
by signal tracing, by making in-circuit voltage checks, by
measuring the transistor resistances, or by the substitution
method previously described.
When troubleshooting by using a voltmeter, measure
the emitter-to-base and emitter-to-collector voltages to de ­
termine whether the voltages are consistent with normal
circuit voltages. Voltages across a transistor vary
type of device and its circuit function. Some of these volt ­
ages are predictable. The base-emitter voltage of a con ­
ducting germanium transistor will normally be 0.2 V and
that of a silicon transistor will normally be 0.5 V to
The collector-emitter voltage of a saturated transistor will
be about 0.2 V. Because these values are small, the best
way to check them is by connecting the voltmeter across
the junction and using a sensitive voltmeter setting.
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 junc ­
tion is back-biased or the device is defective (open).
in excess of those given for emitter-collector could indi ­
cate either a non-saturated device operating normally, or
a defective (open) transistor.
voltage will be developed in resistances in series with it,
whereas if it is open, no voltage will be developed in re ­
sistances in series with it unless current is being supplied by
a parallel path.
Transistor Resistance Checks
Ohmmeter
Connections '
Emitter-Collector
Emitter-Base
Base-Collector
'Test prods from the ohmmeter are first connected to the transistor
leads and then the test lead connections are reversed.
effects of the polarity reversal of the voltage applied from the
ohmmeter to the transistor can be observed.
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after a bad component has
WARNING
form of the
developing excessive leakage.
a transistor for these
a transistor curve display instrument
If a transistor checker is
If the device is conducting,
TABLE 4-1
Resistance Reading That Can
Expected Using the R × 1 k Range
High readings both ways (100 kΩ to
500 kΩ, approx.)
High reading one way (200 kΩ or
more). Low reading the other way
(400 Ω to 3.5 kΩ, approx.)
High reading one way (200 kΩ or
more). Low reading the other way
(400 Ω to 3.5 kΩ, approx.)
been An ohmmeter can be used to check a transistor if
ohmmeter's voltage source and current are kept
limits. 1 ½ V and 2 mA are generally acceptable. Selecting
the × 1 k scale on most ohmmeters will automatically
provide voltage and current below these values.
voltage and maximum output current of a specific ohm ­
meter is in doubt, it should be checked by connecting
test leads to another multimeter before using it
sistors.
Table 4-1 contains the normal values of resistance
expect when making an ohmmeter check of
unconnected transistor.
transistor
sockets for pin location purposes.
The
and other
Diode Troubleshooting
Checks on diodes (other than Zeners) can be performed
found
in much the same manner as on transistor base-emitter
junctions. Germanium diodes should have approximately
0.2 V and silicon diodes should have about 0.6 V across
the junction when conducting.
they are back biased or defective, depending on
The ohmmeter precautions pertaining to transistors
also be observed when checking diodes. Some diodes used
in the Type 1A5 are color coded to identify the diode type.
with the
A blue or pink first band indicates that the next
translate to the last three digits of a Tektronix
Diode polarity can be determined by color code position.
See Fig. 4-2.
0.7 V.
Values
Blue band indicates
next three bands
translate to a
Tektronix part
number
Fig. 4-2. Diode
Be
Field Effect Transistor Checks
Field Effect Transistors can be checked in
ner as other transistors. However, it should be
that normal operation in the Type 1 A5 has
source junction reverse biased just as
cathode is biased in vacuum tubes.
Wire Color Code
Insulated wires in this instrument are color coded
cate operating voltages and to make wire
Table 4-2 lists the color code translations for
Thus, the
voltage supplies. It may be noted that the
based on the IEEE standard color code for
Fig. 4-1 illustrates
Higher readings
Sill
Translate to 3 digits
of Tektronix part
number
Example: blue-red-orange-orange
Tektronix 152-0233-00
polarity and color code.
the control grid-to-
the
within safe
If the
the
on tran ­
to
an otherwise
transistors and
indicate
polarity.
should
three colors
part number.
the same man ­
remembered
the gate-to-
to indi ­
tracing easier.
regulated
translations are
resistors.