Transistor Checks - Tektronix 3A74 Instruction Manual

(9} 18" Patch Cords (two required)
Description: Combination banana-plug and jack con-
nectors on each end. Type PC-18R, Tektronix Part No.
012-031.
Purpose: Use in low-frequency signal-tracing setup to
apply the calibrator signal to the Type 3A74 and test
oscilloscope EXT. Trig. input connector.
(10) 30% Patch Cord
Description: Banana plug on one end, alligator clip on
other end. Type W130B, Tektronix Part No. 012-014.
Purpose: Use in low-frequency signal-tracing setup to
ground the Type 3A74 and test oscilloscope together.
(11) Miscellaneous: Replacement tubes, transistors, and
diodes.
Tube Checks
Periodic tube checks on the tubes used in the Type 3A74
are not recommended. Tube testers in many cases indicate a
tube to be defective when that tube is operating quite satis-
factorily in the circuit, and fail to indicate tube defects
which affect the performance of the circuits. The ultimate
criterion of the useability of a tube is whether or not the
tube works properly in the circuit. If it does not, then
it should be replaced. If it is working correctly, it should
not be replaced. Unnecessary replacement of tubes is not
only expensive but may also result in needless recalibration
of the unit.
CAUTION
Turn the oscilloscope power off when replacing
tubes or transistors as this will safeguard their life
expectancy.
Transistor Checks
Transistor defects usually take the form of the transistor
either opening or shorting or developing excessive leak-
NPN or PNP
TRANSISTOR
Emitter-to-
Collector Voltage
Emitter-to-Base
Voltage
Fig. 5-8. In-circuit voltage checks of NPN or PNP transistors.
®
Maintenance — Type 3A74
age. To check a transistor for these and other defects, use a
transistor-curve display instrument such as the Tektronix
Type 575. However, if a good transistor checker is not readily
available, a defective transistor can be found by signal trac-
ing, by making in-circuit voitage checks, by measuring the
transistor forward-and-back resistances using proper ohm-
meter ranges, or by substitution. Voltage and resistance
checks can be made using the following information.
(1) In-Circvit Voltage Checks
To check transistors using a voltmeter, measure the base-
emitter junction bias and the emitter-to -collectors voltage
as shown in Fig. 5-8. Use Tables 5-1 and 5-2 to determine the
condition of the transistor.
When substituting transistors as suggested in the table,
first be sure that the voltages and loads on the transistor
are normal before making the substitution. If a transistor
is substituted without first checking out the circuit, the new
transistor may immediately be damaged by some defect in
the circuit.
TABLE 5-1
IN-CIRCUIT TRANSISTOR VOLTAGE CHECK
Step | Voltage Reading* | Action
1. | Normal base-emitter read-| Go to step 3.
ing.
2. | Abnormal base-emitter read-| Check associated circuit
ing and/or incorrect polar-| first to find probable
ity. cause. Then, if neces-
sary substitute the tran-
sistor.
3. | Normal emitter-to-collector | None**.
drop.
4. | Low emitter-to-collector volt-| Possibly shorted***,
age. Check associated circuit
for proper voltages and
resistances before sub-
stituting the transistor.
5. | High emitter-to-collector| Possibly open**. Check
voltage. associated circuit for
proper voltage before
substituting the transis-
tor.
*See schematic diagram to determine proper voltages.
**Q583 and @590 are exceptions. They could be open. Check
by substitution.
***Q545, Q555, Q565, and Q575 when conducting normally have
very low voltage between emitter and collector. Try substituting
the transistor.
(2) Resistance Measurements
To check a transistor using an ohmmeter, know your ohm-
meter ranges, the currents they deliver, and the internal
voltage used. If your ohmmeter does not have sufficient
resistance in series with its internal voltage source, excessive
current may flow through the transistor under test and may
permanently damage the transistor.
5-7