Tektronix 442, Instruction Manual

Page: 78 / 170

Maintenance—442 Service
emitter-collector voltages could indicate either a non-
saturated device operating normally, or a defective (open-
circuited) transistor. If the device is conducting, voltage
will be developed across resistances in series with it; if it is
open, no voltage will be developed across resistances in
series with it unless current is being supplied by a parallel
path.
When troubleshooting a field-effect transistor, the
voltage across its elements can be checked in the same
manner as for a transistor. However, it should be
remembered that in the normal depletion mode of opera­
tion, the gate-to-source junction is reverse biased; in the
enhanced mode, the junction is forward biased.
Integrated circuits (1C) can be checked with a
voltmeter, test oscilloscope, or by direct substitution. A
good understanding of circuit operation is essential to
troubleshooting circuits using 1C. Use care when check­
ing voltages and waveforms around the 1C so that adjacent
leads are not shorted together. Typical semiconductor
lead configurations are shown in Figure 5-4.
Diodes. A diode can be checked for an open or a short
circuit by measuring the resistance between terminals
with an ohmmeter set to the R X Ik scale. The diode
resistance should be very high in one direction and very
low when the meter leads are reversed. Do not check
tunnel diodes or back diodes with an ohmmeter.
Do not use an ohmmeter scale that has a high
internal current. High currents may damage the
diode. Do not measure tunnel diodes with an ohm­
meter; use a dynamic tester (such as a TEKTRONIX
Type 576 Transistor-Curve Tracer). Checks on
diodes can be performed in much the same manner
as on transistor emitter-to-base junctions. Silicon
diodes should have 0.6 to 0.8 volt across the junction
when conducting. Higher readings indicatethat they
are either back biased or defective, depending on
polarity.
To check other components—resistors, capacitors,
and inductors—disconnect one end from the circuitry (see
Soldering Techniques and Component Replacement in
the Corrective Maintenance portion of this section.
Especially note the material about machine inserted
components.).
Resistors. Check resistors for discoloration. Then
check them with an ohmmeter after disconnecting one
end from the circuit. Refer to the Replaceable Electrical
Parts list for tolerances of resistors used i n this i nstrument.
Resistors normally do not need to be replaced unless the
measured value varies widely from the specified value.
The best check for thick film resistor networks is actual
operation in the circuit.
Inductors. Check continuity of inductors with an
ohmmeter to detect an open conditon. (It may be
necessary to disconnect one end of the inductor from the
circuit to check continuity. Also, be aware that some
inductors are wound on a resistive core and will present a
resistive reading even when the inductive winding is
open.) Shorted or partially shorted inductors can usually
be detected by checking the waveform response when
high-frequency signals are applied to the circuit. Partial
shorting often reduces high-frequency response (in­
creases roll-off).
Capacitors. A leaky or shorted capacitor can be
detected by checking resistance with an ohmmeter (set to
the highest scale) after disconnecting one end from the
circuit. Do not exceed the voltage rating of the capacitor
(some ohmmeters use up to 30 volts as a source voltage).
The resistance reading should be high after the initial
charge of the capacitor. An open capacitor can also be
detected with a capacitance meter or by checking whether
the capacitor passes ac signals.
Switches. The most common cause of cam switch
failure is dust between the contact and the pad. Checkthe
suspected contact for continuity with an ohmmeter. If
open and not obviously damaged, try cleaning (see Cam
Switch Repair and Replacement).
Another frequent cause of cam switch failure is solder
smoke residue. This can occur when replacing a compo­
nent near the switch. This problem is usually indicated by
reduced high-frequency response. Flushing the contact
with isopropyl alcohol usually fixes this problem.
If the contact is physically damaged, replace the
contact strip. Bending the contact is only a temporary
repair. See Cam Switch Repair and Replacement.
9. REPAIR AND READJUST THE CIRCUIT. If any
defective parts are located, follow the replacement
procedures given under Corrective Maintenance in this
section. Be sure to check the performance of any circuit
that has been repaired or has had any components
replaced. Readjustment of the affected circuit may be
necessary. Refer to the Performance Check or Adjustment
Procedure in the Calibration section of this manual.
REV A MAY 1980
5-9