6-4 Maintenance 8525B/8528 Technical Service Manual
Issue 6 Publication No: 15-02036
6.2 Fault Diagnosis
6.2.1 General
The removal and substitution of components may damage the components
and/or the printed circuit boards. In some cases it is impossible to remove
components without destroying them. It is important therefore to carry out as
much fault diagnosis as possible with components in situ. Specific tests are
described later in this section. The general points which follow should also
be of assistance:
• Spare boards—If spare boards are held in stock, they may be substituted
in order to positively localise the fault to one board.
• Transistor tests (static)—Transistor failures are most often due to open-
circuit base-emitter or base-collector junctions, or a short circuit
between emitter and collector.
These types of faults can often be detected without removing the
transistor, using the ohms range of a multimeter. The two junctions
should both give the appearance of a diode, i.e. high resistance with the
multimeter leads one way round and low resistance when the leads are
reversed. (Polarity depends on whether a PNP or NPN transistor is
being tested.) Resistance between collector and emitter should be high
with the multimeter leads either way round. The circuit diagram should
be examined for parallel paths before a transistor failing these tests is
removed.
• Transistor tests (dynamic)—Some transistor faults can be diagnosed by
measuring voltages within the circuit. One of the most significant
voltage measurements is the base-emitter voltage. The polarity of this
will depend on the type of the transistor (PNP or NPN). A base emitter
voltage between 0.5 and 0.9V should be measured on a forward-biased
base-emitter junction.
With its base emitter junction forward biased the transistor should
conduct. Some indication of satisfactory operation of the transistor can
be obtained by measuring the voltage drop across its collector or emitter
resistor and short circuiting its base to the emitter. The short circuit will
remove the forward bias cutting off the transistor so that the voltage
across the resistor will be considerably reduced.
• Integrated circuits—If there appears to be no output from an integrated
circuit, before replacing the device, it should be ascertained whether the
fault is due to the IC or its load. As a general rule, if changes in input
cause absolutely no changes in the corresponding output the IC should
be suspected. If, however, even a very small change in output can be
detected the load is more likely to be the cause. Depending upon the
circuit, further tests should be made by disconnecting resistors,
capacitors, etc to verify this diagnosis before removing the IC.
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