Probe Precautions; Fault Diagnosis; General - Codan X-2 Service Manual

Maintenance 5-4
5.2.4

Probe Precautions

The following should be observed when connecting CRO probes to the
Transceiver:
1.
2.
3.
4.

5.3 Fault Diagnosis

5.3.1

General

The removal and replacement 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 diagnosis as possible with components in situ. Specific tests are
described later in this section. The following general points should also be of
assistance:
Spare boards
If spare boards are held in stock, they may be substituted to 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 an analogue multimeter or diode test on a digital
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.
When connecting probes to the PA assembly, the earth clip lead should
be wound around the body of the probe so that the earth clip just
reaches the probe tip. This reduces stray RF pick-up.
The earth clip should be connected to the ground plane adjacent to the
point of measurement to which the probe tip is connected.
It is not advisable to connect two probes at the same time, particularly
when one is earthed to the PA ground plane and the other is earthed to
the chassis. This may cause earth loop problems.
Probes should be connected after power has been applied to the
transceiver and the test equipment. The earth connection should be
made first and disconnected last.
X-2 Technical Service Manual
Publication No: 15-02047