Maintenance; General - Codan NGT 2010 RF Technical & Service Manual

Maintenance

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. Therefore, it is important to carry out as much diagnosis as possible
with components in situ.
Spare boards
If spare boards are held in stock, they may be substituted in order to positively localise
the fault to one board.
Static transistor tests
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
Ω range of a multimeter. The two junctions should both give the appearance of a diode
(high resistance with the multimeter leads one way 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. The circuit diagram should be examined for parallel paths before a transistor failing
this test is removed.
Dynamic transistor tests
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 0.9 V 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 is obtained by measuring the voltage
drop across its collector or emitter resistor while 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 is considerably reduced.
Integrated circuits
If there appears to be no output from a particular pin on 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 change in the corresponding
output, the IC should be suspected. However, if 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 the circuit that follows the IC.
NGT Transceiver System Technical Service Manual 25