Barrett 900 Series, Technical Manual

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BARRETT 900 SERIES TRANSCEIVERS
5.7 Fault diagnosis
The circuit diagrams show both DC and signal voltage points to enable a faulty section to be located. It
should be noted that given voltages are typical and allowance should be made for variations between
transceivers. The following diagnostic sequences are general guides only. The use of normal trouble
shooting principles and reference to the technical description, circuit diagrams and overlays will be
required when diagnosing faults.
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 diagnosis as possible without removing components. The following general points should
also be of assistance:
- if replacement spare boards are held in stock, they may be
substituted to localise the fault to one board.
, 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 usually 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, that is 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. The resistance between collector and emitter should be high with the
multimeter leads either way round. The circuit diagram should be examined for parallel paths across these
junctions caused by low resistance devices such as inductors before you remove a transistor that fails these
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 of between 0.6V and 0.8V
should be measured on a forward-biased base-emitter junction. 1.2V to 1.6V for a Darlington transistor.
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 removes the forward bias cutting off the
transistor, so that the voltage across the resistor is considerably reduced.
, if there appears to be no output from an integrated circuit, before replacing the device, it
should be ascertained whether the failure is due to the IC itself or the load it is driving into. As a general rule, if
changes in the input cause absolutely no changes in the corresponding output, the IC should be suspected.
If however, a very small change in output can be detected, the load is more likely to be the cause. Depending
upon the circuit, make further tests by disconnecting resistors, capacitors, etc to verify the diagnosis before
removing the IC.
If a fault can be traced to one PCB but no further then consider using Barrett Communications' PCB and
module exchange service.
5.7.1 General
Replacement boards spare boards
Transistor tests - static
Transistor tests - dynamic,
Integrated circuits
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