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Fault diagnosis
General
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:
Replacement boards spare boards
- if replacement 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 usually be detected without removing the transistor, using the ohms range of an
analogue multimeter or diode test on a digital multimeter. In circuit testing will not work on the PA transistors
due to the low DC resistances found in this area.
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:
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 of between 0.6V and 0.8V
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 removes the forward bias cutting off the transistor, so that the voltage across
the resistor is considerably reduced.
If a fault can be traced to one PCB but no further then consider using Barrett Communications' PCB and
module exchange service.
Specific faults
Transceiver will not power up
Check that the DC power supply is present and that it lies between 12 and 14 volts. Check by removing the
power connector from the transceiver end and measure the voltage at the connector. There are no user
changeable fuses in the transceiver as it is protected by automatically resetting fuse types.
Check that the remote head / front panel is plugged in at both ends of connecting cable.
Open the case to gain access to the PA board. With the power applied, press the on button, for about 1
second. If the transceiver will not start up (no front panel indication), Measure the DC voltage between the
junction of resistors R33 and R32 to ground on the PA board. These resistors are located near power relay
RL9, between the two SO8 MOSFETS Q6 and Q7. This should be near zero volts. If it is near 12 volts,