Maintenance
5-10 HF SSB Transceiver 9323/9360/9390/9780 Technical Service Manual
Fault diagnosis
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 diagnosis
as possible without removing components. 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, 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 before you remove a transistor that fails the following 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 between 0.6 to
0.8V should be measured on a forward-biased base-emitter junction (double
this voltage 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.
• Integrated circuits—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 or its load. 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.
Reading the technical description and understanding how the transceiver functions
will assist in diagnosing any possible faults that may occur.
To Next Page
Loading...
Need help?
General
