Watkins-Johnson Company WJ-8718A/MFP Instruction Supplement page 120

WJ-8718A/MFP
4.1.3 GENERAL TROUBLESHOOTING PROCEDURES
MAINTENANCE
The MFP Option consists mostly of integrated circuits (Ies); therefore, the
maintenance technician should become proficient at isolating defective ICs. Paragraphs 4.1.4
through 4.1.7 contain specific information for troubleshooting the key functional areas of the
controller: microprocessor system, front panel switches, audio signal path, and power supply.
The following general information is provided to aid the technician during all troubleshooting
efforts, and should be used in conjunction with the specific information in paragraphs 4.1.4
through 4.1.7.
4.1.3.1
Digital Test Equipment
In many cases, the most efficient method of testing and troubleshooting digital ICs
is to use a Logic Troubleshooting Kit, containing probes for supplying and detecting digital
. voltage levels and pulses. A recommended set of probes, the Hewlett-Pa-ckard 5022A Multi-
Family Logic Troubleshooting Kit, contains the following devices:
545A
546A
547A
548A
Logic Probe
Logic Pulser
Current Tracer
Logic Clip
The basic functions of these devices are described in paragraphs 4.1.3.1.1 through
4.1.3.1.4.
4.1.3.1.1 Logic Probe
The logic probe detects logic levels and pulses without the need for a voltmeter or
oscilloscope. The lamp in the probe tip shows a bright light for a logic high, no light for a logic
low, and a dim light for a floating or faulty logic level. When a pulse train of up to 50 MHz is
detected, the lamp blinks at a rate of 10 Hz. Power for the probe is supplied by the unit under
test.
4.1.3.1.2
Logic Pulser
The logic pulser probe provides pUlses that are compatible with digital circuits.
When the probe tip is placed against a circuit point, the probe senses the logic level pr.esent.
When the pulse button is pressed, the circuit is briefly pulsed to the opposite state. By using
the logic probe and logic pulser in combination, a known input can be supplied to an IC and the
effect can be observed at the output.
4.1.3.1.3
CWTent Tracer
The current tracer probe senses the magnetic field produced by current flow in the
circuit.
It
can be used to trace current from the source to the sink; therefore, it can help find
short circuits, or determine if an IC output is actually sourcing current, and if it is, find out
where the current is flowing.
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