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5-2. Troubleshooting By Signal Substitution
When troubleshooting dead audio equipment, localize the trouble by injecting an
audio signal from Sweep/Function Generator to substitute for the normal signal.
Starting at the nearest speaker and moving toward the audio input area, Step By
Step, sound will be heard from the speaker for each stage that is operating
normally. When signal is applied to the defective stage, no sound will be heard
from the speaker.
MAKE SURE THE DC OFFSET MATCHES THE NORMAL OPERATING
VOLTAGE AT EACH POINT OF SIGNAL INJECTION. IMPROPER DC OFFSET
COULD BIAS A NORMALLY OPERATING STAGE TO CUT OFF AND MAKE IT
APPEAR DEFECTIVE. IMPROPER DC OFFSET COULD ALSO DAMAGE
CERTAIN CIRCUITS. A COUPLING CAPACITOR MAY BE USED TO BLOCK THE
DC OFFSET AND ALLOW THE SIGNAL TO FLOAT AT THE DC LEVEL OF THE
POINT OF INJECTION IF DESIRED.
The signal amplitude should also simulate the normal signal levels used in the
circuit where signal is being injected. This technique is equally applicable to non
audio equipment. Just connect an oscilloscope, voltmeter, or any other device,
which will indicate the presence or absence of output. Inject the type of signal
normally used by the equipment being tested. This instrument can generate almost
any type of signal normally required in the frequency range (FG-8102:0.02Hz to
2MHz, FG-8103:0.03Hz to 3MHz, FG-8105: 0.05Hz to 5MHz, FG-8110: 0.1Hz to
10MHz). If the equipment under test, It can generate unique sounds or signals by
means of sweep that should be easily distinguishable from any other signals that
may be present.
5-3. Troubleshooting By Signal Tracing
This technique is similar to "Troubleshooting by Signal Substitution" except that the
signal is injected at the input of the equipment under test. An oscilloscope is then
used to check for output at each stage, starting nearest at the input area and
moving toward to the output area. Each stage, which has no output, is presumed to
be defective.
CAUTION
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