Setcom IM-900 Installation And Operation Manual page 33

PROBLEM
Steady hum heard in the
headset
Excessively noisy audio in
headsets
Alternator whine is heard in
headsets at all times
Descriptions and Probable Solutions
•The most common reason for a steady hum heard in the
headsets is a ground loop. If during installation, any of the
interconnecting cabling was routed through a hole without a
rubber grommet or over any sharp edges, the protective
insulating jacket can get cut. Once this happens, there is a
metal braided shield on the cable that is connected to the
intercom system ground. When this touches bare metal of
the chassis, it provides a second grounding point and the
resulting ground loop can cause a hum.
•To test for a short to ground, with all of the subcomponents
connected to the IM-900 or IM-900E, disconnect just the
radio cable. If the intercom system is still working, i.e. you
can hear yourself speaking in the headset, you have a short
to ground somewhere in the cabling. From this point you can
start disconnecting components one at a time until you
locate the shorted component. To fix, locate the sharp edge
that is causing the problem and install a rubber grommet or
other protection. Repair or replace the affected cable and
reconnect.
•Make sure all intercom cables and boxes are routed away
from any possible noise sources such as; power cables,
fuse/relay blocks, lights, antennae, or computer boxes.
•Alternator whine may be heard in the headset at all times
with the engine running, or just when a radio message is
being received. Occasionally, whine is heard only on the
transmitted signal being received by another station, but not
at other times. Alternator whine is recognized as a
moderately high pitched rough tone that rises and falls in
pitch as the vehicle engine speed increases and decreases.
First, determine if the noise is coming in through the radio
cable or power cable. To do this, disconnect the radio cable
from the IM-900. Disconnecting the radio cable will remove
the grounding point from the system; simply grounding pin 4
of the "Radio" port on the IM-900 will allow a complete path.
•If the whine stops or is greatly reduced, the FM radio is
probably the source.
•If not, the whine is probably due to excessive AC ripple on
the battery power leads. Check with battery switch set to 1,
2, and both positions, if so equipped, and note any
difference in the level of whine. If one battery is worse than
the other or when on the "both" position, suspect a weak
battery or bad connections.
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