Bose FREESPACE BUSINESS Manual de servicio - Página 19
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TROUBLESHOOTING TIP FOR HUM AND RFI REMOVAL
Hum and RFI (Radio Frequency Interference) are the most common problems when using
unbalanced signal sources and long signal leads. Once you have connected all equipment properly
and made all chassis grounds,adjust the BALANCE TRIM potentiometer located in the junction box.
See the figure below for reference. This shaft can be adjusted with a screwdriver. It is located to the
right of the (
) symbol,just above the shield label in the SIGNAL INPUT section of the junction
box. Be careful not to contact the AC mains terminals,since this task must be performed with the
power on.
First,turn your signal source electronics off,but do not disconnect the signal leads. If the interference
stops,it is not the signal leads,but the equipment. Second,turn the remote volume control to full
volume (or disconnect the volume control leads). Then,adjust the potentiometer while listening for
minimum interference. Since the system has an automatic turn-on circuit,the system will stop
amplification less than 2 minutes after you turn off the source electronics. If adjustment is not
completed in this time,re-trigger the circuit. You can momentarily turn on the source electronics
(with a signal) or sometimes touch one of the audio terminals with a bare finger. Do not assume that
turning the power switch on and off will trigger the circuit. The internal protection circuitry may
interpret this action as a supply fault and keep the system off for at least two minutes.
At some point in a full rotation of the potentiometer,interference will drop then increase again,so you
must adjust to this minimum point. If this adjustment fails to remove interference,more tests should
be performed before blaming the amplifier or the cabling. Disconnect the signal leads from the
source equipment. Temporarily connect a 1 KΩ resistor between the two signal terminals of the
signal cable(s). Adjust the trim pot again. If the interference is eliminated,the fault is with the source
electronics. If cable termination has been done according to recommendations,then the problem
may still be with the type of isolation provided by equipment with unbalanced outputs. The "iron-
clad" solution to isolation problems is to use an audio isolation transformer for each channel. This
converts the unbalanced output into a balanced output with shield connection. If interference is still
audible,then a very large signal is being picked up. This may require re-routing of the audio leads,or
even tracking down defective electrical equipment such as fluorescent light ballasts or large
horsepower motors. Large and very close broadcast antennas as well as high power CB radios can
also be a problem. Fortunately,interference of such severity is rarely a problem if a quality
installation of the Free Space system is done.
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