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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.