5 Troubleshooting Guide
27
Radio frequency interference
Imaging
Lost sound; center is weak or
missing
Center is too loud
near a power amplifier or any big power transformer. Try
increasing the distance to any nearby power equipment.
Sometimes RFI can induce hum in the system. This could result
from a nearby transmitter or a lamp dimmer on the ac line. If
proper grounding technique does not solve this, then an RF filter
for the ac line (such as Radio Shack 15-1111 for moderate
suppression) might be helpful.
Be sure all equipment has its chassis properly grounded to
earth ground. All cables except loudspeaker cables should be
shielded with the shields terminated at a chassis ground point.
For line-conducted noise, an ac line filter (such as the Radio
Shack 15-1111) may be useful. This may be relevant if the RFI
source is known to be other equipment used on the same power
line.
RFI is often picked up from strong fields radiated nearby. If
connections and cables are optimized for grounding and
shielding, and there is still interference, it can often be solved by
connecting 0.01 uF ceramic capacitors from each input and/or
output signal connection to chassis ground. Bridging balanced
signals with a capacitor from + to
S
may be helpful also.
Check that the center loudspeaker is correctly connected. The
polarity of all signals must be correct. Use the setup procedure
to verify correct left-center and right-center phantom imaging, as
well as optimum amplifier gain settings.
If using a bridging connection, check for a polarity problem.
With C/S Image Balance set to Mono (full counterclockwise), the
center loudspeaker should be 6 dB louder than the side
loudspeakers. It should contain a strong center signal; if it
doesn’t, then check the input signal polarity.
Sometimes the program source may have one channel inverted
which will cause this problem even though the imager and
loudspeaker systems are working correctly.
If the program source has poor correlation or extreme phase
shift between the left and right channels (common with
inexpensive cassette tape decks for example) this can cause a
weak center channel. The solution is to compensate by moving
the C/S Image Balance toward Center.
Check the loudspeaker placement. The center loudspeaker
should not be set too far back relative to the side loudspeakers.
Check the amplifier gains and verify their correct setting with the
setup procedure.
Make sure the program source is stereo. Set the C/S Image
Balance to Diff; you should hear a strong difference signal. If the
sound mostly goes away, then the program source is probably
mono, in which case the center
should
be louder.
