5.21 ELECTRICAL (continued)
Problem Isolation: There are several quick easy steps to isolate an audio noise problem. Try turning the intercom volume knob. If
the problem is controlled by the intercom volume the problem is the intercom itself, wiring from the intercom to the headset jacks,
the headset jacks or the headset. As stated above try different headsets in different jacks. Try turning off each device individually
(EFIS, COM, transponder, ADS-B, GPS, unplug an aux music device such as an iPhone and any connecting cable, a car charger
plugged into the 12V power outlet, etc…) to help isolate the source of the problem. Some circuits may only be controlled by
removing a fuse. Remember to shut the master off before removing a fuse. When calling for tech help first isolate the problem, this
will save time.
Problem: The Audio levels are much different using the same headset in the pilot and co-pilot positions.
Solution: If one side is different than the other the problem may be in the headset jacks. Inspect the wiring below the headset jack
that is not receiving/transmitting properly to be sure that no wires are shorted out. Move the wires to check for a loose connection. If
you can hear or transmit but very faintly then the problem could be a loose connection.
Problem: I can't hear myself when transmitting but can hear COM transmissions
Solution: Check the COM side tone setting. For more information see the manual supplied with your COM radio. Remember to
check both sides of the aircraft for this condition. The COM side tone applies to both sides of the aircraft equally so using the same
headset on both pilot and co-pilot headset positions should yield the same conditions.
Problem: Received COM transmissions are fine but my transmitted voice sounds distorted or there is a high level of background
noise.
Solution: The setting of the COM side tone may be set to automatically adjust with the volume level. In some cases this works well
but in most instances the side tone may need to be adjusted manually especially when using modern headsets that are more
sensitive.
Problem: Only when the COM is turned on there are random loud bursts of noise.
Solution: The default setting of the COM squelch should be adjusted. See troubleshooting section on a random noise through the
headset. If you hear Strobe noise only with the COM on read the problem/solution for strobe noise below.
Problem: Only a small portion of the control module volume adjustment potentiometer range is usable.
Solution: Check that all headsets are in stereo mode. If the left and right channels are shorted together with a mono headset it will
be impossible to change the left or right volume level for any stereo item such as aux music or EFIS warnings.
The audio mixer was designed to give the most usable range of the volume setting potentiometers for the most common input
impedances. If the impedance of the device you are trying to connect is significantly different than the most common impedances
the circuit was designed around you may experience a loss of useful range of the volume adjustment potentiometer.
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REVISION:
DATE:
VAN'S AIRCRAFT, INC.
PAGE
REVISION:
DATE:
09/24/13
05-26 RV-ALL
1
In most cases you may only find that 6 of the available 24 turns in a pot are usable. We still find this situation more pleasant than
trying to use a small range of a single turn potentiometer. The usable range will always start at the full clockwise end of the
potentiometer range.
A short note on the concept of an audio mixer: The mixer potentiometer is acting as a voltage divider, dividing the signal and
sending part of that signal to ground and the rest to the audio amplifier. As you turn the potentiometer clockwise the resistance to
ground becomes less and more of the signal is sent to ground thus reducing the volume of that signal. In short you are reducing the
volume of each signal with the potentiometer to a desired level. The signals small in volume are then added together (for example
Aux1 Left, Aux2 Left and EFIS Left) and amplified to a useable volume. The amplified signal is sent out of the control module on a
single wire to the intercom or audio panel.
The volume range may also be dependent on the type of intercom in use. In recent years there has been an increased use of music
players designed to drive small ear bud headphones. This results in low volume output from older intercom designs. To
compensate, some intercom manufactures have increased the gain level (oversimplified they have turned up the internal output
volume knob within the intercom as high as possible). As the mixer output is increased the high level of gain in the intercom causes
the signal to become distorted (the top of the signal is being chopped off because the intercom cannot handle that high of input).
Problem: Constant background hiss noise when the aircraft engine is turned off.
Solution: It is normal to hear a faint ocean sounding noise with the engine turned off with the electrical system powered on. When
the engine is running this noise will disappear. The noise will disappear not only because of the sound of the engine which is much
greater in magnitude but also because the voltage regulators (especially 12V regulators) in the system will have a greater
differential to regulate from when the engine driven alternator or generator brings the bus voltage up to 14V. The greater the
difference between the bus voltage and the regulated voltage the better the voltage regulator will be able to maintain a steady
voltage for a device such as an intercom. A fluctuating voltage can introduce noise into the system. Note for this reason adding a
battery charger may introduce noise into the system.
Problem: Rhythmic jjgt jjgt noise or banjo noise in the background.
Solution: This noise can be caused if a battery charger has been placed on the battery. Temporarily remove the charger and see if
the noise goes away.
The noise may also be caused by serial data transmission along a wire. Try turning off devices that transmit and receive serial data
one at a time (for example autopilot, transponder, com radio, ADS-B, GPS, etc...) Although Van's Aircraft has done their best
through the use of twisted wires, shielded wires and the use of ground planes this still can be a problem. Try removing tie-wraps
from your harnesses and moving wires relative to each other and see if the sound changes. Using an alternate routing path for a
noisy wire or shielding wires may be a solution. RS-232 data lines, especially GPS signals, using 9600 baud (9.6 kHz frequency)
are particularly susceptible to this; the range of human hearing goes up to 20kHz. If possible use a higher baud rate for the
offending RS-232 data line.
If the problem noise is not related to the solution above try the troubleshooting steps listed for a random static noise.
