Cable Harness Wiring; Electrical Noise Issues - PS Engineering PM3000 Installation And Operation Manual

2. Insert the PM3000 from behind the instrument panel, aligning the holes for the knobs, LED, and
switch.
3. Place the aluminum plate over the knob shafts and secure, using the two # 4-40 round head screws
provided.
4. Install the knobs over the volume and squelch control shafts.
5. Complete a wiring harness in accordance with Appendix D.
2.4

Cable harness wiring

To complete the installation, a wire harness must be made as shown in Appendix D.
PS Engineering can make a custom-tailored wiring harness for the installer. All harnesses use
Mil-spec quality components with professional techniques, and are fully tested before shipment.
If the aircraft already has pilot and copilot headset jacks installed, you may re-use them. Remove and
discard all wires from the copilot headset jacks. You may use the existing pilot headset jacks as the
Auxiliary Aircraft Radio Headset Jacks, but they should be moved to a new location to avoid confusion
with the pilot's headphone jacks. In the event the intercom has to be removed for any reason, these jacks
provide access to the aircraft radio system.
To connect intercom into the aircraft audio system, parallel the appropriate set of cables from the intercom
to the Auxiliary Aircraft Radio Headset Jacks. Finally, install new headset jacks into the aircraft and
connect them directly to the appropriate pins of the PM3000. See the wiring diagram for all details of the
wire harness interconnect.

2.4.1 Electrical Noise Issues

WARNING: You must use individual shielded cables for the microphone and headphone jacks.
Combining these two wires WILL cause loud oscillations and degrade the intercom function. The
oscillation is caused by the cross-coupling between the large headphone signal and the small microphone
signal. The resulting feedback is a high-pitched squeal that varies with the volume control.
Due to the variety of the radio equipment found in today's general aviation aircraft, there is the potential
for both radiated and conducted noise interference. The PM3000 has a specially designed power supply to
reduce conducted electrical noise on the power bus of the aircraft by at least 50dB. Although this is a very
large amount of attenuation, it does not eliminate all noise when the amount is excessive. There must be
at least 13.75 Volts DC present at the PM3000 for the power supply to work within its designed
regulation. Otherwise, it will not be able to attenuate noise properly.
Shielding can protect the system from radiated noise (rotating beacon, electric gyros, switching power
supplies, etc.). However, installation combinations can occur where minor interference is possible. The
PM3000 was designed in an interference -protected chassis and has internal filter capacitors on all input
lines.
Ground loop noise occurs when there are two different return paths for the same signal, such as airframe
and ground return wire. Large cyclic loads such as strobes, inverters, etc., can inject audible signals onto
the airframe return path. Follow the wiring diagram very carefully to help insure a minimum of ground
loop potential. Radiated signals can be a factor when low level microphone signals are bundled with
current carrying power wires. Keep these cables separated.
Insulating washers are required on all microphone and headphone jacks to isolate them from aircraft
ground. The use of a conductor instead of a shield for ground return eliminates these ground loop paths.
200-193-0000
PS Engineering Incorporated
PM3000
Installation and Operation Manual
Note:
Contact PS Engineering for more information.
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Rev. 0, May 1998