Static noise in intercom

After final electrical configuration via VP-X, we wanted to try out the intercom and radio functions of the Garmin GNC255A. Unfortunately, there was definitely more static noise in the background than is normal, especially for a new plane with all new electrical components! Looking at the complex wire connections for the 255A and for the mic and headset jacks, it was clear that a wiring mistake could have easily been the culprit.

Confusing connection of the mic and headset jacks to Garmin

The PTT and headset connections seen above took a good chunk of time to understand. You must use shielded wire, ground the shield appropriately, and then label the wires correctly. Every wire must be shielded for as much of its length as possible - this is a major way that interference noise is kept out of the audio system. It is also important that the outside of the jacks do not contact the airframe - plastic sleeves are provided for this purpose. Troubleshooting for these problems however, did not eliminate the static noise heard inside the intercom.

Troubleshooting the wiring of the GNC255A connector and mic/headset

Here we were testing the continuity between the P2001 connector pins for the Garmin and the mic/headset jacks using an ohmmeter and d-sub test lead. Removing the radio tray and connector would not have been very easy, so doing it this way was the only way. After checking that all pins were properly connected to the mic and headset jacks, we still had a problem...

Finally, a solution arose! Up to this point, the nav portion of the GNC255A had no antenna hooked up yet. There was just the bare coax connector at the back of the radio. I decided to reconnect the laptop and run the VP-X Configurator application to see if some other device (such as the nav circuit of the radio) was causing the interference I was hearing in my headset. The 255A has two power input pins, one for the com and one for the nav. Power is normally switched to the radio via the avionics master, but in using VP-X, each function of the radio (com and nav) was designated its own power pin. Because we did not have the VP-X page function available on the EFIS (see Dynon EMS issue), the laptop was required to control power to the devices. When the VP-X screen is available on SkyView, this function can be done via the EFIS; no computer required.

With a simple click on the laptop to turn off power to the nav radio, the static noise was gone and the intercom sounded crystal clear. A subsequent test will be needed once the nav radio has the VOR antenna connected, but this test shows that when the system is wired correctly, it will work perfectly.

The source of the static noise was isolated when using the VP-X Configurator application to manually turn off power to the nav board of the 255A