Audio jack issues

TLDR

• Conducted wire-to-wire continuity test on audio jacks (still in progress)
• Fixed the wiring after the test

Detail

I’m on a business trip in Taiwan recently so I didn’t have much time doing work on the airplane before the trip. I managed to find a few hours to look into the ground loop issue so I started to conduct wire to wire continuity test on the audio jacks.

In the time I had, I was able to go through about half of wires on the rear passenger audio jacks, and discovered a few issues. The process is not done yet, so I haven’t decided what’s the best course of action.

I continued the testing when I got back home.

Expected behavior

According to Midwest Panel Builders, the audio jack barrel seeks ground through GMA245R.

• When the harness is not connected to GMA245R, everything should be open. ie, barrel should have no ground to the airframe.
• When connected, it should show continuity with airframe.

Test

According to my wire diagram, audio wires connects to GMA245R through HC connector.

So I designed my test to 3 parts:

1. HC disconnected, test continuity between audio jack and HC connector
2. GMA245 disconnected, test continuity between audio jack and GMA245 connectors
3. All connected, test continuity between audio jack and airframe.

All connected

5 out of 8 shows ground. The ones not showing ground are

• Right passenger Mic
• Left passenger Mic
• Left passenger Headset

This is the original state from last week when the whole situation drew my attention.

HC disconnected

No ground to airframe. This is expected, good.

Wire to wire. This is where things get interesting. I discovered 4 pins mismatch according to my wire diagram. See table below

I think HC pin 4 and 5 are swapped, and 7 and 8 are swapped.

MPB sent me a new set of reference to match the wire color coding on the two ends of the connector. I took apart the connectors to check. The colors are actually all matching. So it shows the connectors are fine.

I then showed a picture of the audio jack wires to MPB. They confirmed the soldered wires on the jacks are indeed incorrect. Mystery solved!!

GMA245 disconnected

With HC connected but GMA disconnected, the test covers end-to-end audio jack wiring up to the GMA unit itself.

In this test, the passenger headset pins should go to J2401 connector pin #40 #41 #42. However my #41 #42 slot on the connector has no pin at all. Instead, the pins are inserted to #38 #39. I confirmed with MPB that these need to be moved.

Test conclusion

The not-so-good news is that some wires are misconfigured.

The good news is that MPB has been very helpful with the debugging process, and now I know what to do to fix the wires. Even though this is some unanticipated work, I am feeling pretty good about the progress so far.

Fixes

J2401

I opened up the d-sub connector J2401, and rearranged the wires #38 #39 #40 to #40 #41 #42.

Audio jacks

I cut the wires on both the left jacks and used crimp connector to reconnect them the correct way

• Mic: swap the two wires
• Headset: swap the ring and top wires

Then I put the jacks back to the rib and permanently secured them to the airplane.