I tested my GNS 430W in another aircraft today, and the radio check was perfect. Thanks Dave! So a GNS 430 failure is ruled out. On the one hand it would have been good to know where the failure was, but I’m happy it must be something a lot cheaper than a 430 repair. I didn’t try his 430 in my aircraft, just in case I have some wiring issue that could damage his unit. I didn’t relish spending gas money to repair someone else’s avionics.

The intercom harness was quite tight after I had replaced one of the connectors on the weekend to correct an intermittent fault, so I loosened up some Adel clamps and repositioned the harness so it wasn’t under tension at the connector. I inspected all the pins in the connector, and they seemed to be properly seated.

I then rechecked the mic audio hi and lo between the intercom and GNS 430 rack, and they were fine. I reconfirmed good power and grounds at the 430 rack. I measured the resistance between the ground pins and aircraft ground as less than 0.5 ohm. The resistance between the power pins and the fuse bus was also less than 0.5 ohm.

I confirmed the centre pin in the COM coax connector at the back of the rack wasn’t shorted to ground.

I redid the GNS 430W rack engagement check from the Installation Manual. The results were identical to what they were when I first installed it - 4 turns of the Allen key from power on to fully seated. The minimum allowable value is three turns, so I am OK there.

I unplugged the COM coax at the back of the 430 and plugged in the coax for COM 2, which was working on the weekend. A radio check did not work, but I also noted that I was no longer hearing sidetone when transmitting, nor was I hearing myself when talking in the intercom. I tried my other headset, but no difference. Something has changed here, as the intercom was working on the weekend.

At this point, more confused than before, I stopped and came home.

Next visit to the hangar, I am going to check continuity between the headset jacks and the intercom. I’ll also check intercom power and ground, but its lights were lit up, so they must be OK.

I’m wondering if maybe the intercom box has died. Maybe the COM 1 outputs died first, and now the intercom portion has died too. I’ll make up a test plug that connects to the two intercom connectors and directly ties the headset earphone, mic and PTT lines to COM 1, bypassing the intercom completely.

I also got an e-mail from from another builder saying that he had a bad GPS antenna connection that had somehow killed his COM TX. I don’t understand that one, but I learned a long time ago to not try and tell someone that what he just saw couldn’t possibly have happened. Next time I’m at the hangar I’ll unplug the GPS antenna and see if that makes any difference.

I think my next chance to attack this problem is late Thursday afternoon.