GNS 430W TX Problem

[Kevin Horton]

I'd appreciate some advice as I troubleshoot a TX problem with a GNS 430W.

My setup:
COM 1 - GNS 430W
COM 2 - MicroAir 760
Intercom - PS Engineering PS 4000

Symptoms:

1. This radio worked fine for over two years. Suddenly, starting two weeks ago, COM 1 transmits carrier only. No modulation.
2. COM 1 RX is OK.
3. TX indicator indicates when I key the mike.
4. COM 2 transmits fine, and I can hear myself in the intercom, so I know the headset and wiring to the intercom are OK.
5. Selecting the intercom to OFF makes no difference (this supposedly ties the headset directly to COM 1, bypassing the intercom's circuitry).

What I've done so far:

1. I've checked the mic audio continuity between the intercom and the GNS 430. I found an intermittent open on mic audio low, and fixed it, hoping that was the problem. But, it made no difference, so perhaps it never went open in service.
2. I've checked COM power and ground (2 pins each) on connector P4002.
3. I've made sure the GNS 430 was fully seated in the tray by giving it a good rap.
4. I've tried another headset, even that seemed to be already ruled out.

What I'm planning to do:

1. I'm going to try my GNS 430W in someone else's aircraft tomorrow.
2. I'm going to redo the test on connector engagement specified in the Garmin Installation Manual (turn unit ON while it is out of the rack. Slowly turn allen key to pull it into the rack. You should be able to turn the allen key three more turns after the unit powers up).
3. I'm going to recheck the mic audio wiring between GNS 430 and intercom.
4. I'm going to recheck the resistance between ground pins and aircraft ground and between power pins and the power buses.

Antenna? I haven't checked antenna or coax yet. What do I look for there?

What else? What else should I check?

Thanks for any advice. This is driving me crazy.

[Ron Lee]

Checking in another aircraft is an excellent test.

[dmaib]

Kevin, I had the exact same symptoms last year. It turned out to be the COM power circuit on my VP-200. Due to a gremlin, or my fat fingers, (more likely) the programming for the power to the COM side of the 430W had "disappeared". So, the unit gets enough power to give you the carrier, but no modulation. The TX indicator works in this scenario, and RX is fine, as well. We tried my unit in another airplane and found that it worked fine. We then tried another unit in my airplane and found that it had the same problem. So, we finally zeroed in on the fact that we were not getting COM power. Two minutes to reprogram the VP-200, and all was right with the world. I know you don't have VP, but the unit appeared to be getting power to the COM side, but it really was not. Checking your unit in another airplane will be a good test.
Good luck.

[Ironflight]

Hmm...some good suggestion so far Kevin. If you're sure you're getting carrier but no Mod, then i doubt that it is an antenna problem. Testing in another aircraft is the best next step - and while you have the other guy's 430 out of his ship, slip it in to yours, and you'll get double the information as to where the problem lies - the radio box or the airframe. if it's the airframe, then I am guessing it is an open in the Mic circuit to the 430 - a recessed pin would be most likely. Just eliminate things one by one until you get there. can't wait to hear the results of a 430 swap!

Paul

[GregM]

I had the same thing happen to me when I first tested my comms out. It did turn out to be a broken wire near the pin on the mic output. I would pay close attention in that area.

[deene]

Sounds like you have lost your mike audio to the radio...chase that signal and its ground (lo)...particularly the path from the audio panel to the radio.

[Kevin Horton]

Thanks for the advice guys.

I tested my GNS 430W in another aircraft today, and the radio check was perfect. So that 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 message 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.

[az_gila]

Try this test for the PS 4000 - from the data sheet -

The "Off" mode is part of the automatic fail-safe interconnect to the aircraft systems. If power to the intercom is disrupted, the pilot's headset is automatically connected to the aircraft radio. This permits continuous radio communications.

It may show a problem in the intercom/switching portion.

[Richard Connell]

Gday, I posted this about a year ago...your sypmtoms sound identical. There are 3 power inputs to the 430W. i bet one of them has an issue (or a ground)


"430 would recieve fine but no transmit.
The TX would flash, but nobody could hear me.
Swapped out the entire radio, no difference.
Checked continuity of all pins between it and the audio panel. all fine.
Gave up and booked it in with the local avionics shop. This is after 2 months or trying everything. They had a 1m wait (or just werent interested...)
Went away on a trip using com2 for transmit. Blew the nav light fuse because my rudder bottom filled with water.
Was under the panel replacing the nav light fuse and noticed a blank in the fuse block of the main buss. Wire leading into it says GNS430???
Light bulb moment. the 430 has 3 power inputs. Id split and fused them as directed by the install manual. didnt fuse one of them - com TX. Unbelieveable. Live and learn.
Are all of your 430 Power inputs powered? are all of your grounds grounded?
Cheers
Richard"

[Kevin Horton]

Originally Posted by az_gila

Try this test for the PS 4000 - from the data sheet -

The "Off" mode is part of the automatic fail-safe interconnect to the aircraft systems. If power to the intercom is disrupted, the pilot's headset is automatically connected to the aircraft radio. This permits continuous radio communications.

It may show a problem in the intercom/switching portion.

I already tried that Gil. But thanks for the suggestion. This function seems to be designed to address a power failure. I'm not 100% sure that it would always work if the PS 4000 had an internal failure. I'm going to wire up a bypass harness to rule out the PS 4000 as a possible cause.

Originally Posted by Richard Connell

Gday, I posted this about a year ago...your sypmtoms sound identical. There are 3 power inputs to the 430W. i bet one of them has an issue (or a ground)

I had found your post during a search, and I have confirmed all three power inputs are good. I also did a test that confirmed that there are no obvious signs if the COM power inputs are unpowered.