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Intercom / Transmit Issue

Hi Everyone,

I had an issue surface today with my intercom / radio and I am trying to figure out what the likely issue is.

My RV6A has a FlightCom 403 with digital clearance recorder for intercom, and a KX125 radio. Push to talk on pilot side only.

The intercom system and radio have been working in my new to me plane, but today I could not use the intercom / radio with the following symptoms:

1) Plugged headset into pilot side headset jack:
1.a) when I spoke into headset without keying the mic, I could not hear myself.
1.b) when I keyed the mic button on the pilot stick, I could not hear myself on the intercom, and ground did not seem to hear anything ... l got no response.

2) Plugged headset into copilot headset jack:
2.a) when I spoke into the headset without keying the mic, the intercom seemed to work because I could hear myself
2.b) when I keyed the mic button on the pilot stick (with headset in copilot plugs), I could hear myself on the intercom and the ground controller said "aircraft calling ground, all I hear is static"

3) Tried steps 1 and 2 with different headset make/model with same results.

4) Removed the female microphone plug jacks from the panel on each side (so they are dangling without touching panel).

5) Removed and reinstalled kx125 radio from slide in mount ("re racked").

6) Current state: Intercom seems to work now for both sets of headset plugs. Radio can receive ok. No transmit possible / push to talk does not seem to do anything / no "key up" noise when pressing push to talk button.

Any ideas on where I should focus my troubleshooting efforts? The pilot side headset jack? Push to talk button?

Thanks
 
1. Are you getting a transmit light on the radio? If not: problem with PTT line.

2. Can you pull the fuse or breaker for just the intercom? If so: most intercoms 'fail safe' to directly connect the pilot mic & headphones to the comm if power is lost to the intercom. If comm works normally with the intercom powered off, you have issues in the intercom.

3. Have you reseated the connector on the back of the intercom?
 
Thanks rv7charlie!

Quick update:

- Put contact cleaner on headset and inserted into both sides of panel repeatedly
- Turned on master and avionics

1) No transmit indication on radio when ptt button pressed

2) Pulled breaker for intercom. Intercom went off. No transmit on radio when ptt pressed. Reset intercom breaker. Intercom came back up. Behavior same as 1, above.

3) Have not tried re seating intercom. Will look into this but seems like may be a ptt issue

4) With aircraft power off I did a continuity check between ground and tip of pilot mic jack. When pressing ptt button no continuity between mic tip and ground.

My current thought ? Could this just be a broken ppt push button?
 
Yep; could be. Or, depending on how the plane is wired, could be a bad connection at the intercom connector. If the PTT wire goes in/out of the intercom, then it could be anywhere along the path from the PTT input on the radio, through the intercom, to the PTT button, to ground. You should see somewhere between 5 & 14 volts on the PTT terminal of the switch until the button is pushed. When the button is pushed, voltage should drop to near 0 volts.

Do you have only one PTT switch? If you have two and neither works, that would point to a wiring issue instead of a switch issue.

edit: If the radio has that old "aircraft quality" card edge connector instead of a subD connector, that's a prime problem area. I've seen them so corroded that you almost had to sand them to get them clean enough to work. That's a joke; don't literally sand it. But you may need to go after both the PC edge in the radio, and the terminals in the connector, with emery paper and contact cleaner.)
 
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Thanks again, rv7charlie!

Yep; could be. Or, depending on how the plane is wired, could be a bad connection at the intercom connector. If the PTT wire goes in/out of the intercom, then it could be anywhere along the path from the PTT input on the radio, through the intercom, to the PTT button, to ground. You should see somewhere between 5 & 14 volts on the PTT terminal of the switch until the button is pushed. When the button is pushed, voltage should drop to near 0 volts.

Thanks for this. I will measure this tomorrow to see if its operating properly. If I were to take an wire and connect the tip of the microphone to ground (essentially bypassing the PTT switch / shorting the tip to ground) I assume it should also trigger the transmitter if its only the PTT switch that is bad?

Do you have only one PTT switch? If you have two and neither works, that would point to a wiring issue instead of a switch issue.

Only one PTT switch on pilot side...

edit: If the radio has that old "aircraft quality" card edge connector instead of a subD connector, that's a prime problem area. I've seen them so corroded that you almost had to sand them to get them clean enough to work. That's a joke; don't literally sand it. But you may need to go after both the PC edge in the radio, and the terminals in the connector, with emery paper and contact cleaner.)

Yep, it is a KX125 with the edge connector. I did pull it and re rack it the other day, but will do it again and look more closely at the edge connector and the spring finger connectors in the radio tray. I am planning on taking a multi meter to pins 7 (mic gnd) and 8 (mic key) on the P100 connector in the tray to see if they are connected when PTT pressed.
 
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Yes, shorting tip to ground should trigger the transmit line in the radio.

And corrosion on the board edge and connector pins can be surprisingly difficult to remove. The connector is a real pain because it's so hard to get to the pins, if you're having to reach into the tray.
 
Update: So I believe I figured out what the issue was. Now I need to figure out how to fix this permanently.

There is a shield on the mic cable that needs to be connected to the ring of the pilot mic jack. This enables the ptt button to short the mic tip to the mic lo ground. See attached picture.

znVycta.jpg


As depicted, I was able to use the intercom and communicate on the radio with ground control.

I am not sure how this solder joint was accomplished before as I can’t find where the solder joint must have broken away from. It does not look like it pulled out of the ring connector terminal on the jack (which also has another wire soldered in there at the moment; other side of jack to alligator clip shown in picture).

Ideas for how to fix this connection in a permanent way are appreciated.

Unless other better ideas -- my current no building experience thought is to:
1.a) solder a wire (what type?) to the shield (where alligator clip is),
1.b) then cover the solder joint with heat shrink tubing.
2.a) Then de-solder the ring connector on the jack and
2.b) resolder the ring connector with both the existing wire and the new wire and
2.c) cover the ring connector and the two wires soldered to it with heat shrink tubing....

I do not want to have to take the panel out to do this. Are there good tools to do this while in the plane?
 
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Terminology (pardon the unavoidable pun): tip, ring, shield.

The wire's shield connects to the jack's shield; not the ring. :) The ring gets mic audio, in this application.

Your test clip is on the shield terminal. That's where you want to solder it. It's likely that the builder just got a 'cold' solder joint there when he built the plane. All that's needed is to resolder the shield to that terminal. If you've never soldered anything, just ask for help at your local eaa chapter. It will take longer to get the iron hot than to repair the joint.

Have fun with the 'education' half of experimental aviation....

Charlie

Edit: it looks like the builder did everything right except the actual soldering, so no need for any extra wires, insulating, etc.
 
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Charlie

Thanks for making sure I was using the right terminology. I appreciate it - would have been bad if I had told avionics guy solder wrong thing...

Discussed with an avionics guy that was able to solder everything back together correctly.

Tested with master on, it worked when calling ground ? ?loud and clear?. Tested with co pilot side headset with in line push to talk and it worked when calling ground ? ?loud and clear?.

Funny thing, next day I go to fly with transition training CFI? and with engine and alternator on it doesn?t work again. Intercom works. Radio receive works. Transmit / ptt does not. No tx indicator and no sidetone when hitting ptt. Both the pilot side and the co pilot external line ptt do not trigger transmit.

I put flightcom 403 to isolate, no transmit. I pulled intercom breaker and still no transmit.

I think my plane is playing tricks on me at this point ...
 
Are all symptoms repeatable? (important...)

If you shut down the engine, can you again transmit, etc?

If you restart the engine, can you again *not* transmit?

If all symptoms are consistently repeatable with the two conditions, and your only issue is loss of transmit ability *along with* loss of the transmit indicator on the radio, that's an indicator that you have a ground routing issue.

Get a clip lead on the *tip* of the pilot's mic jack. Leave the other end free, not touching anything. Power up, make sure transmit works. Now, crank the engine and retry. If transmit still doesn't work (remember, repeatable...), take the free end of the clip lead and touch it to the chassis of the plane somewhere convenient. Shouldn't really matter where, but be sure you're touching bare metal that's tied back to battery negative. if you now get a transmit indicator and you can actually transmit, you know you have a ground issue.

It's pure speculation on where the problem actually is, but based on the limited info available 'long distance' like this, it would seem that the ground line is finding ground through some component that gets powered when the engine is running (which would then hold the ground line HI and keep it from pulling the radio's PTT input low when you push your PTT button).

Rather than hunt for that issue, I'd just make follow the ground wire (whatever is tied to the shield terminal of the jack) back to the ground terminal of the radio itself. Might be simplest to just add a wire to make that path solid.

If the symptoms are not repeatable per condition, then you almost certainly have an intermittent connection somewhere. Good luck with that.....

Charlie
 
After shutting down Engine I still had the same peoblem.

I let the plane sit for about an hour and came back.

Just turning on master still had the problem.

I then removed the kx125 and put it back into the rack (card edge connector). With master on, it worked.

By then it was too hot for me (AZ), but I want to start the engine again and see if it has the problem.

Tomorrow I will try to repeat everything again.

Thank you again, Charlie.
 
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