unable to transmit...when engine running

[air cooled dad]

1975 F

gns430w radio

gma-340 audio panel

 

Just finished a round trip from Georgia to Michigan and back.  Last 30 minutes of the flight I couldn't transmit from my push to talk.  Assumed it was just the pilots ptt so I snagged the headset from the right seater and used the right seat ptt.  It worked but the sound feedback in my headset was scratchy.  ATC assumed the scratchy sound was due to an issue on their end.  I cancelled IFR before I couldn't transmit at all.  

Fast forward a couple days...  I head down to the hangar to isolate the problem.  With the engine OFF I can transmit.  A few radio calls to the FBO using multiple headsets, both installed ptt switches and an old velcro ptt worked great.  Fired up the engine to taxi out and tried a few more radio calls.  Nothing...just crickets...  Once again I tried multiple headsets, both pilot and copilot ptt switches and the old velcro ptt and nothing worked.  FBO said the could only hear some clicking and no voice.  Shut down the engine and voice over the comm worked again.  Ugh...  Any ideas before I head off to the avionics shop in the morning?

Thanks,

Jason

 

[ArtVandelay]

Try pulling the alternator circuit breakers.


Tom

[M20F]

Make sure your jacks are tight on the panel.  They loosen sometimes and will ground out (probably not it but easy to fix/check). 

[Guitarmaster]

What is running with the engine on that is not with the engine off....

Alternator
Mags
Regulator

I would definitely be looking to one of those three as the culprit.




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[Andy95W]

13 hours ago, ArtVandelay said:

Try pulling the alternator circuit breakers.


Tom

Like Tom was getting at, you need to isolate the source of the noise/interference.  The most likely sources are alternator/generator, magnetos, or vibration.

Pulling the alternator/generator circuit breaker will basically turn it off.  Turning your magneto switch to L or R will turn off the other magneto.  Vibration is more difficult, but changing RPM might be able to point you in the right direction. 

[Yetti]

The PTT completes a to ground to turn on the radios and such.  So it is a problem with ground.   floating ground

[air cooled dad]

Thanks everyone.  Drove over to Aircraft Spruce and bought a handheld radio.  Heading down to the hangar to chase this thing down.  Bad news, we cancelled our trip to St. Augustine.  Good news, we will spend a few days camping at the lake instead.  The hangar is 10 minutes away from the lake!  First world problems.  Lol.  

[air cooled dad]

Problem solved. 

 

Turns out that the avionics shop that looked at my plane a couple months ago forgot to secure the 430. Over time, the vibration wiggled the radio out just a wee bit. Simple fix to reseat the radio.  

[carusoam]

Great follow-up ACD!

Thanks for sharing the details.

Best regards,

-a-