Radio Transmit Whine

[jameslovesairplanes]

Recently had my RV-4A annualed and afterwards i know have a whine anytime i transmit on the radio. There is no whine unless transmitting and none on the ICS. So far for troubleshooting...

I've tried multiple headsets from both seats to no avail.

Have removed, cleaned and reinstalled the antenna and grounding wire.

Have pulled off the cowling and checked for any loose wires on the alternator.

Couple of other details:
The battery recently went bad and had to be replaced, but before jumping on the alternator as the source of the problem here is another weird one:

with the master switch and radio on (engine off) the whine is still present.

if i pull the T&B circuit breaker (which i believe is the turn and bank indicator CB) the whine will go away during transmission (but only with the engine off - with the engine running the T&B CB position has no affect on the transmission whine).

Any and all help and hints appreciated!!

 

[Dan Thomas]

It's alternator noise, and it's most likely due to the alternator's ground current travelling through the airframe and finding its way into the microphone circuit. Some folks get relief by installing insulators on the mic and headset jacks and running a separate ground wire from the jacks back to the radio rack. If it's a portable intercom, insulate it from the airframe. Don't screw it down.

 

[jameslovesairplanes]

It's alternator noise, and it's most likely due to the alternator's ground current travelling through the airframe and finding its way into the microphone circuit. Some folks get relief by installing insulators on the mic and headset jacks and running a separate ground wire from the jacks back to the radio rack. If it's a portable intercom, insulate it from the airframe. Don't screw it down.

so even with the engine off, the alternator is still potentially the problem?

 

[wilkersk]

You hear it on side-tone. But, do other people hear it in received audio?

 

[jameslovesairplanes]

Yes. I hear it and both tower/grd hear it too. Again only there when engine on, or when the engine is off with the T&B CB in (with engine off and T&B CB pulled out, no whine for me, tower or ground).

Thanks a ton for the advice!

 

[Hang 4]

Picking up electrical noise from gyro in T&B when it's spinning and alternator when engine running. Still likely a grounding issue. I'd start with your main battery and alternator grounds being tight and clean as well as connections to the main buss.

 

[Raymo]

Along with other grounds, clean the headset jacks and confirm they are not grounded at the mount. They should be grounded back to the other radio/audio equipment, then to airframe/battery ground.

 

[weirdjim]

Well, let's see now. As I understand it, the following are true:

1. There was no whine in any configuration prior to the condition inspection (experimentals don't get annuals).
2. Sitting on the ground, engine off but battery on, there is a whine.
3. (Same as 2 above) but when you pull the Turn & Bank breaker, the whine disappears.​

So, what happens when you fire up the engine with the alternator on but pull the T&B breaker?
What do you suppose happened during the inspection? What did the inspector remove/reposition/dick around with?

Some T&B come with a little capacitor filter to cut down the whine. Looks a little like a tiny lipstick can or possibly a chicklet. Does your T&B have one?

Normally the T&B doesn't get a dedicated breaker. Get out your aircraft documents and see what else is connected to that breaker.

Jim

 

[bluerooster]

Also check shielding to ground on mic wiring. Should be grounded on one end only, and usually at the electronix end.
The T&B in my Cherokee is on it's own breaker, from the factory. So some are.
Bein's how it's heard in your "sidetone", as well as by other stations, leads me to believe that it's probably not in the headset wiring.
But some sort of interference being picked up by the transmitter, through one or more circuits feeding it. It seems that you may have isolated the cause of the interference, (T&B), now, what has happened/changed/been done, during the inspection, that may have allowed it to be picked up by the transmitter?

 

[weirdjim]

Hey, folks, if it didn't whine before the inspection and started it afterwards, why are we checking headset jack insulation when that is nearly impossible for an inspection to change?

But if it was the T&B getting onto the power lines, how come it isn't being picked up by the intercom? Unless the intercom is running from its own battery. But then you'd think that the COM radio receiver/audio would pick it up also. Somehow it is only getting into the transmitter modulator, probably from the mic line. But how? Especially when he can hear it in the sidetone, it has to be in the audio transmitter modulator chain somewhere.

Or, as you say, the inspector could have bumped the mic jack or stressed it so that it USED to be insulated from the airframe and now it isn't.

How buried are those jacks on an RV? Did they take the jacks and build the airplane around them, or are they fairly easy to pull loose?

Interesting problem.

Jim