Radio noise while transmitting ?

[777drvr]

Hi Guys,

I have an old Narco 810/Flightcom 403 combination that's worked well for almost 50 flights . Recently , I've started getting a buzzing noise while transmitting . Reception is 5/5 .

Here's what I've tried so far:

- Re-racked the radio and confirmed that that BNC connectors are mated properly . Did not actually check continuity .

- Turned one mag off at a time while transmitting . No change . Can a mag still make noise if it's not turned on ?

- Turned off internally regulated ND alternator . No change . Can an alternator still make noise if not supplying voltage to the bus ? Bearings etc ?

- Turned off all lights and other avionics . No change .

So , I've tried a number of things and am running out of ideas . I have made a few observations about when and how it makes noise . I have a CS prop and there is very little noise at 2700 but the intensity of the buzz increases and the frequency decreases when I lower prop RPM . Also on final with the power pulled back almost to idle there was very little noise .

A few thoughts for continued trouble shooting ......

-Try different MP with a given prop RPM . May indicate an ignition system link but I thought I ruled that out by isolating mags ...

-I'm wondering if the different RPM affect might have to do with a difference in the vibration frequency affecting a loose conection , wire , antenna etc .
I may try putting my hand under the panel to check for vibration and maybe dampen it if there is .

Any suggestions are appreciated ! Unfortunately I have to jet off to the Orient so I won't be able to try all your very helpful suggestions until the weekend . Ultimately I want to modernize my panel but I would like to get the present system back in order until I do.

Marc

[fl-mike]

The mic input is a very low level signal, so it is likely that's where the noise is getting in.
Suggestions:
1. Try a different headset/mic.
2. Try the other input if you have it wired and a PTT.
3. Try going direct to the radio.
if 2 or 3 help, check the cable shield connections on the aircraft mic harness.

Check the mag wire shields.

If it's a RPM related whine, it is probably alternator noise. Mine failed at about 85 hours. Was still charging, but got a fault light and heard a whine only when transmitting. (unlike the constant whining I usually get from my passengers)

Good luck

[Dayton Murdock]

I had the same problem with my 403 intercom
I ran a new ground wire from the PTT switch to my radio ground and the noise is gone.

[777drvr]

Some great advice so far . The noise not a traditional alternator whine but more of a low pitched buzz . Still happens with the alternator off .

I can easily try transmitting through the rear seat PTT . Didn't think of that one last night when I actually had another pilot helping troubleshoot .

Marc