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Phantom Noise Problem Solved

rv6ejguy

Well Known Member
About 4 years ago, my crystal clear radios started to get muddy and noisy, both receiving and transmitting. I had to abort some flights as I was unreadable in some cases. It was very intermittent though so very hard to track down. Each time I thought I'd found and fixed it, it was back a couple flights later.

Took me a year of fooling around to discover that something was going bad inside the old intercom. That replaced, things were back to normal for the most part but I had a different noise intermittently again.

I was up flying this morning and had this noise back again. Moving the headset jacks slightly cleaned it up as usual but I know everything there is 100% and brand new. Ticked off again, I happened to move the low voltage warning buzzer wires which run very close to the headphone jacks/wires- problem solved. If I moved them close, noise was there, moved them away, gone. Did it 5 times to verify.



I unplug the white connector when I work on the plane with the master on to avoid listening to the buzzer. Each time I reconnect, the wires here are in a slightly different place which explains the intermittent thing. I've tucked the wires up and away now as shown here.

There is no current flowing through these wires unless the buzzer is on so I never even thought of this before as being able to cause a problem like this. The buzzer probably has a small coil of wire inside it. Must be some odd form of capacitance coupling going on when the wires are very close.

I learn something new every day...
 
It is amazing what puts out stray interference.

If you have unexplained interference, just see if you have recently installed a USB power socket..... Ask us how we know :D Filthy emitters of RF which was causing us all sorts of issues on our radio - disconnected, cured.
 
I've seen several cases of digital electronics and gauges causing interference with other electronics, either through the wiring or in close proximity to each other.

Had a new digital temp gauge which was sometimes screwing with my MAP sensor, giving me a couple of good frights in the process as the engine burped...
 
Ross,

What drives the buzzer? Is it a simple switch (like an oil pressure switch), or is it driven by an EFIS/electronic engine monitor? (hint...)

Charlie
 
Yup. Seen some odd things with electronic stuff over my many years in the business but this one is right near the top.
I have a BSEE and I'm flummoxed over that one. Then again, in school we learned that that the one component common to all electronic equipment was "magic". I think you're gonna' need an O-scope to track this one down...
 
Perhaps a case for shielded wire? At least to the buzzer.

I just went ahead and used shielded wire for all mic and phone jacks. All shields grounded at the radio (IIRC)
 
The shielded wire would probably do the trick and maybe changing to a peizo type buzzer too. For now, just moving the wires away 1/2" seems to totally cure it. I've wasted enough time on this over the last couple years and I'm kinda done with it now.

I posted this mainly for informational purposes in case someone else has a similar weirdo nose problem, they can think back and remember this post.
 
I'll try again. If the buzzer is switched by an efis or engine monitor, the 'clock' noise from the driver's computer could be riding on the switched line. Getting it really close to a mic line might allow the intercom to 'demodulate' the clock frequency into audio.
 
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