REFLECTOR: Intercom noise

[Laurence Coen]

I once had an intercom noise came from the engine.  It was non electrical 
and caused by an unused headset with the mike touching the airframe.  It 
sounded a lot like electrical noise.  Just a thought.

Larry Coen
N136LC

--------------------------------------------------
From: "Brian Michalk" <michalk at awpi.com>
Sent: Thursday, January 06, 2011 1:14 PM
To: "sbjknox" <sbjknox at earthlink.net>; "Velocity Aircraft Owners and 
Builders list" <reflector at tvbf.org>
Subject: Re: REFLECTOR: Intercom noise

> I forgot to ask about your spark plug wire routes.
>
> Since the automotive wires are not shielded, one should keep sensor wires 
> away from them, and avoid running them through current loops.
>
> A current loop is the area bounded by current running in a wire.  Picture 
> a piece of lamp cord, the kind that's easy to unzip into two single wires. 
> Imagine connecting two leads at one end to a noisy source like a badly 
> oscillating DC/DC converter, and twist the two leads at the other end 
> together.
> If one were to try to sense the magnetic field generated, it would be very 
> small.  The current loop area is the distance between the two wires.  Now, 
> unzip the cord so that it's a big circle.  The current loop is much 
> larger, and the magnetic fields do not cancel each other out as they did 
> before.
>
> On the engine, the spark current goes through a wire to the plug, across 
> the spark gap, into the engine block, to the other spark plug(from the 
> same coil pack, since it's wasted spark), across that spark gap, then 
> through the plug wire, eventually returning to the coil pack.  Picture 
> this as a large current loop.  Try to keep the plug wires as close to each 
> other as possible, as well as close to the engine block.  Also try not to 
> run any sensor wires through this loop, or near the plug wires.
>
> But, as you mentioned, you disconnected the EI, so this shouldn't be your 
> problem.
>
> This is the reason why I twist all of my high current circuits in the 
> ducts.  The EI power is twisted, making current loops as small as 
> possible.  Shielded wire only protects against capacitive coupling, which 
> is rarely a problem with low frequency, low impedance circuits like these.
>
>
> On 1/6/2011 11:56 AM, sbjknox wrote:
>> I have been following this thread and also am interested to know what you 
>> find.
>>
>>>  On 01/05/2011 06:30 PM, aminetech at bluefrog.com wrote:
>>>  I'm still trying to eliminate noise on my intercom.  It occurs when the 
>>> engine is running,
>>>  even while starting.
>>
>> As I understand the current situation based on the discussions between 
>> you and Brian, the noise is *not* caused by:
>> 1) Radiation or conduction from the primary side of either the mag or the 
>> EI.
>> > I've tried turning off the mag, and the EI.  I even disconnected the
>> power to the EI.
>> > I disconnected the ground and P lead to the mag.
>> 2) Alternator
>> >  I removed the alternator belt.
>>
>> That does not leave much... What  remains is:
>> 1) Secondary ignition (high voltage to plugs).  When you turn off one of 
>> the ignitions to listen to the other, does the characteristic of the 
>> noise change (ie., does one sound different than the other)?  If the 
>> sound is different, then perhaps there is a problem with plug wires (?)
>> 2) Some other source... something else running that is generating noise.
>> Does the frequency of the noise follow engine rpm?
>>
>>> Should I consider a mag filter?  (links to two filters, one for P-lead, 
>>> one for power bus).
>>
>> Since you have disconnected the P-lead at the mag for no improvement, the 
>> P-lead filter would probably do nothing for you.  The power line filter 
>> 'might'  help if there is something on the power bus that is generating 
>> the noise.  On the other hand, if there is something (#2 above), it might 
>> make more sense to fix the noise source.
>>
>> How confident are you that the intercom wiring is correct... proper wire 
>> type (shielded) and shields connected where they should be,  etc...
>>
>> One thing you said  "It occurs when the engine is running, even while 
>> starting." that seems odd to me.  I think it is bad practice to have the 
>> avionics powered-up while cranking the engine.  I'm not suggesting that 
>> this is related to your intercom noise... just a side observation.
>>
>> Please keep us informed of your efforts.
>>
>> Sid Knox
>> Oklahoma
>>
>> Velocity  173 RG  N199RS
>> Starduster  N666SK
>> KR2         N24TC
>> W7QJQ
>>
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