REFLECTOR: Intercom noise

[Brian Michalk]

I forgot.  Aviation plug wires are shielded (providing their own return 
path).  Check those for frayed ends.

On 01/05/2011 08:38 PM, aminetech at bluefrog.com wrote:
>
>
> --- michalk at awpi.com wrote:
>
> From: Brian Michalk <michalk at awpi.com>
> To: Velocity Aircraft Owners and Builders list <reflector at tvbf.org>
> Subject: Re: REFLECTOR: Intercom noise
> Date: Wed, 05 Jan 2011 20:13:00 -0600
>
> Hmm.
> if the shield is properly connected to the mag, it should be catching 
> any radiated electrical noise.  There are two possibilities.
> 1) the shield is not well connected
> The shield that connects to the mag ground?  I cut the ends off and 
> put new ones on, so that's good.
>
> 2) the return path from the plugs-to-engine-to-magneto is not well 
> connected.
> What is the engine-to-mag path?  I don't think I have one.  I'll chech 
> the resistance from mag case to engine.
>
> In 1) the shield floats with the core conductor and radiates
> In 2) the magneto case experiences a voltage differential (relative to 
> ground) because of high resistance in the return path from the plug, 
> causing the shield to radiate.
>
> This is all assuming it's magneto noise.  If your test is positive for 
> magneto noise, then check for 1) or 2).
>
> On 01/05/2011 07:43 PM, aminetech at bluefrog.com 
> <mailto:aminetech at bluefrog.com> wrote:
>
>
>
>     --- michalk at awpi.com <mailto:michalk at awpi.com> wrote:
>
>     From: Brian Michalk <michalk at awpi.com> <mailto:michalk at awpi.com>
>     To: Velocity Aircraft Owners and Builders list
>     <reflector at tvbf.org> <mailto:reflector at tvbf.org>
>     Subject: Re: REFLECTOR: Intercom noise
>     Date: Wed, 05 Jan 2011 18:54:08 -0600
>
>     Does the noise stop when that mag lead is grounded?
>     No.
>       What about when you disconnect the mag wire altogether?
>     Good idea.  I'll try that tomorrow.
>       Try easy tests first, don't try to fix anything yet.  Take your
>     hypothesis and test it.
>
>     Is it shielded twisted pair, or coax?
>     Coax
>       Does the mag P-lead return back to a ground at the mag(correct),
>     Yes
>      or through some other ground path(wrong)?
>
>     What is the frequency?  Mags will hum at 1/2 the RPM.
>     Probably 1/2 rpm, definitely not the alternator frequency.
>     John
>       an Alternator will hum at 3X of the alternator RPM, which is
>     several multiples above engine RPM.  Can you eliminate based on
>     that frequency?  If it's the alternator, disconnect battery and
>     field (or belt) and test again.  If it is the alternator, do you
>     have a ground strap from that case to your starter ground cable
>     ... or at least a very good confirmed electrical connection?
>
>     On 01/05/2011 06:30 PM, aminetech at bluefrog.com
>     <mailto: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.  My mag wire
>         going to the switch is in the overhead duct along side the
>         wires for the headset jacks.  Can this close proximity be the
>         problem?  The mag wire is shielded.
>         John
>
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