REFLECTOR: Intercom noise

[Brian Michalk]

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
2) the return path from the plugs-to-engine-to-magneto is not well 
connected.

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 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 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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