REFLECTOR: Intercom noise

Thu Jan 6 13:16:16 CST 2011

I'm no expert (Brian is -- sure glad he's my partner) --  but can every thing be  disconnected and start with a clean slate, then start adding loads and check for noise?

Juanita

--- On Thu, 1/6/11, aminetech at bluefrog.com <aminetech at bluefrog.com> wrote:

From: aminetech at bluefrog.com <aminetech at bluefrog.com>
Subject: Re: REFLECTOR: Intercom noise
To: "Velocity Aircraft Owners and Builders list" <reflector at tvbf.org>
Date: Thursday, January 6, 2011, 11:06 AM

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.  I removed the alternator belt.  Nothing has worked.  Should I consider a mag filter?  I have a slick mag, so I don't know if it will work with this.

http://www.aircraftspruce.com/catalog/elpages/lsMagFilter.php

or this?

http://www.aircraftspruce.com/catalog/elpages/smoothieFilter2.php

My Rose EI runs 2 plugs per coil.

--- 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: Thu, 06 Jan 2011 09:23:03 -0600

Ah.  First try running 1) without mags, then 2) without EI.  See if the noise goes away.  If the EI is the noisemaker, there's a couple of places that can cause problems.  Depending on the coupling mechanism, it can be RF radiated from the wires, or it could be voltage spikes on the power leads, or it could be a return path problem described earlier.  Is your EI a wasted spark setup (two plugs per pack)?

On 1/6/2011 8:39 AM, aminetech at bluefrog.com wrote: 

OK.  What about my EI? (auto wires)

--- 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 21:16:26 -0600

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 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 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.

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