REFLECTOR: Intercom noise

Brian Michalk michalk at awpi.com
Thu Jan 6 11:30:42 CST 2011 

I don't think we've found the source yet.  If we can't find the source, 
can we find the receiver (besides the intercom).

What other wires go to the engine?  RPM sensor?  Oil pressure?  CHT/EGT 
probes?  Is there a big connector somewhere that when disconnected 
removes all of these sources?

On 1/6/2011 11:06 AM, aminetech at bluefrog.com wrote:
> 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 
> <mailto:aminetech at bluefrog.com> wrote:
>
>     OK.  What about my EI? (auto wires)
>
>     --- 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 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
>     <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 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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