REFLECTOR: Intercom noise
Brian Michalk
michalk at awpi.com
Thu Jan 6 13:29:19 CST 2011
Yeah. Grounding a shield at both ends is terrible practice, and there
are *so* many schematics that indicate to do just that.
Sometimes they even call for using the shield as the return path.
Seriously. It's just making a mess of gremlins to catch.
For those not knowing what a ground loop is, consider the following
scenario:
In the panel, two an intercom and RMI share a common ground. The RMI
CHT probe is mounted as a gasket thermocouple, and for some reason has a
shield that is grounded at the RMI case and the spark plug. For some
reason, the spark plug anti-seize compound electrically insulates it
from the engine block. When the plug fires, a very high dv/dt pulse,
trying to find its way back to the coil pack, instead travels up the
shield, to the RMI, to the battery, to the big ground wire to the
starter, and then back to the coil pack. That big spike causes a
voltage fluctuation on the intercom, not coupled through any microphone,
headset or other connection, but enters via the power bus. The incoming
filters can't chop it all off and that can be heard in the headsets.
That fictional ground loop could cause this sort of problem.
Additionally, more grounds is not always a good idea. Force the
electricity to go back along the same path it came in on. Don't let it
"choose" several paths back to the battery. Those other paths are all
ground loops.
On 1/6/2011 1:08 PM, Jeff Howell wrote:
> I would venture to say it could be either a ground loop problem or EMI
> and or RFI interference. I would also ask about the routing of the
> wires. If your wires are run parallel to either power wires or high
> voltage wires (such as coils or strobe wiring) the possibility for
> noise is great. Another thing I would do is make sure your shielding
> is grounded at connector end at intercom and floating at the other
> thus eliminating any ground loops. Never run wires parallel to power
> or high voltage wires and if you have to cross these types of wires
> then cross them at a 90 deg angle thus lowering the chance of cross
> talk. Try to keep a minimum of three feet separation between power and
> com. wires when possible. hope this helps Jeff
>
> ------------------------------------------------------------------------
> Date: Thu, 6 Jan 2011 09:45:20 -0800
> From: aminetech at bluefrog.com
> To: reflector at tvbf.org
> Subject: Re: REFLECTOR: Intercom noise
>
> Rpm signal comes from the EI. When I turn off the EI, I have no rpm
> reading on my RMI monitor, the noise remains. The oil pressure sensor
> is at the oil cooler, not on engine. I'll try disconnecting the
> CHT/EGT connector. Would there be any benefit in removing the mag
> drive gear, so it won't turn?
>
> --- 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 11:30:42 -0600
>
> 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
> <mailto: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 <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: 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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