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Yet Another Magneto Noise Thread

What does the noise sound like? The noise in Gary's example is described as a "chopping static". Is this your noise, or is it more of a buzzing sound? The chopping sound is indicative of spark plugs or leads since they are intermittent in their use. A buzzing sound is more indicative of a rotating electric motor type of constant noise. I've never seen a capacitor cause a noise (that doesn't mean it can't). They generally show failure evidence with pitting on the points.
It's a chopping sound. The plug leads of course are brand new, all tight and no chaffing (only 30 hours on the engine).
 
I haven't seen you respond to the suggestion to "Un-Ground" one end of your shielded P leads...this is one I have seen a lot of. shielded ends terminated in ring terminals and some A&P's screw one end onto a mag screw and the other to a ground bus, thus creating a delightful ground loop, that keeps perfect whine with rpm's...annoying as hell.

I tend to like the shield grounded to a panel bus ground and terminate with a solder shield at the mag end, which I pull the pigtail out of to just keep the end tidy.

That has solved a lot of whines over the years.

Steve
Maybe I don't understand this well enough but if you disconnect the ground how will the mag ground with the switch? I did follow previous suggestions of removing the airframe ground from the key switch and only having the P-Lead ground connected. New P-Leads come today which will allow me to remove the ancient filters.
 
Where is wireweinie when you need him? I'm about out of ideas. If I had a hands on, I would remove one lead at a time from it's plug with the engine running while listening for any changes.

Just to confirm, the P-leads are shielded wire. The shields are grounded to the ground lug on the mag switch at one end and to one of the screws on each respective mag at the other end. There should be no ground connection from the switch or the shields to the airframe other than to the case screw on the mag.
 
Where is wireweinie when you need him? I'm about out of ideas. If I had a hands on, I would remove one lead at a time from it's plug with the engine running while listening for any changes.

Just to confirm, the P-leads are shielded wire. The shields are grounded to the ground lug on the mag switch at one end and to one of the screws on each respective mag at the other end. There should be no ground connection from the switch or the shields to the airframe other than to the case screw on the mag.
Yes, correct on all counts.
 
Not sure you saw or read the link that Gary posted, but over there they narrowed it down to an ignition harness issue, which was also a suggestion here.
 
If you still have the old one may be worth swapping just to eliminate it as a variable.
 
I use a handheld aircraft radio to find noise sources. Wrap the antenna with foil if too loud and everywhere. A headset adapter and headset helps, but isn't required at idle.

Gary
 
If you wire the P-Lead and Shield as mentioned above, (at both ends) you will end up with a ground loop.

Try to think of them as two different things. Shield and Conductor.

To avoid this...The P-lead...meaning the actual conductor wire INSIDE the shielded wire, ( the one you will put a ring terminal on) goes from panel-bussed ground, to one pole of your mag switch via ring terminal. The other end of the conductor wire goes to your magneto via ring terminal or splices to the old cigarette on old mags, as the source to activate or deactivate your mag. The other Pole of the mag switch, can be a non-shielded piece of wire going from your bussed ground, to the switch itself.


When you turn your mag switch ON...you are breaking the ground connection...and the Mag will run. When you turn the switch OFF...you are completing the circuit to GROUND and shutting the mag off or grounding it.


The SHIELD....DOES NOT connect to both the Panel-Bussed ground AND a screw on the mag....That is a ground loop. (because the engine is grounded and the panel/bus is grounded...so electrical noise flows in, on and around this loop.

INSTEAD:

You terminate the shield, 2 inches or so, from the end of the P-Lead wire going to the mag. I do this by using a solder sleeve and pull the pig tail out of it and throw it away. OR...you can do it in reverse...where you connect it to the mag screw end and leave it un-connected at the switch end...I like the cleaner first method.

ONLY ONE END OF THE SHIELD IS CONNECTED TO Ground at one end or the other but not both. The second end of the shield just stops, connecting to NOTHING. But the conductor wire is connected to Mag, then switch, then ground, when off.


The Ground Loop is formed, when you connect your grounded engine and mag, to your panel grounded buss, via the shield...All the electrical noise on the shield, can and will flow to your radio and intercom, via the panel bus, which feeds your radio gear and which electrically, you connected to your mag, by hooking the shield up to your buss and engine.

This is what you DO NOT WANT.

Sometimes, it can sound choppy, sometimes just a whine. Electro Magnetic Interference...likes loops. No amount of filters, will help, since you created a significant path of least resistance, if you hooked both ends of your shield to bonded grounds.

I hope that helps.

Steve
 
If you wire the P-Lead and Shield as mentioned above, (at both ends) you will end up with a ground loop.

Try to think of them as two different things. Shield and Conductor.

To avoid this...The P-lead...meaning the actual conductor wire INSIDE the shielded wire, ( the one you will put a ring terminal on) goes from panel-bussed ground, to one pole of your mag switch via ring terminal. The other end of the conductor wire goes to your magneto via ring terminal or splices to the old cigarette on old mags, as the source to activate or deactivate your mag. The other Pole of the mag switch, can be a non-shielded piece of wire going from your bussed ground, to the switch itself.


When you turn your mag switch ON...you are breaking the ground connection...and the Mag will run. When you turn the switch OFF...you are completing the circuit to GROUND and shutting the mag off or grounding it.


The SHIELD....DOES NOT connect to both the Panel-Bussed ground AND a screw on the mag....That is a ground loop. (because the engine is grounded and the panel/bus is grounded...so electrical noise flows in, on and around this loop.

INSTEAD:

You terminate the shield, 2 inches or so, from the end of the P-Lead wire going to the mag. I do this by using a solder sleeve and pull the pig tail out of it and throw it away. OR...you can do it in reverse...where you connect it to the mag screw end and leave it un-connected at the switch end...I like the cleaner first method.

ONLY ONE END OF THE SHIELD IS CONNECTED TO Ground at one end or the other but not both. The second end of the shield just stops, connecting to NOTHING. But the conductor wire is connected to Mag, then switch, then ground, when off.


The Ground Loop is formed, when you connect your grounded engine and mag, to your panel grounded buss, via the shield...All the electrical noise on the shield, can and will flow to your radio and intercom, via the panel bus, which feeds your radio gear and which electrically, you connected to your mag, by hooking the shield up to your buss and engine.

This is what you DO NOT WANT.

Sometimes, it can sound choppy, sometimes just a whine. Electro Magnetic Interference...likes loops. No amount of filters, will help, since you created a significant path of least resistance, if you hooked both ends of your shield to bonded grounds.

I hope that helps.

Steve
I appreciate what your saying and it has been done this way to no avail. Photo attached show what I believe you to be saying. Switch is grounded to bus and the P-Lead shields are not connected at this side.
 

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I don't recall operating static free small older aircraft radios. They may exist, but not in my experience. While Skywagon185 works through their issues, it might help if others with interest that op similar Cubs disable their squelch then note and report the level of their radio(s) static level. It can be measured with spectrum analyzers but no need, just some subjective info to share. We might learn something?

Gary
 
some confusion here on the definitions of grounding the shield. in many older planes, the shield provides two different functions depending on the position of the switch. the P lead is connected to the mag and the switch as expected, and the ground/shield is connected to both the case of the mag (shield ground) and the ground terminal on the switch. this way, when the switch is on BOTH, the switch is open circuit, allowing the mags to fire; therefore both ground shields are open circuit at the switch, thus making them a shield ground at the mag only.

when the switch is OFF, the shield becomes the conductor to ground to shut off the mags.

it’s important to note that the ground terminal on the switch is not connected to chassis ground at all, so when the switch is on BOTH, the shields are not grounded to anything at the switch end.

a clever way to wire old planes, but dedicated shield and signal grounds is probably better.
 
Somewhat off the trend, but....

From Post #1: "I have done the following:
  • Wired a ground for the antenna"
Normally the antennas on fabric aircraft are attached through the fabric to a metal backing plate of minimum size (like 18x18"), or a smaller backing plate that has a fanned series of wires (1/4 wavelength or ~22" ea.) attached. I assume the static was unaffected by adding an additional "ground" for the antenna? Also try to keep any antennas apart. The distance from wing root to wing root is on a Cub is common. An ELT or ? antenna can have static pickup from it's wiring the Com antenna then hears.

Gary
 
Somewhat off the trend, but....

From Post #1: "I have done the following:
  • Wired a ground for the antenna"
Normally the antennas on fabric aircraft are attached through the fabric to a metal backing plate of minimum size (like 18x18"), or a smaller backing plate that has a fanned series of wires (1/4 wavelength or ~22" ea.) attached. I assume the static was unaffected by adding an additional "ground" for the antenna? Also try to keep any antennas apart. The distance from wing root to wing root is on a Cub is common. An ELT or ? antenna can have static pickup from it's wiring the Com antenna then hears.

Gary
Yes, no difference. I grounded the antenna to the airframe in the wing root.
 
Update. Removed filters. Don't hear the chopping now when I break squelch but still hear it on weak radio calls.

Question: I believe FatCub300 says to ground the shield on the mag but on the switch end remove and terminate. Then ground the switch to the airframe. Others have said to remove the airframe ground from the switch and terminate the shields on the mag and the switch ground post.

I have tried both of these methods and nothing has changed. I may need to defer to an expert. I have a lead on a local guy as well as the guy in Twin Bridges.
 

Here's a suggested aircraft wiring diagram from B&C Specialty Products. See Note 2: "Ground P-lead shield on magneto end only." The individual magneto switch appears to be ungrounded, and when active internally grounds the inner P-lead wire (= magneto off) via a return current via the shield to ground at the magneto. Opinion only.

Edit: If you remove the coax feed at the antenna....what do you hear for static or weak signals? If there's still static even at low level it's probably entering the radio or intercom via other wiring, not the antenna. I'd not expect to hear weak signals with the antenna disconnected.

Gary
 
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It doesn't really matter which end you choose to terminate vs. ground. The point is to give electromagnetic interference an easy path away from your avionics bus or chassis ground....or wherever you ground your radios and intercom to. There are folks who swear you must ground at both ends, but I have had better luck not doing that.

Given all that you have done, and given that none of us can hear the noise...you may have reached the point of soliciting assistance. I have never had good luck with those rotary switches and generally throw them away and put a starter button switch and a pair of toggles for mags. Personal opinion maybe, but I just feel better about toggles and they're easy to keep a spare in my kit for backup.

I have used ferrite ring filters on my shields on the P-leads with success. Again...I prefer to have my shield grounded at the switch end...tied to chassis ground or bonded buss ground and then terminate them 2" from the end of the P-lead. I could be wrong, but it seems to me that the magneto case itself will shed interference back to the engine, due to its lower resistance connection than if you connect the shield and a direct path to a ground source closer to your radios. You also see lots of times, where shields rub on other metal bits, which further muddies the water.

The thing to remember is that when the engine is running, the mags are separated from ground...so if the noise is for sure showing up on your ground system somewhere...you can track it down.

I have in the past dis-connected mags from ground completely, and ran the engine as a test, shutting down with mixture only...obviously you have to be very careful doing this, but I have sorted out a very noisey mag that was internally arcing, sending god awful noise everywhere...but it made it easy to troubleshoot, as you hook things back up, one by one and voila!! the noise is located, isolated and resolved.


Steve.
 
I agree with Steve’s post above.

As a further example, you never ground both ends of a shield on any audio wiring. You’re trying to stop the flow of any noise and the full loop will add noise. So while shields are typically grounded on all data lines on both ends, it’s never done on audio wires.
 
A bit of new data. I disconnected the P-Leads from the Mags completely, no change. I then had someone in the plane and run it and confirm they could hear the noise and I stood outside with a handheld and headset and could hear the same noise. Only way to hear this is with a weak radio reception or to break squelch. The clarify that, I tune in the ATIS for KGPI while on the ground at my home airport. You can only receive it if you break squelch. You can here the interference very clearly when you do this. I had my buddy shut down the engine while keeping the ATIS tuned and sure enough the noise goes away. Not sure at all what would cause this to even be heard by the handheld.
 
If able move the handheld over the running engine to locate the source. Remove the antenna if too loud. Try to get next to plugs, wires, mags, and all wiring w/o touching the rotating prop. What you hear are radio waves transmitted at the listening frequency. You may be able to focus on a source, but if a mag internally all wires attached to one or both may act as transmitting antennas. You may not be able to completely quiet the source(s).

Edit: I didn't reread all but make sure your alternator is shut down during the testing. Open the field wire switch or breaker. Make sure the charging wires from the alternator are away from the spark plugs and leads.

Gary
 
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If able move the handheld over the running engine to locate the source. Remove the antenna if too loud. Try to get next to plugs, wires, mags, and all wiring w/o touching the rotating prop. What you hear are radio waves transmitted at the listening frequency. You may be able to focus on a source, but if a mag internally all wires attached to one or both may act as transmitting antennas. You may not be able to completely quiet the source(s).

Edit: I didn't reread all but make sure your alternator is shut down during the testing. Open the field wire switch or breaker. Make sure the charging wires from the alternator are away from the spark plugs and leads.

Gary
Removed all wires from the Alternator, no joy. Swapped back in the old mag harnesses, no joy. Moved handheld over all of the engine, no source found, same noise everywhere.
 
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