---------------------------------------------------------- AeroElectric-List Digest Archive --- Total Messages Posted Fri 08/12/11: 4 ---------------------------------------------------------- Today's Message Index: ---------------------- 1. 07:15 AM - One Noisy Mag (stearman456) 2. 10:05 AM - Re: One Noisy Mag (Robert L. Nuckolls, III) 3. 03:02 PM - Re: One Noisy Mag (stearman456) 4. 10:07 PM - Re: One Noisy Mag (stearman456) ________________________________ Message 1 _____________________________________ Time: 07:15:25 AM PST US Subject: AeroElectric-List: One Noisy Mag From: "stearman456" This past week I decided to tackle some radio noise in my '49 Aeronca Champ. Although I can transmit just fine (Narco Comm 111) there was always some background ignition noise when receiving (you need to be about only 15 miles away to hear an ATIS clearly). But when you switch over to just the right mag (Bendix mags) the noise goes away. The noise is ignition noise, it varies directly with the rpm, and I believe it's radiated noise as it varies with the radio volume control. If you disconnect the P lead from the left mag's filter (Lone Star Aviations Magneto filters) the noise goes away completely and the radio is perfect. The P leads were shielded wire, probably 16 gauge, and the shields were grounded to the mag cases. Checking the mag switch (AAF A-7 type) I found that the shields were also grounded at the switch. "Aha..." says I. "exactly what Bob said never to do." So I reached in with a pair of side cutters and cut the connections to the shields. Rolled her back outside, started her up and... no change - still a noisy left mag. Deciding that the P lead itself must be chafed or the shielding was open somewhere I removed both leads and made up two new ones from 18 gauge shielded wire, and carefully added a second wire (not just a pigtail) to connect the shielding to the mag cases on the respective mags. Installed them last night, rolled her back outside and... no change! Same quiet right mag/noisy left mag. I couldn't see it being the switch itself but even so I swapped the leads on the switch and the noise followed the lead, so it's not the switch. The only difference I can come up with between the mags is the left mag has the impulse coupling on it (re-read your article about mag switches in the EAA magazine, Bob. Lots of good info there - thanks) but that should have anything at all to do with it. I'm stumped. Should I perhaps use heavier wire for the P leads, or is this an actual mag problem? Thanks in advance. Dan warbirds@shaw.ca Read this topic online here: http://forums.matronics.com/viewtopic.php?p=349246#349246 ________________________________ Message 2 _____________________________________ Time: 10:05:50 AM PST US From: "Robert L. Nuckolls, III" Subject: Re: AeroElectric-List: One Noisy Mag At 09:10 AM 8/12/2011, you wrote: This past week I decided to tackle some radio noise in my '49 Aeronca Champ. Although I can transmit just fine (Narco Comm 111) there was always some background ignition noise when receiving (you need to be about only 15 miles away to hear an ATIS clearly). But when you switch over to just the right mag (Bendix mags) the noise goes away. The noise is ignition noise, it varies directly with the rpm, and I believe it's radiated noise as it varies with the radio volume control. good point If you disconnect the P lead from the left mag's filter (Lone Star Aviations Magneto filters) the noise goes away completely and the radio is perfect. If I read this experiment correctly, the left mag filter is still wired to the p-connection on the mag itself . . . but the wire going to the magneto switch is disconnected from the other end? The P leads were shielded wire, probably 16 gauge, and the shields were grounded to the mag cases. Checking the mag switch (AAF A-7 type) I found that the shields were also grounded at the switch. "Aha..." says I. "exactly what Bob said never to do." Not so much a problem for noise. But it provides a second ground path between crankcase and airframe that parallels all other grounds . . . there is a potential for burning your p-lead wire as described in the 'Connection. So I reached in with a pair of side cutters and cut the connections to the shields. Rolled her back outside, started her up and... no change - still a noisy left mag. So this experiment has the shielded wire grounded a the magneto (probably to the filter mounting screw) at one end and to the GRD terminal of the magneto switch at the other end? Deciding that the P lead itself must be chafed or the shielding was open somewhere I removed both leads and made up two new ones from 18 gauge shielded wire, and carefully added a second wire (not just a pigtail) to connect the shielding to the mag cases on the respective mags. Okay Installed them last night, rolled her back outside and... no change! Same quiet right mag/noisy left mag. I couldn't see it being the switch itself but even so I swapped the leads on the switch and the noise followed the lead, so it's not the switch. The only difference I can come up with between the mags is the left mag has the impulse coupling on it (re-read your article about mag switches in the EAA magazine, Bob. Lots of good info there - thanks) but that should have anything at all to do with it. Good deduction I'm stumped. Should I perhaps use heavier wire for the P leads, or is this an actual mag problem? No, actual size of the wires is unimportant. 24AWG would 'function'. The general rule of thumb for operational wires (not instrumentation) under the cowl is 20AWG or heavier . . . for mechanical robustness. What we may be witnessing is the innate perversity of shielded wires . . . at least in the eyes of many builders. Shielding is 99.9% effective in breaking the electro-static coupling mode and about 1% effective in breaking a radiated coupling mode depending on the frequencies involved. EXCEPTION: Plug wires with shields connected to ground at both ends behave more like transmission lines . . . like your coax from antenna to radio. In this case, there is considerable attenuation of the electro-static coupling of a fast-rise high- voltage spark pulse. Further, they break the wire's ability to be an efficient antenna. Modern plug wires will have resistance wire as the center conductor which has little effect on spark energy but really whacks the wire's performance as an antenna. This is the ONE place where shielding is indicated for both coupling modes . . . if you put an effective RFI filter on a plug wire, the plug wouldn't fire! The shields behave more as an 'enclosure' since the wires are short and the shields grounded a very low-impedance ground (crankcase). The filters on both p-leads that break the radiated coupling mode before the trash gets out onto the p- leads. So if your noise goes away when the engine is run without a p-lead connected to the filter . . . then a reasonable deduction is that the filter is not doing its job. The fact that noise goes away with a p-lead disconnected also says your plug wires are good. I've never personally seen a filter go bad but they're a spares item in most inventories so it must happen now and then. Try swapping the filters between mags . . . If this makes a difference as to which mag is noisy, you might want to replace both. One tired ol' filter going t-u may be a portent of things to come for the other one as well. Good detective work! Bob . . . ________________________________ Message 3 _____________________________________ Time: 03:02:22 PM PST US Subject: AeroElectric-List: Re: One Noisy Mag From: "stearman456" Hi Bob, Thanks for the help. If I read this experiment correctly, the left mag filter is still wired to the p-connection on the mag itself . . . but the wire going to the magneto switch is disconnected from the other end? Yes, the P lead was disconnected from the filter, so when the engine was running with no P lead to the switch the filter was still connected to the mag itself. So this experiment has the shielded wire grounded a the magneto (probably to the filter mounting screw) at one end and to the GRD terminal of the magneto switch at the other end? The P lead wire itself is grounded at the mag switch (the switch itself has a ground wire to the fuselage frame) but the shielding for the P lead is only grounded at the mag case to one of the screws. I'll swap the filters around tonight and let you know how that goes. Really appreciate the help. Dan warbirds@shaw.ca [/i] Read this topic online here: http://forums.matronics.com/viewtopic.php?p=349285#349285 ________________________________ Message 4 _____________________________________ Time: 10:07:47 PM PST US Subject: AeroElectric-List: Re: One Noisy Mag From: "stearman456" Hi Bob, That was it - I swapped the mag filters and now I have a noisy right hand mag and a quiet left hand one. Don't know why I didn't think of that, though they're not all that old. Must have had a dud right out of the box because it's always been like that and I installed those filters about five years ago, but... that happens sometimes. Thanks for the help - sure appreciate it. Dan warbirds@shaw.ca Read this topic online here: http://forums.matronics.com/viewtopic.php?p=349300#349300 ------------------------------------------------------------------------------------- Other Matronics Email List Services ------------------------------------------------------------------------------------- Post A New Message aeroelectric-list@matronics.com UN/SUBSCRIBE http://www.matronics.com/subscription List FAQ http://www.matronics.com/FAQ/AeroElectric-List.htm Web Forum Interface To Lists http://forums.matronics.com Matronics List Wiki http://wiki.matronics.com Full Archive Search Engine http://www.matronics.com/search 7-Day List Browse http://www.matronics.com/browse/aeroelectric-list Browse Digests http://www.matronics.com/digest/aeroelectric-list Browse Other Lists http://www.matronics.com/browse Live Online Chat! http://www.matronics.com/chat Archive Downloading http://www.matronics.com/archives Photo Share http://www.matronics.com/photoshare Other Email Lists http://www.matronics.com/emaillists Contributions http://www.matronics.com/contribution ------------------------------------------------------------------------------------- These Email List Services are sponsored solely by Matronics and through the generous Contributions of its members.