Bad RFI in Icom A-24

[geraldvg]

I posted this note to the bottom of an old thread and got no replies, so am hoping a new thread will get some "expert" and/or "been there done that" responses.

I have a C-85 equipped L-16A (no electrical system) and am using an IC-A24 with the "rubber-duckie" antenna on an extension feedline with the antenna suction cupped to the back window. I have Bendix mags with shielded ignition cables, but without any type capacitor filter. I checked the harness today and it (all leads) checked out clean. Now comes the real puzzle. The loud popping noise is not RPM dependent and mostly goes away on left mag only. Both mags have inpulse couplers. I am at a loss to figure out where RFI sufficient to keep the squelch, at any usable setting, open but not vary with engine speed could be coming from. In addition to the really annoying noise, with the squelch open all the time the radio eats the battery. I will probably replace the ignition harness, just for good measure, but I could sure use some good advise.

Gerald
N2292S

[Ed Clark]

While not the only thing to check, I would check the wires from the mag switches to the mags. they need to be shielded too.

My 46 champ with an A65 had Eisemann mags to start with and had an awful time with RFI. When I put the shielded harness in with the same mags...no change. Once I put the slick mags in that were shielded and a new shielded harness the problem went away like magic. While doing that job we checked the mag wires going to the switches and discovered they were shielded too so did not have to replace them (someone on this site had told me to check them). I use an A-24 radio but do use an external antenna mounted on the left wing root fairing.

Good luck.

Ed

[Dennis]

Gerald,

Apparently Gerald, you have done all the usual things without success. But you state that you do not have a mag filter. They are quick and easy to install. If you do not want to buy one, see if you can borrow one to install just to elliminate that from the equation. If it fails to produce a change, see if you can borrow a Bendix mag to install, just to check it out.

OK guys, I am guessing here so check out my thoughts:

What is throwing me is the lack of change in the frequency of the popping. So here is a "guess" at a way to search for the problem. This approach may give you a clue as to the source. If the Pop is not frequency dependent, with the engine at a fixed RPM, count the number of pops in a given time period, like maybe 5 minutes. Then take the average for 1 minute. For instance, at 600 engine RPMs, the engine will fire 300 times in a minute or each cylinder will fire 75 times in a minute. If you get 75 pops, I would remove the lead to each cylinder, one at a time and place them such that they can't ground out, looking for a change in the Pops.

The mag, on the other hand, must fire all the cylinders so it will make and break 300 times at 600 engine RPMs. So you would get 300 pops if the noise is associated with the mag itself.

Need a little help from you guys here. I think the mag RPMs is different than a given engine RPM. Is that right? Anyway, my point is if the number of pops correspond to a different mag RPS, the problem may be with the mag shaft/magnet.

I guess it would be possible for the mag to trigger the squelch as a carrier frequency but the popping be from somewhere else. Purely a guess.

Oh, I left out the easiest and most obvious, try another radio.

Dennis

[DELCOM]

You give reference to a "feedline" extension. Is this feedline a poor quaility coax or the double shielded good stuff? If you are using an RG58 A/u or the likes it will not properly shield the antenna system and will allow even the tinyist amount of rfi to affect the reciever. These magneto based ignition systems are horendous producers of efi. Just a thought.

Del

[geraldvg]

Some really good ideas to check and follow-up on. I've been planning on installing a fixed antenna mounted to the back panel of the baggage area. I'll plan on using a premium coax feedline for that. Further work with the mags and harness leads to attempt to isolate a problem there will just take some ground time. I don't have great expectations there, except that I already know that the noise seems to be associated with just one of the mags. Digging deeper in that area may make something unexpected show up. I just can't at this point imagine what since the mags must be turning at the same RPM as the engine and the popping is definitely not engine RPM dependent. Not sure I can count the popping frequency. I think it will prove to be too rapid. I'll give it a try, though, and see if there is correlation to anything I can identify. Trying another radio might also be interesting, if I can find one around here anywhere. (I'm kind of out in the boonies here in the Arkansas Ozarks.) I'll also see if I can borrow a mag filter. Almost hope that isn't what it takes to fix the problem, however, because those capacitors reduce the spark intensity some. I'm also going to have a good look at the leads from the switch. Not sure what the source of the noise could be, but those do sound like a likely weak spot.

If anyone has any more ideas, please pass them along. I will post the results of my investigations. Thanks to everyone so far. Hope everyone had a great Thanksgiving, but don't now have any "gross weight" problems.

[Dennis]

Gerald,

Have you ever heard of Mtn. View, Ar or 7M2? Fly down here, put the plane in my hanger and we will play around with it. I got a hand held.

Dennis