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New rf Noise!
I've got about 325 hrs on my STOL CH750/Jabiru 3300. My radio (MGL V6) has been relatively (rf noise) quiet for most of those hours. The Jab 3300 came with quality rf suppressive plug wires - I added Iridium resistor plugs and upgraded the Honda magneto coils with Magnecor rf suppressive leads and this gave me a relatively quiet installation, as far as the radio is concerned!
Lately, however, a new rf gremlin has appeared. The radio is dead quiet during the engine warm-up. Upon takeoff, I get a "Brap!" static noise sound that lasts about as long as it takes to say "Brap!" This may occur in irregularly spaced bursts of 2, 3, or more and then a few seconds later, occur again. Curiously, it seems to continue for the first few minutes of flight and then gradually fades away and is completely gone after 15-20 minutes. If I switch the ignition to "left," it completely kills the noise. If I switch it to "right," it markedly reduces the noise but I "think" I can still hear it. Only when on "both" is it loud and annoying.
The Jabiru 3300 has a unique alternator in that it does not produce charging current until approximately 1900 rpm, so it doesn't produce significant output until full throttle at takeoff. It has been suggested to me that a weak battery, requiring more charge, could be triggering the voltage regulator to make the noise. But, I have a relatively new EarthX lithium battery and it is quite strong and not "weak!"
I have the magnetos' p-leads' shields grounded at the magnetos and not at my ACS switch. The ACS switch's center ground is grounded to the airframe. To the best of my recollection, I've not changed or modified anything electrical recently. I'm at a loss to explain why this noise is just now appearing! Makes one wonder if something is "aging" and failing. Does a bad voltage regulator get "rf noisy?" It appears to be working perfectly as far as maintaining battery charge, etc.
Any ideas?
John
N750A
Replies to This Discussion
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Permalink Reply by Gordon Arbeitman on
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John;
The area that best fits your description of symptoms is the charging circuit. I'd carefully inspect all the the wires at the voltage regulator for burned or loose connections. I'd also make sure that the wires from the alternator are not chafing and are properly secured.
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Permalink Reply by John Austin on
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I have already carefully inspected the alternator and voltage regulator wiring (today!). Although I have the multi-pin connector the VR came with (Pete at Jab USA says to use more robust connectors and a terminal strip), the connectors look "pristine," that is, just as they did as new! No charring, corrosion, etc. All of my alternator wiring is sleeved and carefully secured. David's idea to simply turn off the alternator will rule all this in-or-out quickly! Hope I can squeak in a test flight tomorrow and try that.
John
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Permalink Reply by David Josephson on
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Good set of puzzles here. Jabirus don't have traditional magnetos or P-leads per se, they use magnets on the flywheel and ignition coils from Honda generators, similar idea to rotax except that instead of multiple ignition coils there are only two, and distributors for the spark instead of wasted-spark. They say they're "transistorised" but there is no separate control box, I would expect there's a transistor or two inside the ignition coil to make the trigger point more stable. Anyway I can't see any connection with the alternator. You can confirm this by turning the alternator off and the noise shouldn't change.
There have been a number of changes in the coils used on this engine, see http://www.parts4aircraft.com/PBCPPlayer.asp?ID=1003493 There are also issues with the distributor caps (same as on 1972 Chevy Luv, apparently) leading to the center contact pulling out or losing continuity. This could cause more noise without a significant change in power. I'd look at those areas first -- specifically, inspect the distributor cap center contact inside and out for any sign of erosion, carbon tracks, cracking etc. Sometimes a crack will collect a tiny amount of moisture and arc
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The puzzlement, David, is why does the noise go away after 15-20 minutes??? I'll certainly inspect the distributor caps, etc., but if they were defective why wouldn't the problem persist the entire flight???
Your suggestion to turn the alternator off is one of those "Why didn't I think of that? Duh"" moments! Ha! I'll certainly try that on the next flight and see what happens?
I followed with great interest the recent thread about shield grounds on the p-leads, etc. Do you agree that my set-up is OK: shields are grounded on the magneto ends and not on the ACS ignition switch. The central ground lug on the switch is grounded to the airframe. The noise IS related to the switch in that I can select the L mag and the noise completely is gone and with the R mag it is markedly reduced ... it's only on "BOTH" that I hear it. However, in any case, there have been no recent changes to the electrical system and the noise just started recently, AND it goes away after 15 minutes or so! 'Tis a puzzlement!!!
John
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Lots of things change when the engine gets hot. If there is a crack, it can collect a bit of moisture. When it gets hot, it can expand and/or dry out. Your p-lead setup is OK. I have no explanation for why there is noise when on both but not on either one. The "Brap" implies that something is arcing for an instant and then settling down again. When you turn both ignitions off does the engine actually stop firing?
It could also be an engine grounding issue. Be sure there is a secure ground path from the engine block to the firewall and the negative battery post and the ground return point you're using for your switch.
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I haven't examined the distributor caps yet, but I will! I'll fly again today and try turning the alternator off to see if that kills the noise. FWIW I did try the other ignition shield configuration of grounding the shields at the magneto and switch and removing the central ground from the switch, but it made no difference at far as the "Brap!"/arcing sound.
I have a MGL engine monitor and have dual tach pickups - one off each magneto. When either or both of the magnetos are turned off, it instantly kills that ignition and with both off, the engine instantly dies.
As far as the engine grounding, all grounds are clean and secure - no problem there. I have a very substantial grounding strap between the engine and airframe.
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Permalink Reply by Scott Sutton on
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Just a guess here, but one of the diodes in the diode trio in the alternator could be the issue. After starting, the alternator works very hard to top the battery back off so any alternator issue will be amplified during this time. Also, if the battery is staring to have an issue with a cell it could cause noise in the electrical system. The battery acts very much like a smoothing capacitor. Since the alternators normal output is a clipped DC signal, the battery buffers and smooths out the signal and minimizes this effect.
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The battery is relatively new and seems healthy as far as voltage, etc. However, it is an EarthX lithium phosphate battery with an "onboard battery management system," so it is far more complex that a simple lead-acid battery. A trial battery-swap will be one of my "last resort" tests if I don't come up with the answer.
The Jabiru has a permanent magnet alternator and external voltage regulator. I know this alternator sends AC current to the external voltage regulator, so I "assume" the diodes are in this regulator and if turning the alternator off kills the noise, that would likely actually mean the voltage regulator is suspect?
John
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Well ... it just gets "curiouser and curiouser!" Took a test flight today - the rf noise kicked-in as usual after takeoff, but it seemed more muted and only lasted for 2-3 minutes - in other words, it's getting better! However, while it was still there, I did try cutting the alternator output off by pulling the breaker, but it made no difference.
I was discussing this with a car buddy of mine, and hit upon another possibility. The alternator is a permanent magnet alternator, so it does not need +12V excitation to function. There are two leads from the stator carrying AC current to the voltage regulator. These leads have a connection to extend them so they'll reach the regulator. Once previously, the female spade connectors had gotten hot and were replaced. Jab USA emphasizes to replace them with better-quality connectors, which I did. They also emphasize to thoroughly insulate them as they are prone to arcing to ground on the engine mount otherwise. Seems to me that since a permanent magnet alternator doesn't need +12v excitation, even if the output is "off" (at the voltage regulator), one could still get an arc to ground. So, I think the next thing will be to check those connectors! I may just solder them and heat-shrink sleeve them while I'm at it, since I really don't need removable connectors at that point, anyway.
However, since the problem seems to be less in duration and intensity with each flight - maybe it'll just go away and we'll never know! ;>)
John
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Permalink Reply by James R Depfer Sr. on
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John,
Don't you have a portable radio, or one you can borrow to see if you pickup the same noise ? Do you actually have to takeoff, or can you get the same thing during a run-up ?? Do you hear the same thing through the passenger headset ? I don't know what any of this would prove, but I'm just curious.
Jim
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Update:
I talked to Ben Krotje at the Jabiru USA display at AirVenture. He is really suspicious of the multi-pin connector on the voltage regulator - said that with time, slight corrosion can develop and cause arcing between the contacts with the resultant rf noise. He also agreed the AC output connectors could be suspect and recommended soldering them rather than using connectors. Annual is due next month, anyway, so I'm going to replace the connector with a terminal strip and lugs on the wires so they can be firmly secured and solder the connection between the alternator AC current output and the wire that goes to the voltage regulator. I'll post a follow-up once that's done.
John
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Permalink Reply by Wayne Tyson on
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Howdy, John. Anything new on this?
Wayne
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