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Pmags, audio noise?

jbDC9

Well Known Member
Has anyone had issues with P/Emags causing audio noise over the intercom?

Backstory; I've been running Pmags since new, 13+ years and 1650 hours... my panel was straight from the locomotive era with nothing fancy, just round gauges, a Garmin 496 and a KX125 navcom with audio fed straight to the intercom. Never had any noise. But, I upgraded things and some noise snuck in, boooo. Now I have dual G5s, a GNX 375, a GTR 200 com, an Aera 660 GPS, plus the old KX125. I didn't use a traditional audio panel and instead went with the AP-60 audio mixer which I quite like.

The noise is a low level snap-crackle-pop in the headsets; it initially would occur parked with the engine off, but after start would get worse. I tracked down some of the noise to the Aera 660 audio output; once I disconnected the 660 audio, all was quiet, yaaaay. Then I started the engine and the noise came back. Now it only occurs when the prop is turning; thinking it was a dying alternator, I replaced with a new one. No change. The noise is still present even with the alternator CB pulled. Next up was new NGK spark plugs. Noise still present. The spark plug leads are fairly recent, just three years old...

When I disconnect the AP-60 audio feed to the intercom, all goes quiet, so whatever the noise source is, it's affecting the audio mixer. So, now I'm stumped. I think it's the Pmags, but how do I isolate and fix it? Has anyone else run into mystery noise like this?

Thanks!
 
Is the frequency of the noise proportional to RPM? If no it seems unlikely to be ignition related. The fact that you had it engine off really suggests it?s not ignition related. As airplanes get older ground loop issues tend to emerge. Not fun to trouble shoot.
 
Is the frequency of the noise proportional to RPM? If no it seems unlikely to be ignition related. The fact that you had it engine off really suggests it?s not ignition related. As airplanes get older ground loop issues tend to emerge. Not fun to trouble shoot.

Yep, the noise is RPM proportional. Initially the noise was with both engine on and off, but after removing the G660 audio line, now the noise is only when prop is turning. And no, not fun to troubleshoot as I don't know where to look next... wish I knew what new item introduced the noise, will be a pain to isolate and ops check each one... but, gotsta do it.
 
You probably have a loose ignition terminal at the coil or plug. If you pull a plug wire and see black/arcing evidence on the plug terminal, that's it.
 
You probably have a loose ignition terminal at the coil or plug. If you pull a plug wire and see black/arcing evidence on the plug terminal, that's it.

Sweeeeet. Cowlings are already off to do the SDS induction tube mod (cool upgrade BTW), will double check the leads.
 
Do you now have an aux audio jack in the panel? Is the jack isolated from the panel/airframe? Sounds like a grounding issue somewhere in the aux audio input circuit.
 
A-ha! I found the noise... it's the Pmags, but not the ignition itself or harnesses; it's the power.

My Pmag power is wired straight from the install manual; 18 ga wire (unshielded) from main bus to pullable 3 amp CBs, then to both Pmags. Today while doing a post oil change engine run, I had a flash of inspiration and pulled the CBs while the power was set in the "self generating" range. Lo and behold, all went quiet and the engine kept running...

So the noise has been there all along with no changes made to the wiring since I built it, but my steam locomotive panel never "heard" it. Today while running it, I turned off all items on the avionics bus one by one except for the AP-60 audio panel which has no off/on power; methinks the new install audio panel is picking up the noise.

Okay then, now that I've found it, how do I fix it? Run shielded power wires to the Pmags? Or to the audio panel? Both perhaps? What y'all think?
 
Audio ?noise?

Did you pull the mag cb with the engine off ? Does it make any difference when you pull the cb?
Where is the ground for the mag connected to ?
 
Did you pull the mag cb with the engine off ? Does it make any difference when you pull the cb?
Where is the ground for the mag connected to ?
With the engine shut down there's no noise, so I didn't pull the CBs for that, no need. Pmags are grounded on engine case.
 
With the engine shut down there's no noise, so I didn't pull the CBs for that, no need. Pmags are grounded on engine case.
All ignitions make noise, as do strobes and LED nav lights. Having any of that noise get into your audio is dependent on installation technique.

Carl
 
pmag audio noise

I have this as well from time to time. I've isolated it to one pmag, not the other. I didn't think to see what happens while using only the internal power - will try that on my next flight. My noise is slightly different - random pops/crackle sounds.
 
You might check how the shields are tied to ground for the audio signals. For some installations those shields are only supposed to be tied at one end to ground, normally at the audio panel. That is the way my audio panel is wired. Did you ground both ends?
 
audio grounds - OK

You might check how the shields are tied to ground for the audio signals. For some installations those shields are only supposed to be tied at one end to ground, normally at the audio panel. That is the way my audio panel is wired. Did you ground both ends?
I did take care to ensure that the are only grounded on one end. The audio is perfect when the right PMAG is off. The clicks come back when I turn it back on.
 
Check your ignition wires

I had a similar issue. It was intermittent on Comm1 only. One of the ignition wires on the left PMag coil was loose and it was arcing. Re-secured wire boot and problem went away. From the PMag troubleshooting document:

If a connection is not secure, the cable may not be properly restrained and can result in 1) arching, 2) radio noise, 3) sooting or deposit accumulation on the terminal, 4) plug wire detaching altogether.
 
I had a similar issue. It was intermittent on Comm1 only. One of the ignition wires on the left PMag coil was loose and it was arcing. Re-secured wire boot and problem went away. From the PMag troubleshooting document:

If a connection is not secure, the cable may not be properly restrained and can result in 1) arching, 2) radio noise, 3) sooting or deposit accumulation on the terminal, 4) plug wire detaching altogether.
Thanks for the tip - will check those. Thankfully the right pmag is easy to reach via the oil door.
 
audio noise fixed - bad plug wire

Just to close the loop on my radio noise problem - I found that the problem was caused by a plug wire that seemed to have an "intermittent" short. Swapped with a newly made wire and the noise is gone, and engine is just a bit smoother.
 
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