Noisy transmit on both coms

Coinneach

Coinneach

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Coinneach
1973 Cherokee 180, KMA-20, dual KX-170Bs. We can receive fine on both coms, but xmit on both is extremely weak and white-noisy. Symptoms started last October; we only became aware when tower complained. Since then we've monitored our flights on LiveATC and confirmed. We've also comfirmed with handheld checks on 123.45. Noise is not audible on sidetone.

The noise is only present when the engine is running, on either or both mags, with alternator on or off. We have pulled and benched both radios and the audio panel. Radio check from inside the hangar to DVT ground with the engine off is 5x5. The alternator has been rebuilt and filter replaced. We noticed two badly chafed ignition leads and had them repaired, to no effect. No combination of lights makes a difference, nor does the intercom/squelch.

I want to replace all eight plugs (the last time they were replaced was at the 2020 annual) and the entire ignition harness. My partners and our A&P disagree. Engine is 1800 SMOH.

Am I missing something obvious in our troubleshooting?
 
Have you removed the ELT from your airplane? Strange as it sounds, an ELT, even OFF, can do what you are describing.

Take it completely away............can't hurt!
 
Makes as much sense as anything else. I'll give it a try in the morning. We did replace the batteries at annual last August and we didn't fly again until October, so the timing is there.
 
Are the antennas secure and their airframe grounds clean?

Does the noise disappear if you run the engine to 1500 or so and then shut both mags off and let it die? Transmit while it's spooling down.

Does it have electric oil pressure, temp or CHT? A loose wire at a sensor, or a loose crimp in a terminal at the sensor, will cause noise due to vibration. It arcs a bit.

The transmit weakness pushes me toward the antenna problem. The slipstream will disturb it. But on both antennae? Unlikely.
 
Dan Thomas said:
Are the antennas secure and their airframe grounds clean?
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Antenna grounds were my first thought but isn't there separate antennas for each com? Even if so I guess if one is dirty and poorly grounded then two can be dirty and poorly
grounded ...
 
Not an airplane avionics guy. Have been a bit of a radio guy.
It could be the antennas. My thought, though, that in the general sense it's either conductive or radiated interference. So to troubleshoot, start isolating things. If possible, I'd run the radio and any associated audio panels from a battery. Then if you still have interference, turn off the alternator. Then, I'd disconnect the mic input and run that direct. Then try a separate antenna.

My bet is that it's conducted noise, maybe from the ground, on the audio in to the radios from the mic jacks...but that's just a guess based on symptoms, and an observation that grounds around the panel in older airplanes can be kinda crappy from corrosion over the years. The mic signals are low level, and believe they are shared between radios so they both get mic audio.
 
Coinneach said:
We did replace the batteries at annual last August and we didn't fly again until October, so the timing is there.
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No radio issues before Aug annual?
Any work performed on electrical system during annual up to Oct?

With noise on 2 separate radios, requires engine running, and only heard on xmit, points more to a system/aircraft ground issue. Perhaps review those items in depth before spend more money.
 
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Each com has its own antenna. Avionics guy says they're good and I see nothing obvious from inside the tail.

No radio issues prior to 2022 annual, nor after the 2021 (other than being ancient).
 
Albany Tom said:
Then if you still have interference, turn off the alternator.
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If interference goes away, that maybe a clue about the alternator.
It probably is unlikely since you have had problems since Oct and alternator would probably be dead, but make sure the alternator is not dying, as this would affect the quality of radio communications.
 
MajorTurbulence said:
It probably is unlikely since you have had problems since Oct and alternator would probably be dead, but make sure the alternator is not dying, as this would affect the quality of radio communications.
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You're probably right. My thought was that it's possible for an alternator to be functional at delivering power, but still noisy, and that if there's something else going on in the aircraft that's making electrical noise, that powering down all the electrical stuff except the radio will likely eliminate that.
 
Did the alternator get rebuilt before or after this started??
 
Read the OP. The very first post. He said this:

Coinneach said:
The noise is only present when the engine is running, on either or both mags, with alternator on or off. We have pulled and benched both radios and the audio panel. Radio check from inside the hangar to DVT ground with the engine off is 5x5. The alternator has been rebuilt and filter replaced.
Click to expand...
 
Many times dirty plugs and jacks create problems. Wipe the plugs with a cloth dampened with contact cleaner..then, use a Q tip and gently wipe inside the mic and headphone jacks also dampened with cleaner..rotate the Q tip slightly and be gentle not to lose the cotton swab . You might see the Q tip turn dark with dirt.
 
Doug Reid said:
Many times dirty plugs and jacks create problems. Wipe the plugs with a cloth dampened with contact cleaner..then, use a Q tip and gently wipe inside the mic and headphone jacks also dampened with cleaner..rotate the Q tip slightly and be gentle not to lose the cotton swab . You might see the Q tip turn dark with dirt.
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This may help as the plug corrosion has caused various audio problems when using headsets in my plane, and would affect both radios. Obviously, change to different headsets, change the headset plug in location, and use the hand held mike in the plane to see if anything makes a difference.
 
Does your Nav/Comm stack (trays) use a common ground?
 
I always start at the antenna. As in remove from aircraft and remount. Work your way towards the radios, then back out to the headphones/mics.
 
I had this problem once. The static was regular and varied with engine speed. After many false starts, we figured out that it was a marginal connection between the spark plug wire and one of the plugs. The mechanic re-did the connection and the static was gone.
 
Hmmm

Customer had transmit issues…brought it in….audio panel bad. Replaced and problems went away.

Customer had transmit issues…last IA removed antenna connection to remove the belly pan, didnt reattach.

Customer has transmit issues …coax cable has become chafed and the core was compromised by a pulley.

None of these things had to do with an alternator, spark plugs or twilight zone inflight gremlins scaring William Shatner


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Thanks for the insights, everyone. Seems to be a moot point since I can't even get anyone to look at the airplane. /thread
 
I had a receive problem on my COM. Mostly worked, sometimes got static, other times no indication I was missing calls. Avionics shop tracked it down to the hole for the COM jack was a bit too big and the nylon washer wasn't always able to keep the jack from touching the metal panel.
 
Or a new led light bulb installed somewhere? Happened to me, new led bulbs making all kinds of static on both of my radios.
 
Hiperbiper said:
You don't by chance have a USB power converter do you?
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We do. Removed, made no difference.

Gary Ward said:
Or a new led light bulb installed somewhere? Happened to me, new led bulbs making all kinds of static on both of my radios.
Click to expand...

No new lights, LED or otherwise.

Avionics guys say it's the engine. A&P guys say it's the radios. My previous aircraft maintenance experience was on F-16s, so not relevant and I'm not qualified to gainsay anyone.
 
Coinneach said:
Avionics guys say it's the engine. A&P guys say it's the radios. My previous aircraft maintenance experience was on F-16s, so not relevant and I'm not qualified to gainsay anyone.
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Well it all starts with basic troubleshooting vs changing parts. Maybe find someone who wants to troubleshoot instead of point fingers?
 
Hokay, so. Two days ago the techs removed both 170s and the audio panel, applied contact cleaner to both the radios and tray connectors, reinstalled, ground check OK. This morning I got 5x5 ground and air checks, radios good from DVT to PAN. Why the noise only appeared with the engine running, who can say.

Did have a bit of a weird issue on the way back. Pulled PAN AWOS on COM1, no trouble. Left COM2 on CTAF. 20 miles from DVT, tried to pull ATIS on COM1. No joy until I switched it to TEST. After landing, COM1 received normally again. Guessing there's still something loose in the stack, but for now, I'm happier.

*sigh* Airplanes amirite
 
Coinneach said:
Why the noise only appeared with the engine running, who can say.
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Vibration. The connectors in the trays might be more than dirty. They might be loose and worn out.
 
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