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Troubleshooting radio whine/noise on C essna 182P

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millner(at)me.com
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Posted: Wed Dec 02, 2015 2:40 pm Post subject: Troubleshooting radio whine/noise on C essna 182P

There's a company in Texas that makes an aftermarket part... Aircraft 

Spruce used to carry them.



Paul



On 12/2/2015 11:06 AM, sacha wrote:

 	  Quote: 	
		   



 I took some measurements of bus voltage using a Bluetooth oscilloscope called a Mooshimeter. Here are 3 graphs. I'm also including a fourth one which is picture of the voltage in my car cigar lighter for comparison.



 Two things are noticeable:

 1. The amplitude of the ripple in the aircraft is smaller by a factor of 3 or 4.

 2. There seem to be higher frequency components in the aircraft than in the car.



 Does anyone have any thoughts about this?

 I'm thinking that maybe the noise suppression capacitor is fried and I'd like to test this hypothesis without shelling out $125 for the Cessna part. Can anyone suggest a sensible capacitor I could try out?





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ceengland7(at)gmail.com
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Posted: Wed Dec 02, 2015 5:22 pm Post subject: Troubleshooting radio whine/noise on C essna 182P

On 12/2/2015 1:06 PM, sacha wrote:

 	  Quote: 	
		   



 I took some measurements of bus voltage using a Bluetooth oscilloscope called a Mooshimeter. Here are 3 graphs. I'm also including a fourth one which is picture of the voltage in my car cigar lighter for comparison.



 Two things are noticeable:

 1. The amplitude of the ripple in the aircraft is smaller by a factor of 3 or 4.

 2. There seem to be higher frequency components in the aircraft than in the car.



 Does anyone have any thoughts about this?

 I'm thinking that maybe the noise suppression capacitor is fried and I'd like to test this hypothesis without shelling out $125 for the Cessna part. Can anyone suggest a sensible capacitor I could try out?

Is the Cessna device a simple capacitor? (If so, it'd have one lead 
	


connected to the B lead and the other lead connected to ground.)  A 

simple capacitor could be purchased at any well stocked electronic 

supply; just ask for, for instance, a 2000 mfd 50 volt electrolytic 

capacitor.



If it's an inductor, it will have 2 leads and be *in series* with the B 

lead (one terminal toward the B terminal of the alt; the other terminal 

toward the master contactor/buss). You could check ebay, Amazon, or your 

local parts store for 'alternator noise filter' & find one that's 

intended for hookup like I described.



A third type actually includes both an inductor and a capacitor. It 

would have 3 leads: 1 to the alt B terminal, 1 toward the master 

contactor/buss, and the last one to ground. Again, same search & pick 

the device  you need.



Note that most of the combination inductor/capacitor filters intended 

for automotive use will have limited current carrying capacity because 

they're intended for insertion in line with a single load; not in the B 

lead, where they must carry the entire load of the generating system. 

For testing purposes, just leave all the really big electrical loads off 

(landing lights, etc) and make sure the battery is charged up prior to 

the test. The light-duty stuff I saw was in the $5-$25 range.



You can see images & diagrams by googling 'alternator noise filter' & 

picking google's images display.



BTW, a 3 terminal whole-a/c filter for $125 might not be that bad if 

that's what is broken.

http://www.amazon.com/NewMar-150--150-Noise-Filter/dp/B008P0UPD4/ref=sr_1_2?s=car&ie=UTF8&qid=1449104711&sr=1-2&keywords=alternator+noise+filter



Charlie

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ceengland7(at)gmail.com
Guest

Posted: Sun Dec 06, 2015 1:50 pm Post subject: Troubleshooting radio whine/noise on C essna 182P

On Sun, Dec 6, 2015 at 12:17 PM, sacha <uuccio(at)gmail.com (uuccio(at)gmail.com)> wrote:

 	  Quote: 	
		  --> AeroElectric-List message posted by: "sacha" <uuccio(at)gmail.com (uuccio(at)gmail.com)>

 

 Thanks for your explanation about the radio grounding. It's obvious when one thinks about it.

 

 I'm still struggling with this. I rechecked the capacitors and they measure 5.7 uF for the S1915 which is correct and 3.54 uF for the RFI filter (Cessna part 0770038-2) which seems like a plausible value.

 

 So the only remaining possibility seems to be some bad connection somewhere, most likely on the ground side of things.  But the puzzle remains as to why an alternator that fails can suddenly cause a grounding problem.

 

	




Perhaps it wasn't the failure, but the repair/replacement process.

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sacha

Joined: 04 Jan 2015
Posts: 15

Posted: Sun Dec 06, 2015 2:33 pm Post subject: Re: Troubleshooting radio whine/noise on Cessna 182P

(Thanks to Neal/Bob/Paul/Charles/Joe/Jan and everyone else for patiently continuing to help me troubleshoot this issue).



Charles>Perhaps it wasn't the failure, but the repair/replacement process.  

It couldn't have been the repair process because as soon as I landed I pushed the ALT CB back in and the whine appeared.  I naturally assumed it was caused by a broken alternator but swapping it out didn't make the whine disappear.  (On Friday I took some measurements on the old alternator and confirmed that it does have damage to it's stator coil - this was kindly confirmed to me by my local NAPA store***).



Anyway, today I had the following thought process which makes me think that maybe the audio panel got damaged in some way:



The alternator noise is audible even with the radios off.  Therefore the noise must be entering one of the two wires to the headphone, either the positive or the ground.  



Thinking about the audio ground, I recall that the black wires are all locally grounded at a post below the instrument panel.  So the test I did yesterday (unscrew the jacks from the locally grounded aluminium brackets and see if the noise disappears) was actually useless as a test.  



What needs to happen (as Joe and Jan have pointed out) is that the ground lead for each headphone needs to be run all the way back to the audio panel instead of being attached locally to the airframe.  



But I still can't see how loosing an Alternator could magically bring about this condition since nothing changed with the audio jacks.  Unless of course the audio panel was damaged by this event.  Fixing the audio grounds is cheap, so I guess I'm going to try that first, but I have a suspicion that the audio panel might be damaged.  Do audio panels have failure modes in which they "transmit" alternator whine?



Sacha

*** I got lucky - the NAPA store did this for free.  There was a guy there that managed to look up the equivalent automotive model... it took him a while since what was marked on the alternator (D0FF10300J) did not yield any immediate results.  However he was persistent and eventually he found out that a 65A Alternator for a 1969 Ford pickup truck was practically identical.  Once he had this information, he put the alternator into an automated bench tester which spit out the "stator failed" information a minute later.  For those of you who are interested, I'm attaching the a nice PDF which documents the troubleshooting and overhaul procedure for Ford (now Motorcraft) alternators which I got here: http://vintage.mitchell1.com/PClubData/chassis/elt82/V2D824014.pdf

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Motorcraft Alternators Troubleshooting and Overhaul - V2D824014.pdf

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