Alternator Noise On The Radio

Geico266

Geico266

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Geico
I have 2 buddies that have a noise on their radios when they transmit. It goes away if they shut down the alternator in flight. We have tried hooking up a "noise filter" to the radio with zero improvement. The alternator is charging fine, and tests fine.

Both are home builts, one is a RV-4 and one is a Pitts.

Any thoughts from the avionics gurus?
 
Always look for poor grounds first. Alternator case to engine. Engine to airframe. Radio rack to airframe.
 
JHW said:
Always look for poor grounds first. Alternator case to engine. Engine to airframe. Radio rack to airframe.
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Thanks Jeff, will do. We have checked the radio ground, but not the others you mentioned.

It had been suggested the alternator does not have a "noise filter", or the regulator could be bad.

Thoughts?
 
Possibly a failed (open) diode. It would still put out power but not the rated amount and the voltage would have a much bigger ripple (noise) riding on it.
 
RV10flyer said:
This may help...

http://www.avweb.com/news/maint/182896-1.html?redirected=1
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Graueradler said:
Possibly a failed (open) diode. It would still put out power but not the rated amount and the voltage would have a much bigger ripple (noise) riding on it.
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Thanks for the info!

I had bfast with both plane owners today. We are going to get after the cause as soon as it warms up. Gonna be very cold here the next several days.

We are going to switch out the alternator in the RV-4 and see if it goes away. That would tell us if it is the alternator or a grounding issue.

Reading that article it sure sounds like a bad diode. I will let you all know what works.
 
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Geico266 said:
I have 2 buddies that have a noise on their radios when they transmit. It goes away if they shut down the alternator in flight. We have tried hooking up a "noise filter" to the radio with zero improvement. The alternator is charging fine, and tests fine.

Both are home builts, one is a RV-4 and one is a Pitts.

Any thoughts from the avionics gurus?
Click to expand...

"Noise" covers a multitude of sins. Describe "noise" a little more. A whine? More like random static? Does it go away when you pull the power to idle? Defining a problem well is 50% of the solution.

Thanks,

Jim
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Isolate the ground on the mic jack from the metal of the airframe. The mic audio ground should be at the transceiver, and not also have alternator current mixed in.

Somebody used the aircraft frame to carry ground current. Only wires should carry current or or voltages. You have to isolate them.
 
weirdjim said:
"Noise" covers a multitude of sins. Describe "noise" a little more. A whine? More like random static? Does it go away when you pull the power to idle? Defining a problem well is 50% of the solution.

Thanks,

Jim
.
.
Click to expand...

It is a noise that only happens when the alternator is on line. Pull the circuit breaker and the noise stops. The noise itself is a constant "whine" when at idle, and graduates to a fast high & low pitch back ground noise when at high RPM. The noise is only audiable when the radio is transmitting.
 
nrpetersen said:
Isolate the ground on the mic jack from the metal of the airframe. The mic audio ground should be at the transceiver, and not also have alternator current mixed in.

Somebody used the aircraft frame to carry ground current. Only wires should carry current or or voltages. You have to isolate them.
Click to expand...

Interesting. I will check this out also.
 
Geico266 said:
The noise itself is a constant "whine" when at idle, and graduates to a fast high & low pitch back ground noise when at high RPM. The noise is only audiable when the radio is transmitting.
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Well, there is a constant whine here when I go into idle, but that is another relationship issue :goofy:

So now we know it is alternator whine. You say it is only "audible" when the radio is transmitting. Audible to who? To the pilot as sidetone or to the other radio (s)he is transmitting to?

The isolation of the mic jack is a well known issue. To test it you only need to drop the mic jack down off of the instrument panel (leave it dangling) and then repeat your test. If it does need insulating washers, it will become immediately apparent that you have found the problem.
 
If you have a friend with an oscilliscope have hime look at the output of the alternator and if there is a bad diode , which I am pretty sure you will find , he should see the noise on the output trace. Simpl easy and you do not have to take anything apart. The local alternator repair shop almost certainly would miss a bad diode. I doubt they scope the ouput .
 
weirdjim said:
Well, there is a constant whine here when I go into idle, but that is another relationship issue :goofy:

So now we know it is alternator whine. You say it is only "audible" when the radio is transmitting. Audible to who? To the pilot as sidetone or to the other radio (s)he is transmitting to?

The isolation of the mic jack is a well known issue. To test it you only need to drop the mic jack down off of the instrument panel (leave it dangling) and then repeat your test. If it does need insulating washers, it will become immediately apparent that you have found the problem.
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Good question! Audiable to people listening, hearing the transmission.

Too cold to mess with it now.
 
Alternator whine is nearly always the product of the rectifier (faulty diode) not clipping the entire AC wave to convert it into DC. An AC wave impressed in a DC system can destroy your avionics and should not be taken lightly. Electronics have ZERO protection from AC ripple.

Using a good quality digital multimeter on the bus to detect AC is highly suggested. You don't need an oscilloscope although they do work better than a meter.
 
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philiplane said:
Alternator whine is nearly always the product of the rectifier (faulty diode) not clipping the entire AC wave to convert it into DC. An AC wave impressed in a DC system can destroy your avionics and should not be taken lightly. Electronics have ZERO protection from AC ripple.

Using a good quality digital multimeter on the bus to detect AC is highly suggested. You don't need an oscilloscope although they do work better than a meter.
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Since the overwhelming failure mode of silicon diodes is to fail in the open mode (99+%) would you mind explaining how a diode "clips the entire AC wave"?

Since you must have a strong background in avionics design, would you mind also telling us how you came to the conclusion that electronics have ZERO protection from AC ripple and what that AC ripple might do to a typical piece of equipment?

Jim

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Follow up!!!!!


The alternator had a bad diode.


Thanks to all for suggestions! Thanks to those who nailed the problem too!
 
Vaflier said:
Ok. Now tell us the rest !!!. How exactly did you find it ??.
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Took the alternator into a local auto electric shop. I assume he put it on a scope. You can test it yourself, but it requires desoldering the diodes and testing them with an ohm meter. That doesn't sound like fun to me. :no:
 
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Geico266 said:
Took the alternator into a local auto electric shop. I assume he put it on a scope. You can test it yourself, but it requires desoldering the diodes and testing them with an ohm meter. That doesn't sound like fun to me. :no:
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You can probably diagnose the diodes simply by measuring the AC voltage on the bus with a digital voltmeter but you have to take the battery out of the circuit to do so. In that case you should get 2-3 VAC if the diodes are all good, significantly more than that if not.
 
gismo said:
You can probably diagnose the diodes simply by measuring the AC voltage on the bus with a digital voltmeter but you have to take the battery out of the circuit to do so. In that case you should get 2-3 VAC if the diodes are all good, significantly more than that if not.
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Just for the fun of it - here is a plot of three phase full wave rectified voltage and the same thing with one half cycle missing (like you would get with an open diode). NOTE: this is just from adding the absolute values of sin waves with no attempt to reflect a lot of the physical realities of diode switching, batteries, etc., etc., etc. (Assumes the average DC voltage is 14.5 in each case).
 

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Capt. Geoffrey Thorpe said:
Just for the fun of it - here is a plot of three phase full wave rectified voltage and the same thing with one half cycle missing (like you would get with an open diode)
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And now what it look like with a regular old lead-acid battery across it with a few milliohms of internal resistance?

Jim
 
weirdjim said:
And now what it look like with a regular old lead-acid battery across it with a few milliohms of internal resistance?

Jim
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It would look like more math than I feel like doing just for the fun of it. ;)
Obviously, it would flatten out quite a bit.

But there is still enough ripple to make one of these work.
 
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Question for the electrical gurus - Can you hear the presence of a low frequency subharmonic on the audio if a diode is out?:confused:
 
nrpetersen said:
Question for the electrical gurus - Can you hear the presence of a low frequency subharmonic on the audio if a diode is out?:confused:
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One or more bad diodes will produce "alternator whine" on most airplanes with the pitch varying between a few hundred Hz and a little more than a KHz. You can also get alternator whine with a good alternator and an old battery with high internal resistance but the frequency will be noticeably higher. Better avionics will filter out most of this but even with the best audio panel it's likely you'll hear it if a diode is out.
 
gismo said:
One or more bad diodes will produce "alternator whine" on most airplanes with the pitch varying between a few hundred Hz and a little more than a KHz. You can also get alternator whine with a good alternator and an old battery with high internal resistance but the frequency will be noticeably higher. Better avionics will filter out most of this but even with the best audio panel it's likely you'll hear it if a diode is out.
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How does the whine frequency increase without changing the number of alternator poles? I would think though there would simultaneously be two separate frequency outputs (a whine and a subharmonic) that an ear could identify.
 
nrpetersen said:
How does the whine frequency increase without changing the number of alternator poles? I would think though there would simultaneously be two separate frequency outputs (a whine and a subharmonic) that an ear could identify.
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If you're asking why I gave different ranges for a good alternator vs one with bad diodes, the answer is that when you lose a diode you effectively lose a pole and the energy increases dramatically in a subharmonic of the three phase ripple (see G Thorpe's graphic). But if you're asking why there are ranges rather than single frequencies, that's due to variations in engine (and alternator) RPM.
 
This is an interesting thread but I feel like I only got 2/3 of the story! Was the shop able to replace the failed diode or did the OP's friend have to purchase a new alternator? Sorry if this is obvious to the non-electrically challenged. :)
 
Geico266 said:
I have 2 buddies that have a noise on their radios when they transmit. It goes away if they shut down the alternator in flight. We have tried hooking up a "noise filter" to the radio with zero improvement. The alternator is charging fine, and tests fine.

Both are home builts, one is a RV-4 and one is a Pitts.

Any thoughts from the avionics gurus?
Click to expand...

What kind of 'noise filter'? Typically a properly grounded 25m microfarad condenser will do the job.
 
Same old, same old.........Diodes going bad in the alternator. Take it out and to the automotive repair shop.
 
Geico266 said:
Follow up!!!!!


The alternator had a bad diode.


Thanks to all for suggestions! Thanks to those who nailed the problem too!
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Hello, my Cessna 150 just started having strong audio whine. It went right through my a&r headset. I suspect the alternator probably diode problem. My Cessna doesn’t have a circuit breaker that can be turned off. I will check output voltage dc and for ac for ripple. This is is good post. BTW Jim Weir is a great electronic engineer, years ago, I used one of his magazine articles to help design a remote mode C decoder and display. Thanks Jim !
 
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