Radio comm noise after annual

[BRBENNETT]

Hello, I am hoping to collect some suggestions on how to resolve an issue that just popped up.  I am getting "noise" in the form of alternator whine and others.

The only electrical items touched in the annual were removing the battery and a new set of spark plugs.  Though I know the plugs are really not part of the electrical system, they are grounded to the electrical system.

Here are a description of symptoms:

Whine / noise only occurs when the alternator is online, meaning, the main breaker for the alternator is on AND the enable breaker is on.  When either one of these is pulled, the noise goes away.

I can also hear other noise when switching on/off an item like a landing light or strobes.

The noise remains when switching mags, IE It is present in BOTH, LEFT, or RIGHT.

The noise is barely noticeable when just listening.  it is horrid when I press the push to talk button and of course this noise is transmitted to ATC.

Alternator works perfectly. 

 

Items I have reviewed / inspected:

Inspected all connections to mags, alternator, regulator (even swapped this out with a new spare I had), etc.

Disconnected, inspected, retorqed all battery connections.

Removed, inspected, reconnected all spark plug wires. 

Checked all antenna connections on back of radios.

Alternator -- put RMS digital meter on system, the only AC passing was around 200mVAC.  

 

I am leaning toward the spark plugs.  After I reconnected the spark plug wires and started up the comms were clear.  I immediately shut down thinking I beat the problem.  I started up a few days later only to find it again.  Has anyone experienced this and have any suggestions?  I know shielding / grounding is critical on ignition wires and I have not thoroughly inspected that aspect of the harness.  Could the ignition system be radiating noise to the alternator?  As I mentioned, everything worked perfectly up to annual.

Thanks to everyone in advance!  Feel free to ping me with any questions if you think you can help.

 

[bradp]

This is not necessarily owner PM but you can measure resistance across your ignition leads by removing the distributor cap.  Inspect the shield braid to ensure continuity along the length of the lead.  Ensure that the seal for the leads is intact and you don’t have oil working up the braid.  

Test resistance of the plugs if you have massive.  I’ve had radio noise from a bad plug but this usually sounds like machine gun fire rather than alternator whine.  

[jaylw314]

If it's the alternator, the whine should change pitch with changing RPM's.  IIRC, the usual culprit is bad grounding

Other people have described checking grounding, they're are a lot of places to check.  Other reasonable things to try would be:

• Reseating the radios.  They might be loose in the tray.
• Turning your headset volume down and the radio volumes all the way up.  This keeps you from hearing the whine (although doesn't help with transmitting the whine)
• Having your A&P install an alternator noise filter (a couple hundred $)

If you have the dirty master switch contacts problem, you may hear a warbling or beating of the alternator whine.

AFAIK, if the mags were involved, they would produce noise that sounds like static, not a whine.

[neilpilot]

While this isn't likely your problem, we noticed noise in our C a few years ago that somewhat matched your description.  We replaced the noise reduction capacitor on the firewall and the noise went away. 

[Bob Weber]

Check and rebond all the engine to airframe grounds. You are describing a whine, not a tick, leave the ignition leads and plugs alone. I have a real good story about a Baron project, I built a radio package in it while the engine shop overhauled both engines. Lots of whine when we flew it. Mechanics were convinced it had to be my work because that's where we were hearing the problem. It turned out to be a finger tight bolt on one of the ground straps of the left engine, and I found it!

Noise can weaken the strongest of knees, let me know if I can help you.

WebairConsulting.com 616 822 1999

[carusoam]

Another source of alternator whine... it comes with bad diodes. find the bridge, and visually inspect... diodes often fail leaving ugly marks of heat related damage...

The alternator will appear to work properly, but it is missing some current that is not getting out... if it is a diode...

PP thoughts only,

-a-

[Baker Avionics]

Ok, this is a very common problem and have fixed a lot of these types of issues.  Remove the cowls, look for ANY old wiring that is in the engine compartment that runs through the firewall into the cabin.  That will act as an antenna and will bring all of that noise inside.  Further, verify that the "P-Leads" have their shields grounded.  I believe you find your issue once you fix what you find by doing this.  Good luck and hope to hear from you with good news soon. 

[Yetti]

bad ground.   PTT works by completing the ground.   Might have a big noise reduction thingy on the firewall on the copilot side.   Might have lost a cap in it.

[BRBENNETT]

Mystery still unsolved.  went thru my entire panel looking for loose / broken wires & found nothing.  I decided to take some traces of the DC with an Oscope.  According to everything I have read, this should be a straight line, but, I am seeing 1.92v peak to peak, 387hz.  I have attached pictures of the traces.  Let me know what you guys think.

[ArtVandelay]

Mystery still unsolved.  went thru my entire panel looking for loose / broken wires & found nothing.  I decided to take some traces of the DC with an Oscope.  According to everything I have read, this should be a straight line, but, I am seeing 1.92v peak to peak, 387hz.  I have attached pictures of the traces.  Let me know what you guys think.

I can’t help you, but I’m jealous you have a oscilloscope in your hangar. Makes me wonder what other cool tools you have.

 

Edit: did you try pulling the alternator CB?

 

Tom

[EricJ]

9 hours ago, BRBENNETT said:

Mystery still unsolved.  went thru my entire panel looking for loose / broken wires & found nothing.  I decided to take some traces of the DC with an Oscope.  According to everything I have read, this should be a straight line, but, I am seeing 1.92v peak to peak, 387hz.  I have attached pictures of the traces.  Let me know what you guys think.

What were the conditions for this test?   Was the engine running?  If so, at what rpm?  

[carusoam]

Alternators have a bridge that can be noisy when a component fails... See if turning off the alternator helps...

What else can cause a cyclic noise at 400 or so Hz?

Best regards,

-a-

[BRBENNETT]

Ran AC traces at 1500 RPM and fully loaded electrical system.  1.96 VAC PTP.  At lower load, 1.6VAC.  Plane Power says 1 VAC is max allowed and tells me diode failure.   Trace attached.   

[EricJ]

I was gonna say a bad diode in the alternator might be the culprit and was going to work out the pole frequencies from the rpm, but if you're already headed that way I'll not bother.   Check your alternator, in some it is not  difficult to change a diode.   I don't know whether that's true for typical Mooney alternators or not.

 

[carusoam]

Typical Mooney alternators have a diode bridge that turns the AC into DC... rock on... 

Losing one diode, gets some noise... and can lessen the output voltage... but not really noticeable...

Find the bridge... a visual inspection will probably reveal a burned electrical device... looks sort of like a resistor from the outside... and a Smokey looking stain around it...

PP thoughts only, from experience I had messing up my tug’s motor... had to buy it a new diode...

Best regards,

-a-

[Baker Avionics]

On 7/16/2019 at 3:47 AM, BRBENNETT said:

Hello, I am hoping to collect some suggestions on how to resolve an issue that just popped up.  I am getting "noise" in the form of alternator whine and others.

The only electrical items touched in the annual were removing the battery and a new set of spark plugs.  Though I know the plugs are really not part of the electrical system, they are grounded to the electrical system.

Here are a description of symptoms:

Whine / noise only occurs when the alternator is online, meaning, the main breaker for the alternator is on AND the enable breaker is on.  When either one of these is pulled, the noise goes away.

I can also hear other noise when switching on/off an item like a landing light or strobes.

The noise remains when switching mags, IE It is present in BOTH, LEFT, or RIGHT.

The noise is barely noticeable when just listening.  it is horrid when I press the push to talk button and of course this noise is transmitted to ATC.

Alternator works perfectly. 

 

Items I have reviewed / inspected:

Inspected all connections to mags, alternator, regulator (even swapped this out with a new spare I had), etc.

Disconnected, inspected, retorqed all battery connections.

Removed, inspected, reconnected all spark plug wires. 

Checked all antenna connections on back of radios.

Alternator -- put RMS digital meter on system, the only AC passing was around 200mVAC.  

 

I am leaning toward the spark plugs.  After I reconnected the spark plug wires and started up the comms were clear.  I immediately shut down thinking I beat the problem.  I started up a few days later only to find it again.  Has anyone experienced this and have any suggestions?  I know shielding / grounding is critical on ignition wires and I have not thoroughly inspected that aspect of the harness.  Could the ignition system be radiating noise to the alternator?  As I mentioned, everything worked perfectly up to annual.

Thanks to everyone in advance!  Feel free to ping me with any questions if you think you can help.

 

Check P-Lead shielding

[Browncbr1]

On 8/15/2019 at 11:38 PM, carusoam said:

Typical Mooney alternators have a diode bridge that turns the AC into DC... rock on... 

Losing one diode, gets some noise... and can lessen the output voltage... but not really noticeable...

Find the bridge... a visual inspection will probably reveal a burned electrical device... looks sort of like a resistor from the outside... and a Smokey looking stain around it...

PP thoughts only, from experience I had messing up my tug’s motor... had to buy it a new diode...

Best regards,

-a-

What does the bridge look like and is it in cabin side or engine side?   And where is a diode?

I’ve had an alternator whine for a while and very slight voltage instability and slight drop from previous before the noise.  

Edited August 23, 2019 by Browncbr1

[carusoam]

Mr. Brown,

The alternator typically has some electronic circuits on board, at the back of the alternator...

A search online with your alternator model number will probably reveal the details you are looking for.

What alternator do you have?

The diodes are a typical failure point. The alternators typically are designed to swap the parts out...

PP thoughts only, haven’t done this work on an airplane before... 

Best regards,

-a-

[Browncbr1]

5 hours ago, carusoam said:

Mr. Brown,

The alternator typically has some electronic circuits on board, at the back of the alternator...

A search online with your alternator model number will probably reveal the details you are looking for.

What alternator do you have?

The diodes are a typical failure point. The alternators typically are designed to swap the parts out...

PP thoughts only, haven’t done this work on an airplane before... 

Best regards,

-a-

Thanks, I have the plane power alternator.   I don’t know what a bridge is or looks like.   I just know my VR is on my firewall and all connections are good. 

[jetdriven]

On 7/16/2016 at 10:14 PM, MooneyBob said:

So what needs to be done to fix it ?

Thanks 

Replace the rectifier.  The Plane Power one is The inr-720p.  From here http://www.aspwholesale.com/rectifier-nd-p1359.html

 

if you have the prestolite it's going to be a rectifier from like a 1972 ford f-100.  

Edited July 16, 2016 by jetdriven

[jetdriven]

Your A&P splits the alternator case, inspects the brushes, and replace the rectifier.  or have an alternator shop do it, and your A&P inspects it before reinstal.

[carusoam]

26 minutes ago, Browncbr1 said:

Thanks, I have the plane power alternator.   I don’t know what a bridge is or looks like.   I just know my VR is on my firewall and all connections are good. 

See the great details from Byron, above...  

-a-

[Browncbr1]

35 minutes ago, jetdriven said:

Replace the rectifier.  The Plane Power one is The inr-720p.  From here http://www.aspwholesale.com/rectifier-nd-p1359.html

 

if you have the prestolite it's going to be a rectifier from like a 1972 ford f-100.  

Edited July 16, 2016 by jetdriven

Thanks Byron, 

i just ordered it and looking forward to solving the issue.  

Can this be pulled out the back of the case without removing from the plane?  Hoping to avoid removing lower cowl.  

Craig

[jetdriven]

Nope. Sorry. Alternator has to come off. 

[BRBENNETT]

yep, had to go thru the pain of removing the lower cowling.  Replaced the alternator and life is good!!