Radio noise w/ small voltage fluctuations

[J0nathan225]

Alright MS'ers I just wanted to confirm my findings before dropping .5+ AMUs. Yesterday as I started my decent into KNEW I started to get a loud whine through my headset. I landed at KNEW and upon start up and departure the same noise was again present.

Other info:

Noise changed with RPM, but always present.

EDM900 showed volts fluctuating 13.3-13.9 (usually steady at 13.9-14.0 in flight).

 Noise stops when ALT breaker is pulled.

My thoughts: Alternator diode has failed, replace alternator before it totally fails to provide electrical power.

 

Thanks for any insight.

Jon.

[N201MKTurbo]

Your assessment is sound. 

[carusoam]

Assessment is sound... but....

What is wrong with finding out what diode failed, and swap it out?

In some cases, the whole bridge comes out, replacing all the diodes at once...

I only have done the same thing for a mag on my plane’s tug...

Fried diodes are relatively easy to detect... visually and resistance wise...

Or... swap out the whole alternator.  Keep in mind Diodes cost about a dollar.  
 

Make sure you know where the diodes are... in the event they are not onboard the alternator... that would be important...

PP thoughts only, not a mechanic...

Best regards,

-a-

[N201MKTurbo]

My experience is the diodes don't get "fried", the solder or crimp connections vibrate apart. I usually take the alternator apart and try to fix it. I usually can.

[jetdriven]

The plane power Nippondenso alternators Diodes  will go bad. But they are pressed together and are not replaceable. Diode pn is INR720p

[J0nathan225]

11 minutes ago, jetdriven said:

The plane power Nippondenso alternators Diodes  will go bad. But they are pressed together and are not replaceable. Diode pn is INR720p

It's not much cheaper turning the old one in VA buying new. I figure I'll get a new one and take the old one apart to fix or at least learn how it's put together. 

[jetdriven]

A new alternator is like 500$ while the diode Bridge  is 21$

[Vance Harral]

Anybody have pictures of the diode bridge?  We have a similar problem with alternator noise we're going to try to solve at our upcoming annual.  Baseline idea was to just buy a new alternator, but I'd be happy to replace/repair a capacitor and/or diode bridge instead, if it's practical.  But I can't locate any replaceable components on our alternator, at least not that I can see in-situ with a quick look.  I'm only generally aware that "old school" alternators have external electrical components that can be replaced.  I understand the principles, but have never actually worked on the guts of an alternator, or seen one worked on.  If you talk about this sort of thing in the context of any automobile built in the last 30+ years, you get roll-eyes and an explanation that alternators aren't built that way any more - all you can do is replace the whole unit.

[N201MKTurbo]

24 minutes ago, Vance Harral said:

Anybody have pictures of the diode bridge?  We have a similar problem with alternator noise we're going to try to solve at our upcoming annual.  Baseline idea was to just buy a new alternator, but I'd be happy to replace/repair a capacitor and/or diode bridge instead, if it's practical.  But I can't locate any replaceable components on our alternator, at least not that I can see in-situ with a quick look.  I'm only generally aware that "old school" alternators have external electrical components that can be replaced.  I understand the principles, but have never actually worked on the guts of an alternator, or seen one worked on.  If you talk about this sort of thing in the context of any automobile built in the last 30+ years, you get roll-eyes and an explanation that alternators aren't built that way any more - all you can do is replace the whole unit.

NAPA has most of the internal parts for most modern alternators.

I have the parts manual for the Prestolite alternators. I can post it next weekend. I’m in the boarding gate at SWA right now.

[Vance Harral]

13 minutes ago, N201MKTurbo said:

NAPA has most of the internal parts for most modern alternators.

I have the parts manual for the Prestolite alternators. I can post it next weekend. I’m in the boarding gate at SWA right now.

Meaning that replacing these parts requires disassembling the alternator case?  I understand it's certainly possible to do so, but it changes the nature of the repair with respect to paperwork.

[jetdriven]

The log entry will say something like “replaced INR720P rectifier in alternator” vs “replaced alternator”

what kind of alternator do you have?  A 1965 M20E could have an Interav (Motorola), a  Plane Power (Nippondenso) or something else. 

[Browncbr1]

 

@J0nathan225 Before you go buy diodes or rectifiers and starting the time consuming and expensive process to pull your lower cowl and alternator like I did, crawl under your panel and make sure the power cables (particularly from alternator) and completely tight on the bus bar.  Anything less than right will cause this.   While you’re in there check all grounds.  Do all this first.     Your symptoms mirror what I had exactly.  

Edited February 11, 2020 by Browncbr1

[Vance Harral]

4 hours ago, jetdriven said:

The log entry will say something like “replaced INR720P rectifier in alternator” vs “replaced alternator”

what kind of alternator do you have?  A 1965 M20E could have an Interav (Motorola), a  Plane Power (Nippondenso) or something else. 

Multiple people in the thread are interested, but in my case the aircraft is a 1976 M20F and our alternator is a Hartzell/Prestolite ALY-8520.

Regarding the paperwork, we enjoy a good relationship with our A&P, who frequently allows us to perform our own repairs under his supervision and sign them off.  I'm not sure that courtesy will apply to disassembling electrical equipment and working on the internal components, however.  While there is plenty of debate about what kind of work requires a Repair Station Certificate, the only opinion that matters in our particular case is our mechanic's.  If he is unwilling to sign off on letting us do the work ourselves, the cost and hassle of finding someone who will would almost certainly exceed the savings of replacing internal components vs. buying a whole new alternator.

[J0nathan225]

10 hours ago, Browncbr1 said:

 

@J0nathan225 Before you go buy diodes or rectifiers and starting the time consuming and expensive process to pull your lower cowl and alternator like I did, crawl under your panel and make sure the power cables (particularly from alternator) and completely tight on the bus bar.  Anything less than right will cause this.   While you’re in there check all grounds.  Do all this first.     Your symptoms mirror what I had exactly.  

I'll check my panel maybe my copilot kicked something, the noise happened all of a sudden during my descent though.

Also I have the plane power 10-1051 / SAL12-70 Alternator. 

[Browncbr1]

12 minutes ago, J0nathan225 said:

I'll check my panel maybe my copilot kicked something, the noise happened all of a sudden during my descent though.

Also I have the plane power 10-1051 / SAL12-70 Alternator. 

Yea, same here with all of a sudden.   Vibration frequency changed with I reduced rpm to let down and that’s when I noticed it first.  

I have the same alternator.  It takes about 5-6 hours to pull the lower cowl and alternator and replace rectifier and put all back together.   You don’t want to do that and not solve the issue.  Then, later crawl under the panel and turn a bus bar screw 1/4 turn to tighten.   You should check the connections at the VR and alternator sides also.  

[PhateX1337]

Just curious - did you ever find the issue here @J0nathan225?

I'm having very similar issues with a '77J. There is "whining" in the headset that's variable depending on RPMs. The voltage is bouncing from ~13.8 to 14.3v even in cruise, no power setting adjustments.

Not sure if these are also related, but the STEC-55x autopilot is on the fritz on bootup - either just doesn't start, or the AP disconnect horn blares on startup. Again, also not sure it's related, but the GDL90 appears to be dropping/causing the ADS-B to drop intermittently in the air as well.

My suspicion is the voltage fluctuations are causing these issues ... but I'm not sure if it's the alternator failing, diodes failing, master switch cleaning, bus bar issues, etc. I will check the bus bar & master switch from behind next time I'm out there.

Any other thoughts?

[J0nathan225]

Replaced the alternator, no issues now.  Replaced/upgraded starter while they were in there. @PhateX1337 another sign, for me, was it happened very suddenly. I was on final to KNEW no noise then WHAT IS THAT!?  It continued on subsequent start up changing with power/RPM, my thinking was something had to have failed since it was a sudden change. 

Edited May 6, 2020 by J0nathan225
more info

[PhateX1337]

Helpful context, thank you. It was there when I just bought the aircraft, but it's gone downhill quickly since then - will take a look at master/bus bar and if that doesn't solve it will call in the A&P. Helpful to know that fixed it for you, thanks!

[Ragsf15e]

2 hours ago, PhateX1337 said:

Just curious - did you ever find the issue here @J0nathan225?

I'm having very similar issues with a '77J. There is "whining" in the headset that's variable depending on RPMs. The voltage is bouncing from ~13.8 to 14.3v even in cruise, no power setting adjustments.

Not sure if these are also related, but the STEC-55x autopilot is on the fritz on bootup - either just doesn't start, or the AP disconnect horn blares on startup. Again, also not sure it's related, but the GDL90 appears to be dropping/causing the ADS-B to drop intermittently in the air as well.

My suspicion is the voltage fluctuations are causing these issues ... but I'm not sure if it's the alternator failing, diodes failing, master switch cleaning, bus bar issues, etc. I will check the bus bar & master switch from behind next time I'm out there.

Any other thoughts?

In addition to the above recommendations, check the security of your spark plug leads.  If loose or degrading, they will cause radio noise with rpm.  You can try a mag check to narrow down which side is causing it and to see if it goes away on one mag.

[carusoam]

Expect electrical noise related to rpm is commonly diodes in (or for) the alternator... there is a bridge related to the alternator that when one or more of the diodes goes on hiatus... it causes noise...

Find the bridge test the diodes.... replace the smoky one(s).

PP thoughts only, stuff I read about on MS...

Best regards,

-a-

[J0nathan225]

Revived. 
 

So I got the same noise again, all of a sudden when changing from2500 to 2400 rpm while at 9000.  I had already been at 9000 for 45mins cruising, so I pulled the Alt CB and it went away, tried to “reset” it a few times, but once the Alt CB was pushed in noise came back. I had the same voltage drop/fluctuations as before...

BUT good news, maybe?  

After a 20 minute shut down it didn’t return and Volts were back up to 14Vs. I shut down 3 additional times and flew another 3 hours through the day with it not returning. VERY odd! 

[carusoam]

Start becoming familiar with where the bridge of diodes is for your alternator...

Sounds like one of the diodes may be getting old....

The alternator can work with one missing... but the whine noises get louder as the voltage drops...  

PP thoughts only, not a mechanic...

Best regards,

-a-

[carusoam]

On 2/10/2020 at 10:41 AM, N201MKTurbo said:

My experience is the diodes don't get "fried", the solder or crimp connections vibrate apart. I usually take the alternator apart and try to fix it. I usually can.

Rich is getting to know my level 2 mechanical skills on a scale of 1 - 10...

Fortunately, the diode I fried was on my tug... it’s new battery was physically different than the old battery... the posts were reversed +/-...  somehow normal for some lawn tractors....

In the case of reverse wiring a diode on a lawn tractor.... complete smoke show... fried, gone... dead...   

Best regards,

-a-

[J0nathan225]

Sadly the alternator is ~14 months old.  Here's a question: is a low amount of use or pull an issue?  on average I'm only pulling 11-18 Amps, does that cause more wear or heat build up?  Might be giving plane power a call, though it stopped malfunctioning after the first flight, so we shall see. 

[larryb]

The proper tool to debug a problem like this is an oscilloscope. Very common in the electrical engineering world but sadly lacking in the small airplane service world. Just a few seconds monitoring the buss voltage would tell you if you have a diode problem or not.

 

https://www.amazon.com/Siglent-SHS810-Oscilloscope-2-Channel-Multimeter/dp/B00GQNNSEK/ref=sr_1_9?crid=2WXKPIPQLL9BK&dchild=1&keywords=siglent+oscilloscope&qid=1615183694&sprefix=siglent+o%2Caps%2C236&sr=8-9