Comment

Comment by Tracy Trathen on March 13, 2013 at 5:56am

Hi Bob.
Thought you might like to know that I found the problem with the alternator whine. After cleaning and adjusting all the grounds on the aircraft with no success I replaced the alternator. Problem gone. A bad set of diodes. Thank you for your advice.

Comment by Tracy Trathen on October 24, 2012 at 10:33am

Bob:

Awsome.I have some work to do. Thank you for your help. IIll let you know what I find.

Tracy

Comment by Robert Stacey on October 24, 2012 at 7:01am

Tracey.. 

Ground loops can be quite tricky but the basic idea is to have the shield around the microphone wires grounded in only one place. Usually at the intercom. If you ground it at another location , say at the microphone jack you now can have power or noise electrical current running down the shield which is then picked up by the active microphone wire, gets amplified and manifests itself as noise in your audio. 

Where to start??? The solution may not be to add grounds but to remove or relocate your current grounding system. Probably the fastest test would be to remove the mike connector at your intercom and check at the microphone jack with an ohm meter for ground to the aircraft frame.  If it is grounded ( which it shouldn't be) you will have to use insulating washers or remount the jacks on an insulated mount. When you reconnect your intercom jack you should now have ground at the mic jack because the shield is connected through the jack to the ground at the intercom.  Remember to check the passenger side if you have one. 

As for the alternator whine there may not be anything wrong with the alternator itself but the only sure way to know is to do a swap with another known good alternator.  

I do not know your exact wiring setup but there are a few basic rules to follow to avoid unwanted noise in your electrical system.  The first  is to keep sensitive wires such as microphone inputs and radio antennas physically separated. The second is to keep all shields grounded at only one end( avoid ground loops).  The third is to use large enough wire to carry the current. Ground return wire as well as the positive power wiring.  

The final thing you might want to look at is the actual current return path for your charging system.  I don't know what engine you are using but if it is a conversion the alternator might use it's case as the negative power post which opens up the possibility of charging current using the air frame to pass power from the alternator to any other electrical devices you may have grounded elsewhere in the aircraft. This includes the battery if you have any additional grounds on the negative post in addition to the heavy negative wire going to your engine. Also your alternator, starter and engine block should be well grounded to this return cable.

I may be way off base but I hope this helps you track down the problem. 

Bob

Comment by Tracy Trathen on October 23, 2012 at 11:26pm

Thanks Bob. So where do I start? Do I run a ground wire from the mic to the ground bus? Do I run a ground from the panel to ground bus? I'm a maritimer as well.....I can learn! :)

Comment by Robert Stacey on October 23, 2012 at 5:23pm

Tracey.. 

Sounds like you are picking up electrical system noise through your intercom. One way this can happen is if you have noise current running through the shield around the mike wires. This is commonly called ground loops and usually occurs if the microphone jack is grounded to the panel. It might sound like grounding the mic jack is a good thing but what usually occurs is the shield now becomes a ground path for the intercom itself and anything on the connected ground buss. Check to see if the microphone jack is making electrical contact with the panel and if it is you could try to isolate it and see if helps. 

Bob