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Zodiac 601 with Jabiru engine radio noise revisited.

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1rv4

Joined: 11 Sep 2006
Posts: 20

Posted: Wed Jul 24, 2013 6:52 am Post subject: Zodiac 601 with Jabiru engine radio noise revisited.

Hi Bob and All

   I had a chance to do some work on the Zodiac 601 with a jabiru  engine again. The problem is  the Microair radio has  a very loud hum  when transmitting. The voice is clear but along with the voice is a very  loud  hum (whine). This happens with either the radio, or intercom being  used, makes no difference. So far Iâ€™ve installed a forest of tabs on the  firewall, and rewired all grounds to it, Jabiru recommended twisting the  alternator(PM) leads full length, this was done, phone jack isolation washers  where installed and checked for no errant grounds, regulator/rectifier hot lead  was moved to the battery, ground was moved to forest of tabs big bolt. Mic audio  and mic ground wires where separated as best as I could for most of their  length(short run anyway). Capacitor was installed across the bus to  ground.

   The noise starts to appear at 1600 rpm and increases in volume up to  max engine rpm. The alternator cuts in at 1600 rpm. Disconnected the alternator  and ran it,  no hum,  everything is fine. If it is the alternator  being the culprit, is there anyway to deal with it?

   Thanks for everyone's suggestions so far but Iâ€™m running out of  ideas.

  

                                                                                 Chris F

   [quote][b]

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nuckollsr

Joined: 24 Mar 2009
Posts: 95
Location: Medicine Lodge, KS

Posted: Fri Jul 26, 2013 7:16 am Post subject: Re: Zodiac 601 with Jabiru engine radio noise revisited.

The voice is clear but along with the voice is a very loud hum (whine). This happens with either the radio, or intercom being used, makes no difference.



This suggests that the noise is being injected into the microphone wiring such it is heard on the intercom and carried through to the radio.



So far Iâ€™ve installed a forest of tabs on the firewall, and rewired all grounds to it



ALL grounds? Avionics grounds should be treated as shown in installation diagrams. For example, microphone and headset grounds are generally routed with their signal wires and grounded to the device that get the signals . . . intercom or radio.  This is illustrated and discussed in the chapter on audio systems.



 Jabiru recommended twisting the alternator(PM) leads full length, this was done,



Pointless



Phone jack isolation washers where installed and checked for no errant grounds,



Good



regulator/rectifier hot lead was moved to the battery, ground was moved to forest of tabs big bolt.



Okay



Mic audio and mic ground wires where separated as best as I could for most of their length(short run anyway).



OOPS!  See above.



Capacitor was installed across the bus to ground.



Okay but pointless. Bus filters never help.



The noise starts to appear at 1600 rpm and increases in volume up to max engine rpm. The alternator cuts in at 1600 rpm. Disconnected the alternator and ran it, no hum, everything is fine. If it is the alternator being the culprit, is there anyway to deal with it?



The alternator is the greatest generator of noise in the airplane. It cannot be filtered by any practical means . . . only careful crafting of system architectures will isolated the victim radios from the potential antagonism of the alternator.



Thanks for everyone's suggestions so far but Iâ€™m running out of ideas. 



Run all three microphone wires in twisted trio (shielded too if handy but not a driving requirement) from mic jack to the intercom. Do you have a stick mounted push to talk? Make sure it doesn't get to ground anywhere at connects only to the COMMON and PTT terminals on the mic jack.

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