nuckolls.bob(at)aeroelect
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 Posted: Mon Jul 08, 2013 5:59 am Post subject: Subject: Re: Jabiru alternator noise-what really cause |
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Is the microphone jack electrically isolated
from the airframe? Further, does it's ground wire
route in parallel with mic audio and ptt lines
back to the radio?
Not sure about the mic audio and ptt grounds but will check when I get back from holiday. We did isolate the jacks.Noise is the same whether its intercom or radio, only time it disappears is when we take the alternator out of the mix.
Okay. The high probability condition is
a ground loop that injects alternator
noise directly into the microphone
wiring.
Where are the alternator and battery
grounded? To the Forest=of=Tabs?
The battery is grounded on the engine firewall side to the forest of tabs, the alternator is case grounded to the engine, it has two leads from it that go to the regulator. These leads are twisted for their full length.
Hmmmm . . . the PM alternator should be
totally isolated from ground. I.e. a pair
of leads come off the ends of the winding
and go directly to the rectifier/regulator.
Does the R/R have a case connection? The only
ones I am familiar with feature electronics that
float completely within the case.
It has been described as a dynamo more than an alternator, where the rectification takes place at the regulator?
Yeah, the common vernacular speaks to the PM
alternator as a 'dynamo'. Back in the early
days of belt driven alternators for cars, the
rectifiers and regulators were external too.
These two wires are carrying A/C as far as I can tell.
Check to make sure the R/R does not incorporate
any kind of case ground. Consider an experiment
to connect the R/R(+) lead directly to battery(+)
at the contactor and R/R(-) lead directly to the
forest of tabs.
As microphones go, the voltage levels at the
mic jack for an aviation mic is pretty high . . .
hundreds of millivolts. To inject so strong
a signal from the alternator into the mic circuitry
suggests an extra-ordinary set of physics . . .
especially since your alternator is so limited
in output.
I think we're going to find that something
in the wiring is not as presently perceived.
Bob . . . [quote][b]
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