Maybe I missed it earlier in the thread. But, why have you all ruled out the
possibility of a problem with the alternator?

Of course, there is always some ripple on the DC voltage and a grounding
problem will allow it to be more noticeable. But, it could be that the
grounding is just fine and the ripple got worse. Either a blown diode or a
bad phase.

Changes frequency with RPM? Alternator not regulator. Gets louder with load?
Alternator phase not diode.

I had a whine show up recently. It's was only bothersome with ANR headsets.
If you concentrated, you could also hear it with regular headsets. I checked
grounds and switches. Nothing made the least bit of difference.

Finally, used an oscilloscope to look at the alternator output. Nice
consistent ripple. No spikes. But, it was minimally .5 volt p-p growing to
1.5v p-p with load. Typical alternator output ripple should be more like
20mv to 50mv p-p.

They're rebuilding my alternator now. I'll let you know what happens in a
couple of days.

-------------------------------
Travis
N3094P
Lake Amphib
PWK

wrote in message
...
Tauno Voipio wrote:
: It still has the smell of a ground loop if the intercom
: power supply filtering is OK.

: Maybe a ground loop between the intercom and the radios
: (audio panel)?

Now you're talking. Like I said, I don't know the particulars on the
SPA-400,
but many intercom models are simple single-ended... a ground is a ground
is a ground.
The intercom *does* connect to the audio panel, which connects to the nav,
com, dme,
adf, radio, anklebone, etc, etc.

I suspect if you *truly* were to disconnect absolutely everything that
could
re-ground the intercom (and thus render most of the avionics stack
useless), float the
case and all jacks, you won't hear any whine.

As far as the pilot/copilot side being worse, remember that only the bus
(+14V) goes to the battery on a dedicated line (I don't recall what plane
you're
talking about... if it's composite, I'm wrong). The alternator current
meanders
through the aircraft structure somehow and eventually makes it to the
battery. The
structure could very well favor more of this current on the copilots side
and generate
a few more mV of alternator-induced IR drop there.... or rather where
"there" is the
distance between where the intercom and some other avionics is grounded.

-Cory


--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************