Travis Marlatte wrote:
. . .

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.

Travis, if you connected the alternator Field directly to a 2 to 5V DC
supply, connected a heavy load (like a couple of landing lights across
the alternator output), then you spun the alternator in a drill press
or lathe, while viewing the voltage across the load with a scope, you
would see about 4V p-p of ripple. Is that a reason to re-build the
alternator?

No!! It is actually the aircraft battery which "filters" the output of
the alternator, and removes the ripple. If you see ripple, it is
because there is some resistance in the path between the alternator
output and the battery posts, or a bum battery. It could be either in
the positive path or in the ground return. I predict that you will
still have your ripple when you get your freshly o/h alternator back...

Several hundred mV of ripple measured at the B terminal of an
alternator is perfectly normal...

MikeM