On Nov 28, 5:47 pm, wrote:
Could be diodes, more likely a failed filter. There's probably
one in the output line, and might be another in the field circuit.

The filter in the output line (usually a 0.1uF to 2uF Hypass Coaxial
capacitor) is there for ONE reason only. It is an RF bypass capacitor
to keep alternator brush noise out of the ADF an/or Loran at 200kHz
and above. It does NOTHING to filter the alternator ripple at the
alternator rotation frequency of about 1kHz, never did, never will.
The minimum amount of capacitance that would have be paralleled with
the alternator output that would actually reduce the ripple would be
50,000uF. However the effective series resistance of a large
electrolytic capacitor is so high that such a capacitor is not
particularly effective at reducing the ripple. It is up to the
aircraft battery to act as the filter for alternator ripple; it is
very effective at reducing the total bus voltage ripple to less than
100mV. The large potted HYSONIC type filters are also only for
filtering ADF and higher frequencies.

Back EMF from the rotor can cause whine, hence the field capacitor.

You are probably referring to the INTERAV STCed alternator conversion.
For whatever reason, the original developer of this STC put a
100,000uF electrolytic capacitor to filter the source of the field
voltage upstream of the Voltage Regulator. This capacitor does NOTHING
to reduce the ripple content of the automotive Motorola alternator.
There were tens of Millions of this exact same alternator installed in
automobiles in the 1960 and 1970s without this capacitor. This STC was
poorly conceived; the large capacitor produces no benefit to the
operation of the alternator, but it introduced a single-point failure
mode when that large electrolytic was subjected to the hight
temperatures (for an electrolytic) under the cowling. If you call the
present STC holder, they admit this, but don't want to spend the money
to get the STC re-certified by the FAA if they remove it...

Alternator whine heard in aircraft avionics audio has nothing to do
with "filtering" at the alternator. It has everything to do with how
the avionics wiring harnesses were fabricated and installed. If the
avionics wiring is properly done using a "single-point-grounding", and
to a lesser extent, proper shielding, there is no detectable
alternator whine or strobe squeal, filters or not!