Nathan Young wrote:

Have you measured the bus voltage? Sounds like it might be set too
high, and possibly on the threshold of the OVR cutout. If so, any
inductive spike would be able to push the voltage higher forcing an
OVR cutout.

14v and change. Even if the set point was too high it doesn't explain
the tendency to intermittently show discharge with spiking loads, nor
the occasionally-observed behavior of a largely resistive load (turning
on the landing lights) tripping the thing off (the working theory is
that the halogens have a high cold inrush current but quickly settle
down to a lower operating current -- quick enough that the regulator
overshoots).

Also, what is the voltage at the input to the OVR and the input to the
VR?

Sorry. I forgot to include that all of the voltages in the system are
within InterAv's specs for troubleshooting "common" failures. The
conditions for these measurements are engine not running, master and
field on, 12.5v at the alternator output terminal. Under those
conditions the alternator regulator terminal has 0.9 volts on it, the
regulator's regulator terminal has 0.9 volts, the regulator's field
terminal has 1.2 volts and the alternator's field terminal 1.2 volts.
The OVR is in series between the regulator's field output and the
alternator's field terminal; its sense inputs are tied to the bus and
the regulator ground.

I'm wondering if brush chatter could be contributing. I can't find any
log entries suggesting that they've been changed, and the thing has
about 400 hours on it. I also haven't stuck a scope on it to see if a
diode has given up the ghost, but I'm not seeing or hearing anything in
the avionic stack that would suggest I've got a noisy bus.

What I really want is a 100A conversion, but at this rate I'm going to
end up ripping this alternator and its attendant bits out of the
aircraft and installing Yet Another Copy of the InterAv kit :P

Chris.