On Mar 27, 2:27 pm, Lancair IV-P Flyer wrote:
Would anyone have an idea about the cause of this?

Problem:
Intermittent noise (sounds just like a squelch volume test) on both
communication radios. Lasts from 15 seconds to two minutes. Will
occur at any RPM including idle. Changing frequencies eliminates the
noise but the noise can and most likely will come back shortly on that
same frequency.

I have a back up alternator on this aircraft and when I have the main
alternator turned off to check the operation of the backup alternator,
the noise never seems to occur. Also, when the noise manifests, if I
turn off the alternator the noise always goes away. I have done this
several times on the ground and it has been 100%. The voltage
regulator has just been replaced and the noise is still present. The
alternator has been rebuilt three times in the last 30 hours of
operation. The alternator shop is tired of hearing from me. This
squelch break noise is new however in the last 15 hours of
operation.

The alternator has been having so much trouble because we have been
trying to locate an alternator field circuit breaker popping issue.
Typical scenario is the aircraft must fly for at least 40 minutes
before the breaker will pop. Usually, it pops in conjunction with a
small static crack in the headset and a voltage excursion of one to
two volts. This most often happens in level flight. This same
voltage excursion has been noted at least once when the main
alternator is turned off and running on the backup alternator.

All connections in the alternator/breaker/battery/ground circuit have
been cleaned, tightened and cable ends replaced when suspect. All to
no avail. This is a 24 volt single battery system. Running the 35
amp hydraulic pump to actuate flaps or gear will on occasion trigger a
field alternator breaker pop but many times it won't either.

Thanks in advance for any light you might be able to shine on this
problem.

Mike,

The OV protection is part of the voltage regulator product. The one
we are using is an LR3C 24 volt from B&C Specialties. It is widely
used in the experimental market and has a bullet proof history of no
problems. Since we had tried everything else the company sent us a
replacement regulator to try just in case we had a problem. I flew
the airplane last week with the new regulator and saw no change in the
symptoms. So, I am pretty confident the OV protection is not causing
the problem.

Regarding the field wire integrity, I hooked up a multimeter in series
to the field breaker and looked at the amperage to the breaker during
a flight. I was hoping for a building amperage which would have
indicated resistance building then I could have begun searching for
what was building resistance. But the multimeter amperage reading was
dead solid at 1.5 amps which is quite a cushion from the 5 amp
rating. Something is causing a voltage spike. I just have to find
it.

I am grateful for your help please forward any additional ideas you
may have on this.

Steve