On Mar 29, 7:57 am, Nathan Young wrote:
On Fri, 28 Mar 2008 12:03:30 -0700 (PDT), Lancair IV-P Flyer

wrote:

The handheld radio had no static. However, it gave me an idea to
power up the radios without the engine running to isolate whether it
is alternator based. I can get the static without the engine
running. The alternator switch is off so the VR is not powered up
either so the static is coming from a powered instrument it would
seem. So, why does the static go away when the engine is running and
the alternator switch is turned off?

Hmmm. It is odd that the shutting off the alternator field current
seems to fix the static, but meanwhile you also get the static while
on the ground without the engine running.

I think your idea to pull breakers while on the ground without the
engine running is a good next step.

You mentioned a voltmeter in the plane. Does it oscillate or is it
fairly stable? I don't know if you have access to one, but it would
be really helpful to see an oscilloscope on the main bus when the
static condition occurs.

BTW - where are you based?

Nathan,

I have two voltmeters on board. They both show very steady voltage
within 0.1 volt except when there is a voltage excursion which occurs
at least 40 minutes into a flight. The voltage excursions are usually
less than two volts but sometimes there is enough of a spike to trip
the alternator field circuit breaker. I replaced the circuit breaker
but no change in the problem. I now have a new voltage regulator but
have not flown the plane yet to see if that made a difference. I have
a couple of trips planned for this week and will get a good look at
things with the new VR.

The oscilloscope idea is a good one. I will have to see if I can find
access to one.

I am based at LWS (Lewiston, Idaho on the Lewis and Clark trail).

Steve