nuckolls.bob(at)aeroelect
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 Posted: Fri Feb 21, 2014 5:52 am Post subject: noise problem on radio - seems to be correlated to Autopil |
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The noise is intermittent but seems to be strongly correlated to two things
- when the Autopilot (Trio Avionics ProPilot) is switched on I get much more
noise. This noise is present even when the engine is off. It's like the
squelch breaking.
The image (or sound) that pops to mind with the term
"squelch breaking" is the sort of white noise you
get from an receiver when no useful signal is present
but the volume is turned up. The noise is a combination
of atmospheric and internally generated noises. Is
this the kind of noise you're hearing?
For some reason I've noticed that it happens more when I
move the stick back and forth... I have no idea why this may be.
- engine RPM - when I increase the throttle the noise becomes more and seems
to increase in pitch and intensity.
But it's present even when the engine is not running?
I'm suspecting at least one faulty ground connection.
Not sure there is foundation for this supposition yet . . .
It's possible that it
could also be due to the fact that the magneto wires (well, I'm not sure
they are real magnetos, the engine is a Rotax 912UL) are not shielded as I
didn't know any better when I wired them the first time.
The 912 has a form of capacitor discharge ignition system.
See . . .
http://tinyurl.com/mmwemuu
The capacitors are charged via separate coils on
the PM alternator stator stack . . . the larger two
of ten coils visible here.
http://tinyurl.com/lndjw9n
The Rotax manual calls for shielded wire on the ignition control
lines . . . which is fine . . . but it also calls for the shields
to be grounded at both ends which is not fine . . . at the
engine end only and use the shield as a ground for the
ignition switch. See:
http://tinyurl.com/n3oy37f
However, since you say the noise is present with the
engine not running . . . then the wiring of the ignition
switches is not a likely source of your difficulties.
(Most of the wires that come from the panel go through connectors; in this
way, I can remove the panel from the aircraft by detaching three connectors
and the wire harness to the Autopilot.).
My question is: how do I go about isolating what the fault might be? I'm
dreading having to take apart every single wire in the avionics
interconnections.
Study the chapter on noise and formulate a plan . . . it's
like playing Clue (Mr. Mustard with the Pipe in the Library).
This is an exercise in hypothesis development and elimination
which starts with identifying the antagonist. The antagonist's
nose signature is your first clue . . . so we first need to qualify
the "breaking squelch" term.
Bob . . .
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