Today's Message Index:
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1. 08:08 AM - Re: Magneto Noise (Robert L. Nuckolls, III)
2. 08:32 PM - When are diodes needed? (Thomas Blejwas)
Message 1
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Subject: | Re: Magneto Noise |
At 12:04 PM 4/6/2014, you wrote:
I flew w/ a friend recently and when trying to get awos for our
destination airport (which is a pretty weak signal) it was almost
impossible to hear due to magneto noise. The radio is a Garmin 430W
in a Cherokee and we were around 10 miles from the airport. When we
got within 3 or 4 miles the signal was much clearer. All other
transmissions with ATC & towers were acceptable but I noticed at high
volume I could clearly hear magneto noise in the background.
I'm wondering if that's just the way it is or if there is a problem
with the magnetos or P lead wiring.
Was this a new condition or has it been that
way 'forever'. It's difficult for the neophyte
observer to tell the difference between ignition
noises getting into the system by way of plug wire
radiation versus p-lead radiation.
First, make sure that the perceived noise goes up
and down with volume control settings on radio.
This all but guarantees that it's coming in through
the antenna.
Then, disconnect both p-leads from the back of the
magnetos and run the engine. See if the noise is
still heard . . . turn to unused frequency . . .
open squelch.
If you can still hear it, then look for bad plug
wire. If not, hook up one p-lead at a time and
repeat experiment . . . see if noise is predicated
on either or both of the p-leads being hooked up.
Once you've identified the offending radiator,
you need to figure out why. If this is a 'new'
condition, then you're looking for something that
has changed. Bad cap in mag (probably not both)
broken ground lead on p-lead shield. If possible/
practical wire p-lead wires per Figure Z-27
http://tinyurl.com/n3oy37f
Ground p-lead shields at magneto end only . . .
use p-lead shields to ground mag switch. Remove
and existing ground to the mag switch.
If the noise is coming from both p-leads,
there are 'approved' magneto noise filters
http://tinyurl.com/l3s4mrj
that most mechanics will install with a simple
log-book entry.
Bob . . .
Bob . . .
Message 2
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Subject: | When are diodes needed? |
In adapting Z-07 and/or Z-19, I'm confused about the use and purpose of dio
des.- As I understand them, diodes allow current to flow in one direction
, with a small voltage drop, but not in the opposite direction, except for
a small leakage.- The voltage drop times the forward current yields a pow
er loss that results in heat, which may need to be dissipated to keep the h
eat from damaging the diode or other nearby equipment.- The power lost ma
y also be an issue in a battery-only situation after loss of an alternator,
for example.- O.K. so far?=0A=0APairs of diodes are used in Z-19 for the
two battery feeds for the main fuel pump, for example.- When both feeds
are on, this would keep a "good" battery from feeding a "bad" one.- Is th
at the primary purpose?- In Z-07, only one feed to the bus with the fuel
pump has a diode because judicious switching can isolate either battery.-
Correct?- Is there something more that I'm missing?- For example, if I
'm not concerned about one battery "feeding" another, do I still need diode
s to "smooth" the feed to a bus from two different batteries?- Thanks.=0A
=0ATom=0A=0ASent from my iPad
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