> Alan Stewart wrote:
>
> Unfortunately, with the engine running there is a 'rev-related'
> howl, both on transmit and receive which is quite loud and
> distracting.
> Should I have an 'electrical choke' fitted into the circuit,
Probably yes, if not a faulty alternator regulator.
> or should I attempt to achieve 100% shielding
> of all wires and connectors for the unit.
Probably no.
> (There is in fact an exposed
> area around a 'D' connector) ?
Audio frequency noise can't enter there.
I assume you're hearing it through headphones, and that this is
alternator whine or howl. The only thing common to both operating
modes is phone audio, but inductive pickup (wire bundle) into the low
impedance phone audio line shouldn't happen. If the Rotax regulator
is operating normally, the noise should be clamped by the battery to a
low enough level.
However, if it's entering through the Microair's 14V input, then we're
common to both modes and a more "modern" power supply design therein
not likely in your clunky old Terra is a possible culprit. The first
thing I'd do is connect just the Microair to a 12V battery of some
sort; it needn't be much, like even AA-cells, since we won't need to
transmit. Then start the engine and listen on receive.
If no noise, then at a store or online where they sell audio equipment
for motor cars should have a choke/capacitor filter arrangement to
insert into the 14V line. Just guessing that this might be your
problem, as I can't recall other Microair/Rotax users have reported a
problem. I'd thus be prone to slap an oscilloscope on your 14V bus to
see the magnitude of the noise. Typically I've seen only about 200
millivolts on automotive systems, which should be silent in the
junkiest of radios, but from what I now know about the internals of
the Rotax regulator, anything can happen.
I had to do this fix for an audio panel in the other plane, where it
cropped up all of a sudden, but was a more muted "whine." Probably a
bad diode in the alternator, but she still charges fine to this date.
The problem there was line transient response in the audio panel's
voltage regulator that ain't worth a hill of beans at frequencies well
beyond 60Hz household hum. Much cheaper was just a little toroid choke
and a tantalum (low series resistance) capacitor from my supply of
electronic junk, and it works just like electronical theory says it
should...'til the alternator fails for good.
Reg,
Fred F.
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