Bob Harrison wrote:
> ...
> It seems to me that I need to establish what the damn noise is from first
> ..... how? Bill Bell has heard it but can't identify it , although he did
> say it varied as he walked round the aircraft with the engine running,
> that's where I started applying nickle based paint.
If you can scare up somebody with a little hand-held, LCD screen
oscilloscope, I'd check what's on the 14v supply to the radio and in
the air. Here, they're sort-of cheap now (Vellemam HPS5), and some of
us just can't resist.... Not a grown-up scope on an auto-battery AC
converter, unless a good friend with a cheap scope. Those converters
can fry a laptop battery charging brick (switching regulator) real
good, w/o a clean sine wave; even ruined the lithium-ion ($350) batt.
I just now tossed the LCD scope on the battery of my Dodge, which got
one of them new-fangled solid-state alternator rag-a-lators, but still
there's a .2V p-p low-freq, sinusoidal-like wave there (alternator
output), which shouldn't hurt VHF, but there is also high-frequency
hash along the sine wave, of about 5mv. Lord knows what that is,
maybe the commutator brushes or CDI ignition primary. 5mv on the
radio's supply shouldn't hurt, but if the source is broadcasting it
into the radio's antenna.... Like scope disconnected, there's the
spark plug spikes of healthy amplitude, picked up by the scope cable
with one's body as antenna.
Avionics are designed to be immune from a reasonable amount of junk
---From buzz-box alt regulators, too, but it might help to see what's
there. Still, my FM radio is unaffected. It's not a Blaupunkt, but
not made by an avionics mfr either. Just don't know which way that
slices though, except that cost bears no relation!
Best,
Fred F.
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