>I had originally located the mic and phone jacks to the comm radio to the
cage structure behind the pilot's left shoulder, using a pair of insulated
adel clamps to attach the simple angle bracket.. This turned out to be a
nuciance, and reminded me uncomfortably of John Denvers problem. So I
decided to move the jack assembly to a more convenient position, which
turned out to be at the rear of the seat back rail tab. I simply drilled a
hole in the angle and used a slightly longer bolt to mount the seat rails.
>
>The next time I turned on the radio, planning to go flying and test a trim
change, I heard the unmistakable noise of the aircraft alternator in the
head set. Not only that, I discovered that there was suddenly an
interaction between the strobe lights and the Apollo SL60 nav/comm, and
that the nav display blinked on and off in response to the strobe flashes!
>
>It was apparent that I had unintentionally picked up a lot of ground
currents in the shield of the headset cable, and introduced some
undesirable coupling to other parts of the electrical system. Mounting the
headset jack bracket on a standoff solved the problem, and my radios are
once again quiet.
I've reposted this item to several list-servers . . . including
some of the plastic airplanes interest groups. It's important to
avoid any local "grounding" of headset and microphone jacks, even
to a metal instrument panel in a composite airplane. Hard paper
extruded washers are available to provide insulation of the
jacks from their mounting plate, or you can mount the jacks
on a thin sheet of glass-epoxy like etched circuit boards.
We stock the washers too @ $1 per jack, postage paid.
Bob . . .
////
(o o)
< A mind abhors a vacuum . . . >
< When deprived of facts, >
< our fantasies are generally >
< much worse than reality. >
|