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Date: Sun, 10 Dec 2006 14:31:08 -0500
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Subject: Re: [LML] Observed Comm Mic Interference with JPI and TITs
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From: "John Schroeder" <jschroeder@perigee.net>
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Jeff -

We are having the same troubles with RF feeding into the Blue Mountain engine instruments and into the Tru Trak DFC-250. One of the antennas, the most egregious, is on the right wall of the aft part of the baggage compartment. It is a Bob Archer  "E" shaped com antenna. Bob feels that pretty high levels of RF is getting into the two DC power circuits, probably from the battery feed cables that are routed below the antenna in the bottom of the fuselage. His suggestion is to install 50 pico farad capacitors between positive & grnd leads on these two DC circuits on the firewall - before the DC buss feeds.

Another suggestion from a couple of HAM gurus is to install toroid suppressors on all of the cabling feeding power instrument signals into the EFIS.

I'll try to get some from JPI. The interesting thing is that the JPI EDM has absolutely no reaction to this problem.

Keep us posted on what works for you. I can't imagine getting the same reaction with the radios and audio panel off. Emissions from the alternator/s?

Cheers,

John


On Sun, 10 Dec 2006 13:39:24 -0500, Jeffrey Liegner, MD <liegner@earthlink.net> wrote:

> I have a comm bug that needs some ideas on cause and solve.
>
> Toggling mic button (transmit) on the joystick causes the displayed
> TIT2 and (to a lesser extent) TIT1 values (on EAU/Chelton) to
> promptly increase and the separate JPI ammeter/voltmeter readings to
> go screwy.
>
> I plan to place a JPI provided feritte suppressor between each of the
> two bussses and the JPI Shunts.
>
> Interestingly, in addition to when actually transmitting on the comm,
> this observed screwy change occurs when all comm units are OFF and
> their circuit breakers are pulled, and even when the PMA7000B
> intercom is also off with its CB pulled.  Note that the observed
> effect is the same when everything is OFF or ON, when the mic button
> is toggled.
>
> It happens on both joysticks, but appears more pronounced with the
> pilot side mic compared to the copilot side mic button.
>
> Any wisdom on this ghost?
>
> Jeff L
>
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