Flight computer -> radio interference

[cernauta]

One of my friends is bothered by his Becker radio, emitting terrible
noises through the speaker, whose origin seems to be the flight
computer (LX7000).
The power supply to both devices is fitted with ferrites.

I tried a few tricks, like wiring the metal case of one or both
devices directly to the negative or to other metal parts of the
cockpit. No effect.

One thing is a bit disconcerting for me: as I removed the antenna
cable from the radio, the noise stopped immediately. The antenna is
integrated in the fin, pretty far away from the panel. The antenna
cable looks like new, it is not bundled together with other wires and
isn't too long.

Anybody have any suggestion?

thank you very much!

Aldo Cernezzi

[Darryl Ramm]

On Jul 30, 5:22*pm, cernauta > wrote:
> One of my friends is bothered by his Becker radio, emitting terrible
> noises through the speaker, whose origin seems to be the flight
> computer (LX7000).
> The power supply to both devices is fitted with ferrites.
>
> I tried a few tricks, like wiring the metal case of one or both
> devices directly to the negative or to other metal parts of the
> cockpit. No effect.
>
> One thing is a bit disconcerting for me: as I removed the antenna
> cable from the radio, the noise stopped immediately. The antenna is
> integrated in the fin, pretty far away from the panel. The antenna
> cable looks like new, it is not bundled together with other wires and
> isn't too long.
>
> Anybody have any suggestion?
>
> thank you very much!
>
> Aldo Cernezzi

A penny's worth...

It sounds like this is happening when not transmitting (But if only
when transmitting it is likely the microphone wiring).

How does the RF connector look at the antenna cable, especially the
shield connection. If you give it a good wiggle when connected does it
change what you hear? Does it use a screw nut to hold the braided
shield? if so test by wiggling the nut, but I would replace it with a
crimp on connector as a matter of principle. BTW many people seem to
assume Becker wiring harnesses are somehow shielded - as you've no
doubt noticed the shroud on the harness is just plastic netting.

Are the ferrite isolators as close as possible to the power supply
cable entry on the Radio and LX7000? The isolators should roughly be
as big as the first part of your thumb or forefinger and the wire
should loop through several times. Large size split style/snap
together isolators are handy to use to test on cables even if too
large to leave in the glider.

The earth wires should be as short as possible and bond to a common
tie point (probably better in the confined space of a glider panel) or
thick bus bar/cable. Run both the earth and positive side of the power
through the ferrite cores, at least to start with. Check the tie point/
connections for even the slightest amount of corrosion. Anything not
pretty and shiny should be replaced.

Try disconnecting each data and other non-power cabling from the
LX7000 and see if that makes things better or worse. If so look at
those cables carefully for problems, or try putting ferrite cores on
them.

Inspect all the wiring, remove and replace any PVC insulated wire with
Tefzel insulated MIL spec wiring :-) (my real reason for posting was I
just wanted to say that).

Darryl

[cernauta]

On Wed, 30 Jul 2008 18:28:52 -0700 (PDT), Darryl Ramm
> wrote:

>It sounds like this is happening when not transmitting

Correct .
>
>How does the RF connector look at the antenna cable, especially the
>shield connection. If you give it a good wiggle when connected does it
>change what you hear?

No, it doesn't.


>Are the ferrite isolators as close as possible to the power supply
>cable entry on the Radio and LX7000?

Yes.


>Inspect all the wiring, remove and replace any PVC insulated wire with
>Tefzel insulated MIL spec wiring :-) (my real reason for posting was I
>just wanted to say that).


:-))
I'll check everything you kindly suggested

thanks!

Aldo

[JJ Sinclair]

Sounds like the problems I fought for years with the B-100, ie your
computer and or GPS is pumping out RF on or near 123.3/5. Try
disconnecting the GPS and see if your radio stops breaking squelch. If
so, try moving the GPS away from the computer and or radio. If your
running a model-20, Cambridge can fix the RF emissons. Try moving the
GPS display away from the computer/radio.
I never did get it completely fixed and finally gave up and bought an
SN-10.
JJ

On Jul 30, 5:22*pm, cernauta > wrote:
> One of my friends is bothered by his Becker radio, emitting terrible
> noises through the speaker, whose origin seems to be the flight
> computer (LX7000).
> The power supply to both devices is fitted with ferrites.
>
> I tried a few tricks, like wiring the metal case of one or both
> devices directly to the negative or to other metal parts of the
> cockpit. No effect.
>
> One thing is a bit disconcerting for me: as I removed the antenna
> cable from the radio, the noise stopped immediately. The antenna is
> integrated in the fin, pretty far away from the panel. The antenna
> cable looks like new, it is not bundled together with other wires and
> isn't too long.
>
> Anybody have any suggestion?
>
> thank you very much!
>
> Aldo Cernezzi