And did John mention that he feels it's a GROUND problem? Also, how
could you have the strobes installed at the factory without shielded
wire? Were they added at some point? If you're able, check all
grounds before spending money with a shop.

On Wed, 27 Oct 2004 02:20:37 GMT, (John_F) wrote:

On Wed, 27 Oct 2004 00:43:50 GMT, Jonathan Goodish
wrote:

I know that this has been beaten to death, and I have searched the
archives, but I'm trying to narrow my specific case down a little bit.

I have a Cherokee with factory-installed wing tip strobes. I have an
original (1977) King Silver Crown avionics package, including audio
panel, KX170Bs, etc. I also have a Sigtronics panel-mount 4 place
intercom that was installed at some point before I purchased the
plane... I believe it is a SPA-400.

Everything is fine, except that the wing tip strobes can be heard like a
siren in the headsets. The siren sound is also occasionally transmitted
over the radio when we make transmissions, because I have been told to
"check my ELT" after making transmissions. Here are my observations:

1 -- Noise is present with intercom on or off.

Conclusion, it is not coming in within the intercom.

2 -- Noise is WORSE when any radio, but especially marker beacon, is
selected on the audio panel.

The noise is coupling into the marker beacon audio. Is the marker
beacon ground connection the same point as the strobe ground? Is the
marker beacon audio lead shielded wire?

3 -- When all radios are unselected on the audio panel (i.e. nothing
selected to either speaker or headsets) noise goes from very noticeable
to barely noticeable.

Sounds like a nosy ground problem.

4 -- Noise is much WORSE when things are "cold." After about 20 minutes
of flying time, the noise is still noticeable, but not nearly as bad as
it was when the flight began.

Sounds even more like a loose ground problem. Aluminum srinks more
with cold temperatures than the steel screw that holds the wire to the
airframe.

5 -- Unless I'm hearing things, which is possible, I can SOMETIMES hear
the noise through the headset with the strobes on but avionics bus OFF.

If the ground resistance were bad enough it would do this.

I am not sure whether the strobe power pack is crying out for help,
whether I have a ground problem elsewhere in the system, or whether I
have an improper installation of something (like the intercom), or
whether the audio panel is shot.

Find out where the ground of the strobe pack is connected to the
airframe and clean it.

The strobes were factory-installed, so I assume that the installation of
the strobes is correct.

Don't bet on this.

The SPA-400 intercom was obviously not factory
installed, and I am not convinced that insulating washers were used on
the headset plugs, so I do need to check that out.

Unless you have a large current through the airframe or have some poor
ground points insulating the jacks may be a wasted effort. If it does
work you need to look at the ground connections for the radios ,
intercom and audio panel because they will almost certainly be not
tight and will be high resistance.

I do know that shielded cable was NOT used on the intercom installation.

VERY VERY BAD!!! This is how the noise is coupling from the strobe
power leads on to the audio leads if they are in the same wire bundle.
ALL of the audio leads MUST be shielded wire.

Does anyone have any ideas? I've basically narrowed it down to three
things, in no particular order:

1 -- intercom
2 -- audio panel
3 -- strobe power pack

A big 1000uf cowpasture on the strobe power pack input power may help.

Thanks,
JKG

Find a avionics shop that knows what they are doing and have them fix
it. They are very hard to find and generally will NOT have the lowest
rates but they will generally save you money.

John