Jonathan Goodish wrote:
: 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.

: Yes, I know that this is bad, but it probably could be worse (and
: remember, I'm not responsible for the intercom installation). The leads
: to and from the intercom don't appear to be bundled with the strobe
: power leads, or even close to them. I am hoping that this isn't the big
: problem, and I can let this ride until I have the interior replaced and
: then replace the wiring at that point.
Not that big of a deal, actually. Shielding gets an over-used emphasis in
most installations. There is not that much coupling between adjacent wires at audio
frequencies (4kHz). Not that there isn't any, but for any signal it's small. For
any signal above mic level, it's fairly insignificant. Also, only the inductive
coupling has any important coupling for a wiring bundle with the types of impedances
used in a typicaly aircraft system, so only wires carrying current can couple noise.

: 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.

: I don't want to give someone a blank check to troubleshoot a problem
: without some idea of what it may be. In other words, I need to try to
: troubleshoot as much as possible before engaging the avionics shop for
: more in-depth troubleshooting. I do believe that I have a good avionics
: shop in the local area, and they've done work for me before, but I
: haven't classified this problem as more than a nuisance at this point.
: However, it may be getting worse, and I certainly don't want it to reach
: a point where it becomes a show-stopper.

Good idea. It will depend on how friendly your A&P is with you crawling under
the panel, removing things to get at wiring, and tracing the wiring itself. It's
probably about 95% certain that it's a ground loop issue... in particular with the MB
it sounds like. Since many installers will ground different avionics willy-nilly
throughout the airframe wherever it's convenient, you get ground currents causing
voltage drops everywhere and coupling into the audio system.

Since you said things change as they heat up, it might help a bit to try to
locate the grounds on all the audio equipment and clean them. Most likely, however,
it's due to things being grounded at different places.

You might be able to band-aid the specific problem of strobe noise by running
a dedicated ground wire to it from "some good ground." That "some good ground" sorta
depends, but good places to try for troubleshooting would be the battery negative, and
alternator casing. That doesn't fix the multipoint ground problem in the audio
system, but it might reduce the structural ground currents enough to reduce the noise.

Cheers,
-cory


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* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
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