UPDATE -- Headset Noise

[Jonathan Goodish]

Inspired by the many helpful suggestions of group participants, I
finally went searching for the potential ground loop issue--and I
believe that I found it. The tip terminal of the copilot mic jack was
contacting the airframe from below, creating the loop through the
copilot circuit of the intercom. Unfortunately, this was by far the
least accessible set of jacks in the airplane (Cherokee), but I was able
to get a dental pick up there, hook the terminal, and pull it down away
from the point where it was contacting the airframe. The other
terminals have wire connections and were soldered and covered with heat
shrink, but since the tip terminal wasn't connected, it was missing the
heat-shrink.

The intercom system is now almost completely silent, though I've only
ground-run the airplane to test at this point. There is still a very,
very slight whine that is audible through the system when the headset
volume is at maximum, but it is not obvious and it is now equivalent
between the pilot and copilot positions--the nasty whine that was in the
copilot headset appears to have almost disappeared.

I have no idea whether this will have an effect on the issue of static
in the sidetone when the pilot mic is keyed, but who knows. I'll try to
fly the airplane over the next couple of days to see what happens, but
hopefully this problem is now resolved.

Thanks to all who responded with input on this problem.



JKG

[Michael Ware]

"Jonathan Goodish" > wrote in message
...
>... but since the tip terminal wasn't connected, it was missing the
> heat-shrink.
> JKG

So how does the co-pilot transmit?

[Jonathan Goodish]

In article >,
"Michael Ware" > wrote:
> >... but since the tip terminal wasn't connected, it was missing the
> > heat-shrink.
> > JKG
>
> So how does the co-pilot transmit?


Only the barrel and ring of the intercom mic jack are live. Ring
connects to the intercom, and barrel to ground. Tip is not connected.
Note that this is different than the aircraft mic jack, where tip is
used to key the mic for transmit.



JKG

[Montblack]

("Jonathan Goodish" wrote)
[snip]
> Inspired by the many helpful suggestions of group participants, I finally
> went searching for the potential ground loop issue--and I believe that I
> found it. The tip terminal of the copilot mic jack was contacting the
> airframe from below, creating the loop through the copilot circuit of the
> intercom.


I know zip about electronics ...but am still (miraculously) correct 75% of
the time with this type of post.

It's "always" the ground. <g>


Montblack-wire-goes-where?
"Dance, mailman."

[mikem]

Good Going, Jonathan!

I feel good that I actually might have provided a clue when I wrote:
"I'm thinking a wiring error, or short which occurs only when headphone
plugs are pushed in, or an internal fault inside the SPA400."

Always very satisfying when you can find a real problem and fix it.

[Jonathan Goodish]

In article . com>,
"mikem" > wrote:

> Good Going, Jonathan!
>
> I feel good that I actually might have provided a clue when I wrote:
> "I'm thinking a wiring error, or short which occurs only when headphone
> plugs are pushed in, or an internal fault inside the SPA400."
>
> Always very satisfying when you can find a real problem and fix it.

Mike,

Your expertise and insistence (along with others) did motivate me to
look again for the ground loop. It actually didn't have anything to do
with the plug being pushed in, but you wouldn't believe what a freak
event it was--the panel hole sits right under a rear lip on the panel,
and the way the jack was mounted (turned) in the hole caused the tip
terminal to make contact. All of this mess was right above the
electrical bus bars, right behind the cabin heat control cables, and
about halfway up the back side of the panel, so it was nearly impossible
to see. I almost gave up before I decided to give it one last look, and
the light finally hit in the right spot. The dental pick was about the
only thing that I could get up in there to pull the terminal away from
the airframe, because I really, really didn't want to take the jack out
of the panel.

Thanks again to all who responded.


JKG

[[email protected]]

On 17-Apr-2006, Jonathan Goodish > wrote:

> The dental pick was about the only thing that I could get up in there to
> pull the terminal away from
> the airframe, because I really, really didn't want to take the jack out of
> the panel.


I know what you mean. Our Arrow has a 4-place intercom system, and the
headphone jacks for all but the pilot went silent a couple of days ago.
Troubleshooting was actually pretty easy -- we could see that the two wires
normally soldered to a lug on the copilot headphone jack had broken off.
One comes from the intercom, the other carries the same audio signal to the
rear seat jacks. Looked like a simple repair, but in the tight confines
behind the panel it was a nightmare to get the wires stripped, twisted
together, and soldered back on.

If I was made czar of airplane panel design, I'd make it in modular sections
that could easily be removed. Wiring between sections would be through
connectors that would allow a "problem" section to be removed and repaired
on a bench.

-Elliott Drucker

[Tauno Voipio]

David Lesher wrote:
> writes:
>
>
>>Looked like a simple repair, but in the tight confines behind the
>>panel it was a nightmare to get the wires stripped, twisted together,
>>and soldered back on.
>
>
>>If I was made czar of airplane panel design, I'd make it in modular sections
>>that could easily be removed. Wiring between sections would be through
>>connectors that would allow a "problem" section to be removed and repaired
>>on a bench.
>
>
> I envision all avionics in the back, inc. under the rear seat. The panel
> has a small head with display. Mike/headphone jacks have a box as well.
> The boxes connect to the rear via ordinary ST fiber optics jumpers --
> light [boo], sturdy, no ground-loop issues.
>
> And the panel would hinge up, al-la "cab-over" semi tractors.
>
> Of course, it IS just a dream.


Have a look at the Aerospatiale Trinidad / Tobago series.

--

Tauno Voipio
tauno voipio (at) iki fi