f.miskelly(at)imperial.ac
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 Posted: Thu Sep 13, 2007 12:16 pm Post subject: Nav/com 'acoustic feedback' |
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Feedback for the list
This problem was due to the poor intercom on a Bendix/King 155 picking up radio interference and was solved by adding an external intercom
(I've included the correspondence as it contains useful info)
- Frank
1. The radio is a Bendix/King KX155
2. It is approximately 1 year old. Don't know the model # but could find out
3. It definately has an internal intercom but i've no links to describe it
4. I didn't check whether the sound varied depending on where your head/headset is sitting, or by
bending the wires around on the headset, or the wires to the jack but it occurred with 3 separate headsets and on both pilot and co-pilot outputs. Pretty sure its not a headset problem as same headsets work fine in other planes
5. The noise gets worse with higher engine RPM but is very obvious even at 1000 RPM. The character doesn't change - just its loudness
Many thanks
Frank
Interesting.. I wasn't aware that any KX155 has intercom. Okay. I see
the manual says it has 500ohm aux inputs, but don't see any reference to
intercom. We're talking about a Bendix/King KX-155, right? Do you have a
link to a file which describes the feature?
Does the sound vary depending on where your head/headset is sitting, or by
bending the wires around on the headset, or the wires to the jack?
Is the frequency of the whine/feedback dependent on the engine RPM? Or is
it just a steady tone?
I agree with item 3 (assuming an intercom).
Regards,
Matt-
| Quote: | Matt
1. When i cover the mic with my hand (muff off) the character of the
feedback alters significantly but is not overall reduced. If i move the
boom away from my mouth and around to the back of my head then the
feedback reduces but doesn't disappear. Its worse (louder) the closer the
boom to my mouth. These symptoms occur on both left and right hand
headsets
2. My KX155 has an internal intercom. I haven't tried to disconnect it.
3. Plugging in the mic jack should activate the intercom and the transmit
on the radio (when PTT pressed)
4. Remember the problem only occurs when the engine is running.
5. Does anyone know how to adjust the mic gain (intercom volume control)
on the KX155? Couldn't find this info in the installation manual.
Kind regards
Frank
<mprather(at)spro.net>
Does it help if you cover the mic with your hand (both sides - pull the
muff off)?
I suspect that the input stage of your intercom is being overdriven by the
acoustic noise of the engine.
Regards,
Matt-
> Thanks for your comments Ernest.
> I've double-checked all the grounds i could find. Even took the whole
> harness out (which was new) and had it double-checked. Nothing found.
> If its the intercom in the radio why does it not cause the same problem
> when the engine is off?
> It only causes the problem when the engine is running
> Kind regards
> Frank
>
> ________________________________
>
> From: owner-aeroelectric-list-server(at)matronics.com on behalf of Ernest
> Christley
> Sent: Mon 13/08/2007 15:04
> To: aeroelectric-list(at)matronics.com
> Subject: Re: Nav/com 'acoustic feedback'
>
>
> <echristley(at)nc.rr.com>
>
> Miskelly, Francis G wrote:
>> The KX155 nav/com in my Glastar has 2 problems which may be related.
>>
>> 1. Inserting the headphone jack i have near perfect reception.
>> However, when i insert the microphone jack there is a loud whine
>> suggestive of 'acoustic feedback'. Moving the boom away from my mouth
>> reduces the whine. With a different headset it appears as lots of
>> static noise which again changes with position of the boom. I've
>> changed the aerial, power supply and checked all the earths. It occurs
>> with either mag and when the generator is disconnected. It only
>> happens when the engine is running. The radio's been bench tested and
>> passed as good.
>>
>> 2. Radio reception is good with just the headphone jack inserted.
>> However, insert the microphone jack and reception becomes very poor or
>> disappears completely. I suspect the 'acoustic feedback' is raising
>> the automatic squelch which is cutting off the radio station
>>
>> Can anyone suggest an explanation and a solution!
>> Many thanks
>> Frank
> Since there is a problem with multiple headset, I would check the jack
> first. The symptoms seem to imply some feedback from the mic to the
> audio. Look for a short or a broken ground. Beyond that, it would seem
> circuitry withing the intercom is fritzed, and it gets much more
> complicated.
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