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Nav/com 'acoustic feedback'

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f.miskelly(at)imperial.ac
Guest

Posted: Mon Aug 13, 2007 4:15 am Post subject: Nav/com 'acoustic feedback'

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

  

 [quote][b]

echristley(at)nc.rr.com
Guest

Posted: Mon Aug 13, 2007 6:05 am Post subject: Nav/com 'acoustic feedback'

Miskelly, Francis G wrote:

 	  Quote: 	
		   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.

f.miskelly(at)imperial.ac
Guest

Posted: Mon Aug 13, 2007 8:01 am Post subject: Nav/com 'acoustic feedback'

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'






Miskelly, Francis G wrote:

 	  Quote: 	
		   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.

mprather(at)spro.net
Guest

Posted: Mon Aug 13, 2007 9:03 am Post subject: Nav/com 'acoustic feedback'

Ernest,



Your response kind of points to what I was thinking..  I don't think the

KX155 has any intercom, but the description of the symptoms seem like an

intercom problem.  If so, I wonder if turning off the intercom aleviates

the behavior.  If there isn't any intercom, then I might suspect the radio

has a problem.  Plugging in the mic shouldn't have any effect until the

PTT circuitry is activated, but then you wouldn't get _any_ reception

(because the transmitter would be active)..



If the problem is related to the intercom, consider trying to find a way

to adjust the mic gain on the intercom.

Regards,



Matt-



 	  Quote: 	
		   

 <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.

echristley(at)nc.rr.com
Guest

Posted: Mon Aug 13, 2007 9:03 am Post subject: Nav/com 'acoustic feedback'

Miskelly, Francis G wrote:

 	  Quote: 	
		   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



   


	


Where is your squelch set at?

mprather(at)spro.net
Guest

Posted: Mon Aug 13, 2007 11:14 am Post subject: Nav/com 'acoustic feedback'

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-



 	  Quote: 	
		   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.

f.miskelly(at)imperial.ac
Guest

Posted: Mon Aug 13, 2007 12:36 pm Post subject: Nav/com 'acoustic feedback'

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

  From: owner-aeroelectric-list-server(at)matronics.com on behalf of Matt Prather

Sent: Mon 13/08/2007 20:12

To: aeroelectric-list(at)matronics.com

Subject: RE: Nav/com 'acoustic feedback'

--> AeroElectric-List message posted by: "Matt Prather" <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-

 	  Quote: 	
		   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'

 --> AeroElectric-List message posted by: Ernest Christley

 <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.

        - The AeroElectric-List Email --> http://www.--> http://forums.matronics.com

===========

 [quote][b]

mprather(at)spro.net
Guest

Posted: Mon Aug 13, 2007 1:32 pm Post subject: Nav/com 'acoustic feedback'

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





 ________________________________



 From: owner-aeroelectric-list-server(at)matronics.com on behalf of Matt

 Prather

 Sent: Mon 13/08/2007 20:12

 To: aeroelectric-list(at)matronics.com

 Subject: RE: Nav/com 'acoustic feedback'



 

 <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.

rjquillin

Joined: 13 May 2007
Posts: 123
Location: KSEE

Posted: Mon Aug 13, 2007 2:16 pm Post subject: Nav/com 'acoustic feedback'

At 14:29 8/13/2007, you wrote:

 	  Quote: 	
		  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?


	


I do find a reference in the IM for a 155A/165A to an intercom level 

adjustment.

Is this an A model?



Ron Q

f.miskelly(at)imperial.ac
Guest

Posted: Mon Aug 13, 2007 3:43 pm Post subject: Nav/com 'acoustic feedback'

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

________________________________

From: owner-aeroelectric-list-server(at)matronics.com on behalf of Matt Prather

Sent: Mon 13/08/2007 22:29

To: aeroelectric-list(at)matronics.com

Subject: RE: Nav/com 'acoustic feedback'

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

 ________________________________

 From: owner-aeroelectric-list-server(at)matronics.com on behalf of Matt

 Prather

 Sent: Mon 13/08/2007 20:12

 To: aeroelectric-list(at)matronics.com

 Subject: RE: Nav/com 'acoustic feedback'

 <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.

f.miskelly(at)imperial.ac
Guest

Posted: Tue Aug 14, 2007 6:42 am Post subject: Nav/com 'acoustic feedback'

The squelch is set to off. Pulling out the volume control knob takes off the squelch. When i leave the squelch on there is virtually no radio reception at all.

 - Frank



________________________________



From: owner-aeroelectric-list-server(at)matronics.com on behalf of Ernest Christley

Sent: Mon 13/08/2007 17:57

To: aeroelectric-list(at)matronics.com

Subject: Re: Nav/com 'acoustic feedback'






Miskelly, Francis G wrote:

 	  Quote: 	
		   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



  


	


Where is your squelch set at?

mprather(at)spro.net
Guest

Posted: Wed Aug 15, 2007 2:31 pm Post subject: Nav/com 'acoustic feedback'

I'm still not sure whether it's electrical noise or acoustic noise - since

covering the mic has some effect on the behavior.  If it were just

acoustic, covering the mic should stop the noise.  More questions:



 - Is the acoustic squelch on the intercom adjustable?  Can you set the

threshold higher?  Maybe pulling out the volume knob and turning it

adjusts that?  Or is it dynamic/smart adjusting?



 - What kind of engine is it?

 - Does it have separate ignition systems?  Can you turn one off at a time

(to help isolate electrical noise)?

 - Can you turn the alternator off with the engine running?



 - You probably answered this before, but with the engine off, does the

intercom appear to work properly?  Speaking in the mic on one headset

yields sound coming from the other headset - using the radio sidetone?



 - Are you able to transmit/receiver properly when the engine is off?



 - Are the mic leads made with shielded wire?



 - Do other aspects of the electrical system all appear to function

normally - bus voltage, nav signals, etc?



Bob often recommends powering from a separate battery the piece(s) of

equipment that are involved in the problem.  A couple of lantern batteries

could be used to power the radio while the engine is running..  If that

has any effect on the noise, it indicates that the problem is conducted to

the radio via the power bus.



What would be slick for this kind of online debug process would be to have

an online spreadsheet or form which could be used to document the problem,

behaviors, and the things that have been tried in order to fix it..  Have

to ponder that a bit.

Regards,



Matt-



 	  Quote: 	
		   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



 ________________________________



 From: owner-aeroelectric-list-server(at)matronics.com on behalf of Matt

 Prather

 Sent: Mon 13/08/2007 22:29

 To: aeroelectric-list(at)matronics.com

 Subject: RE: Nav/com 'acoustic feedback'



 

 <mprather(at)spro.net>



 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-

> 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

>

>

> ________________________________

>

> From: owner-aeroelectric-list-server(at)matronics.com on behalf of Matt

> Prather

> Sent: Mon 13/08/2007 20:12

> To: aeroelectric-list(at)matronics.com

> Subject: RE: Nav/com 'acoustic feedback'

>

> 

> <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.

>>

>>

>>

>>

>>

>>

>>

>>

>

f.miskelly(at)imperial.ac
Guest

Posted: Thu Aug 16, 2007 4:33 am Post subject: Nav/com 'acoustic feedback'

Thanks Matt

Answers (in italics) to your questions below 

 - Frank



________________________________



From: owner-aeroelectric-list-server(at)matronics.com on behalf of Matt Prather

Sent: Wed 15/08/2007 23:27

To: aeroelectric-list(at)matronics.com

Subject: RE: Nav/com 'acoustic feedback'






I'm still not sure whether it's electrical noise or acoustic noise - since

covering the mic has some effect on the behavior.  If it were just

acoustic, covering the mic should stop the noise.  More questions:

I suspect its not acoustic as it doesn't affect my handheld ICOM and the a/c is not that noisy

 - Is the acoustic squelch on the intercom adjustable?  Can you set the

threshold higher?  Maybe pulling out the volume knob and turning it

adjusts that?  Or is it dynamic/smart adjusting?

I don't know how to adjust the intercom volume. I presume it is adjustible. Pulling out the volume knob takes off the automatic squelch. There is no obvious knob to adjust the intercom vol.

 - What kind of engine is it? Its a Lycoming 0-320 with 150hp



 - Does it have separate ignition systems?  Can you turn one off at a time

(to help isolate electrical noise)? It has 2 mags and i've tried both separately - no effect

 - Can you turn the alternator off with the engine running? Turning off alternator - no effect



 - You probably answered this before, but with the engine off, does the

intercom appear to work properly?  Speaking in the mic on one headset

yields sound coming from the other headset - using the radio sidetone?



With the engine off the radio is perfect. No intercom problems. Perfect reception and transmit



 - Are you able to transmit/receiver properly when the engine is off?



 - Are the mic leads made with shielded wire? Mic leads are not shielded. They are 2 twisted wires



 - Do other aspects of the electrical system all appear to function

normally - bus voltage, nav signals, etc? All other parts of electrics are fine



Bob often recommends powering from a separate battery the piece(s) of

equipment that are involved in the problem.  A couple of lantern batteries

could be used to power the radio while the engine is running..  If that

has any effect on the noise, it indicates that the problem is conducted to

the radio via the power bus. Tried that from a separate 12V battery - no effect



What would be slick for this kind of online debug process would be to have

an online spreadsheet or form which could be used to document the problem,

behaviors, and the things that have been tried in order to fix it..  Have

to ponder that a bit.

I've changed the antenna, power supply, headsets, earthed everything possible and checked all the earths. The problem of feedback only occurs when i plug in the mic jack when the engine is running. I have a handheld ICOM which uses the same power supply and aerial yet it works perfectly. It has no intercom.



I plan to disconnect the intercom this weekend to see whether the problem disappears.



Regards,



Matt-



 	  Quote: 	
		   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



 ________________________________



 From: owner-aeroelectric-list-server(at)matronics.com on behalf of Matt

 Prather

 Sent: Mon 13/08/2007 22:29

 To: aeroelectric-list(at)matronics.com

 Subject: RE: Nav/com 'acoustic feedback'



 

 <mprather(at)spro.net>



 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-

> 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

>

>

> ________________________________

>

> From: owner-aeroelectric-list-server(at)matronics.com on behalf of Matt

> Prather

> Sent: Mon 13/08/2007 20:12

> To: aeroelectric-list(at)matronics.com

> Subject: RE: Nav/com 'acoustic feedback'

>

> 

> <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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