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Decreased Radio and Intercom Volume with Altitude

rockydoc

By rockydoc
in Avionics/Panel Discussion

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rockydoc Community Regular

rockydoc

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Over the last six months or so, I have had five different avionics tech trying unsuccessfully to solve this problem.   My intercom volume is very low.  My radio volume decreases above 3500' and my radio static increases above 3500' to the point that I can't communicate with ATC.   Above 7000' communication is totally impossible.  We have tried cleaning and tightening the antennae connections and even tried switching antennae locations.  Nothing seems to work.  Ideas?

Rocky

1964-M20E Grand Master

1964-M20E

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I’ve had some serious static in certain areas around New Orleans on specific frequencies 119.9 KNEW TWR and 119.5 KMSY TWR and this was down low.  This was on both radios and it happens around an antenna farm and near downtown.  We ended up deciding it was the ELT 121.5mhz causing the interference.  I do not know why it would but it does.  Unplug the ELT antenna and it goes away.

ArtVandelay Grand Master

ArtVandelay

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Same problem with both radios? Static listening or transmitting or both? I assume you try to increase the volume using both radio control as well as your audio panel? Same with copilot side? Did you try switching magneto off to see if it makes a difference?

Skywarrior Proficient

Skywarrior

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I'm not an electronics expert, and I didn't stay at a Holiday Inn Express last night... but it sounds like a 'box' somewhere depends upon being hermetically sealed - but isn't.

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mooniac15u Veteran

mooniac15u

Posted
Posted

Over the last six months or so, I have had five different avionics tech trying unsuccessfully to solve this problem.   My intercom volume is very low.  My radio volume decreases above 3500' and my radio static increases above 3500' to the point that I can't communicate with ATC.   Above 7000' communication is totally impossible.  We have tried cleaning and tightening the antennae connections and even tried switching antennae locations.  Nothing seems to work.  Ideas?

Rocky

 

Most intercoms should be wired so that turning off the intercom connects the pilot headset directly to comm 1.  If you turn off the intercom is the problem still present?  That might help you determine if the problem is the radio or the intercom.

rockydoc Community Regular

rockydoc

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Most intercoms should be wired so that turning off the intercom connects the pilot headset directly to comm 1.  If you turn off the intercom is the problem still present?  That might help you determine if the problem is the radio or the intercom.

Good point.  I'll try turning off the intercom and see if it makes a difference.  I have pulled both radios out of their rack and re-inserted them.  Didn't make any difference.

Piloto Grand Master

Piloto

Posted
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This could be a cooling problem. As you go higher in altitude the air density is less, thus the cooling efficiency of avionics decrease. Check the avionics fan and ducting. You may have an electronic component that has gone marginal with temp. Aim the side cabin vents toward the avionics stack to improve cooling.

 

José

rockydoc Community Regular

rockydoc

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This could be a cooling problem. As you go higher in altitude the air density is less, thus the cooling efficiency of avionics decrease. Check the avionics fan and ducting. You may have an electronic component that has gone marginal with temp. Aim the side cabin vents toward the avionics stack to improve cooling.

 

José

Buen punto José.  Gracias.  Muy amable.

Rocky

N601RX Grand Master

N601RX

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Your plane probably doesn't have a cooling fan, unless someone has added one.  The older ones came from the factory with a cooling louver behind the radio stack hooked up to a ram air vent to the outside.  Not a good idea for cooling modern stuff.  I was having cooling issues in the center of the stack (KNS80). We removed the ram air and cooling vent and added a fan with a tube the the rear of each piece of equipment.  I haven't had any problems since.

rockydoc Community Regular

rockydoc

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Your plane probably doesn't have a cooling fan, unless someone has added one.  The older ones came from the factory with a cooling louver behind the radio stack hooked up to a ram air vent to the outside.  Not a good idea for cooling modern stuff.  I was having cooling issues in the center of the stack (KNS80). We removed the ram air and cooling vent and added a fan with a tube the the rear of each piece of equipment.  I haven't had any problems since.

That sounds like a great solution whether its responsible for my existing problem or not.  You don't happen to have a photo of the cooling fan and tubes as well as a source for the fan.  It sounds like a special fan that I would like to buy.

takair Grand Master

takair

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Pretty bulletproof setup. As I recall, those don't have cooling ports. Suggestion above, to turn off intercom, is a good one. This is a long shot, have you tried different headsets? You may have a ground problem too, don't know how it relates to altitude, but that could be common to all. Do you have a voltmeter, is it possible your alternator or generator is not charging at altitiude? Things tend to arc with thin air, this bad brushes can be worse. A friend recently had a loose lug on the alternator, similar story.

rockydoc Community Regular

rockydoc

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Pretty bulletproof setup. As I recall, those don't have cooling ports. Suggestion above, to turn off intercom, is a good one. This is a long shot, have you tried different headsets? You may have a ground problem too, don't know how it relates to altitude, but that could be common to all. Do you have a voltmeter, is it possible your alternator or generator is not charging at altitiude? Things tend to arc with thin air, this bad brushes can be worse. A friend recently had a loose lug on the alternator, similar story.

The KX-155 does have a cooling port.  I'm not certain about the others, but I ordered the recommended avionics cooling fan with 3 air outlets so I plan on aiming a hose at any tray that doesn't have a cooling port.  I have tried various headsets--no difference.  I have a voltmeter and constantly check my generator output--which is consistently good.  I think your comment about a good ground needs pursuing.  I think I'll run an individual ground wire directly to each radio tray or should I connect it to each radio chasis inside of the tray?  Is the Mooney frame under the instrument panel a suitable grounding site?

Rocky

takair Grand Master

takair

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The problem is that the chassis ground and audio ground within the radio is different. The audio grounds coming from the connector are typically grouped together such that the audio panel, intercom and radios audio circuits have a common connection point to airframe ground. While this will ohm out the same as chassis, it does make a difference in performance. (Noise, volume, quality) In addition, ideally the headset plugs are isolated and the ground is run back to audio ground. The tube chassis is a good ground. The sheet metal panel is screwed to this. It it possible that the radio/audio ground is connected to the sheet metal, but the sheetmetal panel is not well connected to the tube. Check to see if your panels are well grounded. Not sure how a B panel is secured. Frequently people will use the screw that holds the panel to the lower tubing to ground, and these will get loose and cause problems. Still unsure how the altitude would make this worse....so it may be a long shot.

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rockydoc Community Regular

rockydoc

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The problem is that the chassis ground and audio ground within the radio is different. The audio grounds coming from the connector are typically grouped together such that the audio panel, intercom and radios audio circuits have a common connection point to airframe ground. While this will ohm out the same as chassis, it does make a difference in performance. (Noise, volume, quality) In addition, ideally the headset plugs are isolated and the ground is run back to audio ground. The tube chassis is a good ground. The sheet metal panel is screwed to this. It it possible that the radio/audio ground is connected to the sheet metal, but the sheetmetal panel is not well connected to the tube. Check to see if your panels are well grounded. Not sure how a B panel is secured. Frequently people will use the screw that holds the panel to the lower tubing to ground, and these will get loose and cause problems. Still unsure how the altitude would make this worse....so it may be a long shot.

Well, verifying the existence of a good audio ground and headset plugs ground is a good starting point for me to troubleshoot.  Thanks for the detailed input.

Rocky

N601RX Grand Master

N601RX

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If its strictly a cooling problem, I would think that you should notice it getting better or at least happening at a higher altitude now that the weather has cooled down. If it is altitude related it may be an alternator arcing like Rob mentioned or a mag starting to misfire.  I think I would take it up to an altitude that causes the problem and try turning pulling the alternator main and field breaker.  If that doesn't help then I would try each mag separately or possibly even both mags. That will rule out 3 things that will be difficult to rule out on the ground.

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