Radio squeal

I know this subject has come up often in the past and I have checked
back on most the old postings for a cure but still don't seem to be
able to solve the problem.

The aircraft is a Koala (Fisher Products, all wood and fabric). Radio
fitted is a Narco Com 810 with Telex headsets. (I know it's not the
best but I have to make do with what I got here).

The setup squeals intermittently when in TX mode. When I say
intermittently, I mean sometimes its quiet, depending on which
position I'm sitting in or whether I'm moving my arms or depending on
whether the headset cable is draped over my shoulder or lying in my
lap or lying on the seat beside me. If I touch it, it squeals. If I
move it around, it stops. If I touch the coaxial cable it stops
squealing, or starts, whichever way round. It's as if its acting like
an antenna emitting a signal when its being waved around.

This is what I have tried already:
1) re-made the antenna with new coaxial cable and fittings.
2) made up a new groundplane for the antenna.
3) checked the VSWR with a 2 meter ham SWR meter. The reading is just
a touch over 1.5:1 on 122.600 which is my airfield's frequency
4) checked all the ground connections and fitted .01µF ceramic disk
capacitors from the audio line to ground and from the mike line to
ground.
5) fitted a small 470µH RF choke to both the mike and audio lines.
6) replaced the headset mike and audio lines with new, individually
shielded cables.
7) de-soldered and disconnected the microphone from the circuit inside
the earcups.
8) de-soldered and disconnected the headphone speakers inside the
earcups.
9) re-routed the coaxial cable.
10) tried coiling the coaxial cable
11) I have NOT tried another radio, because that will probably involve
a rewiring job on the panel.
12) everything works off a 12V DC power supply (automobile battery)

What seemed to work fine was when I plugged a "rubber ducky" antenna
straight into the radio, thereby eliminating the length of coaxial
cable which runs to the roof of the aircraft. When the "rubber ducky"
is connected to the outside of the aircraft, on the roof, in place of
the proper antenna, it squeals again, which tells me it has something
to do with the coaxial cable and heatset cables radiating, inducing,
feedback,emitting (whatever the right word is) into things.

Anyone got some ideas I haven't tried yet?

Gerrie

Gerrie wrote:

I know this subject has come up often in the past and I have checked
back on most the old postings for a cure but still don't seem to be
able to solve the problem.

The aircraft is a Koala (Fisher Products, all wood and fabric). Radio
fitted is a Narco Com 810 with Telex headsets. (I know it's not the
best but I have to make do with what I got here).

The setup squeals intermittently when in TX mode. When I say
intermittently, I mean sometimes its quiet, depending on which
position I'm sitting in or whether I'm moving my arms or depending on
whether the headset cable is draped over my shoulder or lying in my
lap or lying on the seat beside me. If I touch it, it squeals. If I
move it around, it stops. If I touch the coaxial cable it stops
squealing, or starts, whichever way round. It's as if its acting like
an antenna emitting a signal when its being waved around.

This is what I have tried already:
1) re-made the antenna with new coaxial cable and fittings.
2) made up a new groundplane for the antenna.
3) checked the VSWR with a 2 meter ham SWR meter. The reading is just
a touch over 1.5:1 on 122.600 which is my airfield's frequency
4) checked all the ground connections and fitted .01µF ceramic disk
capacitors from the audio line to ground and from the mike line to
ground.
5) fitted a small 470µH RF choke to both the mike and audio lines.
6) replaced the headset mike and audio lines with new, individually
shielded cables.
7) de-soldered and disconnected the microphone from the circuit inside
the earcups.
8) de-soldered and disconnected the headphone speakers inside the
earcups.
9) re-routed the coaxial cable.
10) tried coiling the coaxial cable
11) I have NOT tried another radio, because that will probably involve
a rewiring job on the panel.
12) everything works off a 12V DC power supply (automobile battery)

What seemed to work fine was when I plugged a "rubber ducky" antenna
straight into the radio, thereby eliminating the length of coaxial
cable which runs to the roof of the aircraft. When the "rubber ducky"
is connected to the outside of the aircraft, on the roof, in place of
the proper antenna, it squeals again, which tells me it has something
to do with the coaxial cable and heatset cables radiating, inducing,
feedback,emitting (whatever the right word is) into things.

Anyone got some ideas I haven't tried yet?

Gerrie

What you did is good. It's been too long since I've done this sort of
work on aircraft for me to feel even remotely qualified; but have you
tried another headset? Microphone pickup from speakers or earpeice(s)
of a headset will always be a common source.

Peter

"Gerrie" wrote in message
om...
I know this subject has come up often in the past and I have checked
back on most the old postings for a cure but still don't seem to be
able to solve the problem.

The aircraft is a Koala (Fisher Products, all wood and fabric). Radio
fitted is a Narco Com 810 with Telex headsets. (I know it's not the
best but I have to make do with what I got here).

The setup squeals intermittently when in TX mode. When I say
intermittently, I mean sometimes its quiet, depending on which
position I'm sitting in or whether I'm moving my arms or depending on
whether the headset cable is draped over my shoulder or lying in my
lap or lying on the seat beside me. If I touch it, it squeals. If I
move it around, it stops. If I touch the coaxial cable it stops
squealing, or starts, whichever way round. It's as if its acting like
an antenna emitting a signal when its being waved around.

This is what I have tried already:
1) re-made the antenna with new coaxial cable and fittings.
2) made up a new groundplane for the antenna.
3) checked the VSWR with a 2 meter ham SWR meter. The reading is just
a touch over 1.5:1 on 122.600 which is my airfield's frequency
4) checked all the ground connections and fitted .01µF ceramic disk
capacitors from the audio line to ground and from the mike line to
ground.
5) fitted a small 470µH RF choke to both the mike and audio lines.
6) replaced the headset mike and audio lines with new, individually
shielded cables.
7) de-soldered and disconnected the microphone from the circuit inside
the earcups.
8) de-soldered and disconnected the headphone speakers inside the
earcups.
9) re-routed the coaxial cable.
10) tried coiling the coaxial cable
11) I have NOT tried another radio, because that will probably involve
a rewiring job on the panel.
12) everything works off a 12V DC power supply (automobile battery)

What seemed to work fine was when I plugged a "rubber ducky" antenna
straight into the radio, thereby eliminating the length of coaxial
cable which runs to the roof of the aircraft. When the "rubber ducky"
is connected to the outside of the aircraft, on the roof, in place of
the proper antenna, it squeals again, which tells me it has something
to do with the coaxial cable and heatset cables radiating, inducing,
feedback,emitting (whatever the right word is) into things.

Anyone got some ideas I haven't tried yet?

Gerrie

Just for clarification, are we talking about squeal in the headphones when
you transmit or squeal in the signal that is being transmitted (and heard by
others over the air)?

Normally, microphone and headphone jacks are NOT grounded to the airframe
ground. The common wires run back to the radio connections and the radio IS
grounded. If you use shielded wires for the connections from the headphone
jacks to the radios, the shields should be grounded at the radio end ONLY
and not at both ends.

Rick Pellicciotti

On Tue, 05 Aug 2003 13:25:15 GMT, Peter Dohm
wrote:


What you did is good. It's been too long since I've done this sort of
work on aircraft for me to feel even remotely qualified; but have you
tried another headset? Microphone pickup from speakers or earpeice(s)
of a headset will always be a common source.

Peter

I have disconnected the microphone and removed the earpieces
altogether!

Gerrie

On Tue, 5 Aug 2003 09:23:49 -0500, "Rick Pellicciotti"
wrote:


Just for clarification, are we talking about squeal in the headphones when
you transmit or squeal in the signal that is being transmitted (and heard by
others over the air)?

Normally, microphone and headphone jacks are NOT grounded to the airframe
ground. The common wires run back to the radio connections and the radio IS
grounded. If you use shielded wires for the connections from the headphone
jacks to the radios, the shields should be grounded at the radio end ONLY
and not at both ends.

I get the squeal on both ends. In my headset and on a remote receiver.

I have removed the earpieces from my headset and disconnected the
microphone, but can still hear the squeal on the remote receiver. The
remote receiver is about 150 ft away. When I stand next to the cabin
or sit in it for that matter, I can make the squealing come and go by
moving my arms about or adjusting my body position.

The microphone and headset jacks are not grounded to the aircraft
ground and I am using shielded cable connected to the radio at the
connector marked "speaker, mike and phones ground return"

And yes, the shields are grounded only at the radio end and not at
both ends.

Been there, done that, got a wardrobe full of teeshirts!

Gerrie