Radio "Squelch-type" Noise

[Kensandyeggo]

Have an unusual problem with a new radio, an ICOM panel mount. Nothing
wrong with the radio, but I'm getting a squelch/static noise at certain
times. It is worse when both mags are on and when the engine goes from
about 2200 rpm down to idle. It is less on one mag and practically
non-existent on the other. It sounds like someone is breaking squelch.

A local radio-guy suggested 2 new "P-leads?" I wonder if he meant
capacitors, which is what the previous owner suggested replacing.
Spruce and Chief have mag noise filters at about $45 each. AmeriKing
has a DC filter for over $100. I ordered 2 capacitors for about 10
bucks each, but there are others for close to 50 bucks each. Anyone
have any experience with this stuff? I figured if I'm going to waste
my money, I may as well try the cheapest things first. Radio is in a
craft with a Lyc O-360 w/Bendix mags. Thanks.

[mikem]

You are experiencing radio-frequency-interference (RFI) being radiated
from a P-lead directly to the COM antenna, or RFI radiated from one or
more of the shielded HV ignition harness leads running from the mag to
the spark plugs.

The ICOM sees the RFI noise pulses at its antenna input, and thinks it
is a valid signal and opens the audio squelch, so you hear it. If the
ICOM had an manual squech control, you could probably just set the
squelch tighter...

There are coaxial RFI bypass capacitors built in to the mags. They are
the first line of defense in suppressing P-lead radiation. The P-lead
running from the stud on the mag through the firewall to the Mag Switch
must be shielded (unbroken), and the P-lead shield MUST be connected to
the body of the mag at one end, and to airframe ground under the panel
near the Mag switch at the other. Rebuilding the mags with new
capacitors may help.

The HV spark leads must have a continuous shield from mag to plug. The
plug end gland nut is aluminum, and sometimes it makes a bad connection
to the outside of the plug. The other end is bonded to the shield over
the HV tower inside the mag. The usual cure for this problem is a new
set of ignition harnesses.

Do not screw with putting external capacitors in the P-leads. The
internal mag capacitors form a series-resonant circuit with the coil
primary, just like an old point and condensor ignitions of old (up to
1960s cars). Putting external capacitors (especially the ones you are
talking about) will greatly degrade the spark energy.

[nrp]

You DON'T want to add any capacitors to the P leads!

Have you adjusted the gain, squelch and mike gain (I think that's the
nomenclature) on your I-Com radio? The I-com I have is a very
sensitive radio. You just may have the squelch incorrectly set (I. e.
too sensitive) for your installation. You have to remove the radio
from the tray to get access to these 3 controls.

A key point - do you get mag noise overriding on normal radio receive
signals? or just when the radio breaks squelch? That would tell you if
the mags etc are radiating excessive noise.

[Kensandyeggo]

Thanks for the great replies. The ICOM radio's squelch is a part of
the volume control. You pull it out and hear the squelch and turning
the knob just adjusts the volume of it. If you push it in the squelch
noise stops. Doen't seem to me that the squelch is really controllable
except to change the volume of it, which also changes the radio volume.

I never knew about a squelch and gain controls on the back of the
radio. I don't have the manual with me. I'm now at a remote airstrip
where there is hardly any radio traffic, so I'll have to get someone to
transmit while I try it out.

I called Aircraft Spruces radio shop and got a little feedback from a
tech. He said to first check out my spark plug connections. He said
loosen them one by one and then retighten them. He said that's a
common cause of noise. And now that I think of it, I had the CHT wire
break and to fix it, one spark plug had to be removed, so I better
start with that one. I guess I could actually disconnect that one
plug and run the engine to see if the noise disappears. If that
doesn't work, he recommends a new ignition harness, which isn't too
painfully expensive. Thanks again for the help, especially forgetting
about using an external capacitor. Is that what those mag filters
basically are?

[Dave Stadt]

"Kensandyeggo" > wrote in message
oups.com...
> Thanks for the great replies. The ICOM radio's squelch is a part of
> the volume control. You pull it out and hear the squelch and turning
> the knob just adjusts the volume of it. If you push it in the squelch
> noise stops. Doen't seem to me that the squelch is really controllable
> except to change the volume of it, which also changes the radio volume.

The radio has adjustable squelch. The adjustment is on top of the radio
IIRC and should have a piece of tape over the adjustment hole. There is
also a sidetone adjustment so make sure you get the correct one.

[Travis Marlatte]

Pulling the knob out is the override for the squelch. That's not your
problem. By pulling the volume knob out, you are simply disabling the
squelch so that you hear whatever is being received. That would be a lot of
static with no real signal for the receiver to latch on to.

Turning that knob adjusts the volume of the received signal regardless of
whether the knob is in or out.

A typical scenario is to listen to ATIS, contact ground, contact tower, etc.
Listening to ATIS allowed you to make sure that your volume was up high
enough to hear responses from the tower. But, if you were at an airport that
did not have an ATIS, you might pull the knob out to make sure that the
volume is up high enough to hear any responses on CTAF as you leave the
area. Adjust the volume so that the static is at a tolerable level, then
push it in.

Or, if you are trying to pick up a weak signal (far away ATIS or ATC), you
can pull the knob out so that you hear whatever is transmitted as best you
can. Of course, there will be a lot of noise as well. If the received signal
is strong enough to quiet the noise, then it is probably strong enough to
break squelch too.

With the knob in for normal operation, the squelch is only broken when the
received signal exceeds the set squelch. If you are in a noisy area or your
installation is noisy, the squelch level needs to be adjusted. nrp posted
the correct information. The limit is set by a "hidden" adjustment.

For a demonstration of this, the ICOM and Vertex handheld radios have
similar capabilities. I'm sure others do too. I just know that those do.
There is a button on the side that can temporarily disable the squelch. To
change the squelch threshold, you have to go into the settings mode.

Ideally, you would adjust the squelch level down so that the normal noise
level breaks the squelch. Then take the squelch level up a number or two.
You want a quiet radio except when there is a transmission that is strong
enough to break squelch and be heard clearly.

If a transmission breaks the squelch but is too weak to be heard clearly,
then the squelch is probably too low. That goes for the handhled and for the
panel mounted. That sounds like what you are describing. Although, just
increasing the squelch level will also supress weak transmissions that you
might want to hear. Trying to make the installed environment as quiet as
possible is a good goal.

Time to get out a hex wrench and a jeweler's screwdriver...

-------------------------------
Travis
Lake N3094P
PWK


"Kensandyeggo" > wrote in message
oups.com...
> Thanks for the great replies. The ICOM radio's squelch is a part of
> the volume control. You pull it out and hear the squelch and turning
> the knob just adjusts the volume of it. If you push it in the squelch
> noise stops. Doen't seem to me that the squelch is really controllable
> except to change the volume of it, which also changes the radio volume.
>
> I never knew about a squelch and gain controls on the back of the
> radio. I don't have the manual with me. I'm now at a remote airstrip
> where there is hardly any radio traffic, so I'll have to get someone to
> transmit while I try it out.
>
> I called Aircraft Spruces radio shop and got a little feedback from a
> tech. He said to first check out my spark plug connections. He said
> loosen them one by one and then retighten them. He said that's a
> common cause of noise. And now that I think of it, I had the CHT wire
> break and to fix it, one spark plug had to be removed, so I better
> start with that one. I guess I could actually disconnect that one
> plug and run the engine to see if the noise disappears. If that
> doesn't work, he recommends a new ignition harness, which isn't too
> painfully expensive. Thanks again for the help, especially forgetting
> about using an external capacitor. Is that what those mag filters
> basically are?
>

[[email protected]]

Travis Marlatte > wrote:
: If a transmission breaks the squelch but is too weak to be heard clearly,
: then the squelch is probably too low. That goes for the handhled and for the
: panel mounted. That sounds like what you are describing. Although, just
: increasing the squelch level will also supress weak transmissions that you
: might want to hear. Trying to make the installed environment as quiet as
: possible is a good goal.

: Time to get out a hex wrench and a jeweler's screwdriver...

Not necessarily a good idea. I thought I'd be clever and adjust the squelch
on my KY-197 to crank it up a bit. Turns out there were two pots, and they were very
sensitive to adjustment (read: I screwed it all up and couldn't get it back
reasonable again). I had to obtain the relevant pages in the service manual, put it
on the bench with an RF signal generator and some attenuators to get the adjustment
back within spec. It's still a little bit twitchy and sensitive to noise and opening
squelch, but I *know* it's adjusted to the book now and I'm not going to touch it
anymore.

Other radios may be more forgiving, but I know this one wasn't. You may be
wishing you hadn't fiddled with that particular "knob."

-Cory


--

************************************************** ***********************
* Cory Papenfuss *
* Electrical Engineering candidate Ph.D. graduate student *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

[nrp]

The capacitors in the mag filters are there to form a resonant spark
(electrical) tank circuit with the inductance of the magneto coils and
protect the ma=gneto points. If you hang additional capacitors on the
P leads, the mag functions will get completely screwed up as the
resonant frequency of the spark tank system will be depressed - if they
even work at all.

I should have mentioned that the three controls I noted are accessible
thru the top cover of the radio once it is removed. Mess with it
gradually and carefully.

The ICom radio is very sensitive. Even with my squelch setting
modified I'm still picking up transmissions from over 125 miles away.
I wish the squelch was more user real time accessible.