"scratchy" radio transmission

I am looking for ideas here; ATC says my radio transmissions sound
'scratchy' or 'staticy' as little as 15 miles out on callup. I have
been going nuts trying to find the cause. The aircraft has 2
independant radios, 2 antenna systems (one on top of the fuselage, one
underneath). One radio replaced as an update recently. Both radios have
been checked out by the factories recently and pronounced fine (one
Narco Mk12D and one Icom). Been through the wiring and grounds several
times and found nothing. Tried different headsets (note: the voice
sideband through the headset sounds perfect). ATC said no difference
with alternator shut off. Reception is always perfect. Complaints seem
to pick up more in the cold weather. Don't know if its important but
maybe once or twice each summer I get alot of static in the headset
that seems to disapate in a minute or two, but never in the winter.
If I were generating a large static charge in the dry winter air
wouldn't it effect my reception? Some days I get complaints even when
taxiing for takeoff. Any ideas what to try next without going on an
expensive fishing expedition?

Jim

Jim,

Could be lots of things, but I'd be willing to bet a cold beer that
its in the audio input path to your radio, and not between the radio
and the antenna given that you receive fine.

Here's a few thoughts that _might_ help:

Loose/broken wire in mic - Try a different microphone
Oxidation in mic jack - Clean both the male and female jacks
Loose/broken wire between mic jack and radio
Cold solder joint in either mic, jack, etc
PTT switch starting to oxidize internally

Good luck finding the problem.

Don W.

wrote:
I am looking for ideas here; ATC says my radio transmissions sound
'scratchy' or 'staticy' as little as 15 miles out on callup. I have
been going nuts trying to find the cause. The aircraft has 2
independant radios, 2 antenna systems (one on top of the fuselage, one
underneath). One radio replaced as an update recently. Both radios have
been checked out by the factories recently and pronounced fine (one
Narco Mk12D and one Icom). Been through the wiring and grounds several
times and found nothing. Tried different headsets (note: the voice
sideband through the headset sounds perfect). ATC said no difference
with alternator shut off. Reception is always perfect. Complaints seem
to pick up more in the cold weather. Don't know if its important but
maybe once or twice each summer I get alot of static in the headset
that seems to disapate in a minute or two, but never in the winter.
If I were generating a large static charge in the dry winter air
wouldn't it effect my reception? Some days I get complaints even when
taxiing for takeoff. Any ideas what to try next without going on an
expensive fishing expedition?

Jim

Knowing what kind of aircraft it is would help a bit.

Jim




wrote in message
ups.com...
I am looking for ideas here; ATC says my radio transmissions sound
'scratchy' or 'staticy' as little as 15 miles out on callup. I have
been going nuts trying to find the cause. The aircraft has 2
independant radios, 2 antenna systems (one on top of the fuselage, one
underneath). One radio replaced as an update recently. Both radios have
been checked out by the factories recently and pronounced fine (one
Narco Mk12D and one Icom). Been through the wiring and grounds several
times and found nothing. Tried different headsets (note: the voice
sideband through the headset sounds perfect). ATC said no difference
with alternator shut off. Reception is always perfect. Complaints seem
to pick up more in the cold weather. Don't know if its important but
maybe once or twice each summer I get alot of static in the headset
that seems to disapate in a minute or two, but never in the winter.
If I were generating a large static charge in the dry winter air
wouldn't it effect my reception? Some days I get complaints even when
taxiing for takeoff. Any ideas what to try next without going on an
expensive fishing expedition?

Jim

Full metal jacketed RV-4

If it was a broken/loose wire in the mic circuit I would think I would
not get a good sidetone in the headset. Have seriously cleaned both of
the mic jacks but now that you mention it I'll bet it could be the PTT
button (duh!). I probably need to make a debounce circuit or something
if possible. At least I could try taking a passenger to try the rear
xmit button. Thanks for the "wake up call".

Jim

If, as you have told us, the sidetone is clear and loud, then the odds of it
being in the microphone/jacks/PTT switch are nearly zero. My bet is an RF
problem of some sort.

And why would you consider some sort of "debounce" circuit? DO you have a
clue what a debounce circuit does?

Jim




wrote in message
oups.com...
If it was a broken/loose wire in the mic circuit I would think I would
not get a good sidetone in the headset. Have seriously cleaned both of
the mic jacks but now that you mention it I'll bet it could be the PTT
button (duh!). I probably need to make a debounce circuit or something
if possible. At least I could try taking a passenger to try the rear
xmit button. Thanks for the "wake up call".

Jim

I would try replacing the coax from Radio to antenna in ONE piece. You
might first try just loosening and tightening the connections. If you have
your coax all tie-rapped down, Just leave it and see if a new coax loosely
installed does the job. Tie-rapping too tightly can damage the coax. Many
times it is just bad ends.

--
Cy Galley - Chair,
AirVenture Emergency Aircraft Repair
A Service Project of Chapter 75
EAA Safety Programs Editor - TC
EAA Sport Pilot
wrote in message
ups.com...
I am looking for ideas here; ATC says my radio transmissions sound
'scratchy' or 'staticy' as little as 15 miles out on callup. I have
been going nuts trying to find the cause. The aircraft has 2
independant radios, 2 antenna systems (one on top of the fuselage, one
underneath). One radio replaced as an update recently. Both radios have
been checked out by the factories recently and pronounced fine (one
Narco Mk12D and one Icom). Been through the wiring and grounds several
times and found nothing. Tried different headsets (note: the voice
sideband through the headset sounds perfect). ATC said no difference
with alternator shut off. Reception is always perfect. Complaints seem
to pick up more in the cold weather. Don't know if its important but
maybe once or twice each summer I get alot of static in the headset
that seems to disapate in a minute or two, but never in the winter.
If I were generating a large static charge in the dry winter air
wouldn't it effect my reception? Some days I get complaints even when
taxiing for takeoff. Any ideas what to try next without going on an
expensive fishing expedition?

Jim

It is odd that he has a clear "sidetone", receives fine, and yet the
transmission is scratchy (apparently on both radios?). Also that both
radios apparently benchcheck fine.

Jim, do you have an audio mix panel, and if so does the mic input go
to it before splitting to the radios? Also, I read your original post
pretty quickly and didn't notice the part where you said you were
hearing the sidetone okay. That eliminates a lot of potential failure
points.

Another thought: Do you have an "intercom" that both mics and the PTTs
connect to before the mic audio gets to the radios? If so, the intercom
could be the culprit. There are so many ways that your aircraft could
be wired up that its hard for me to think of all of them. You could
probably get to the bottom of it all pretty quickly with an oscilloscope
if you have one and know how to use it.

RST, if both radios have the same static problem (with their different
RF XMIT sections, T/R switches, and antennas), it is not very likely an
RF problem.

OTOH, it could be a DC voltage problem, as in the DC voltage supply to
the radios is too low, causing the transmit power to be less than it
should.

Jim, have you tried both radios and verified that ATC hears them both as
scratchy?

Don W.

RST Engineering wrote:

If, as you have told us, the sidetone is clear and loud, then the odds of it
being in the microphone/jacks/PTT switch are nearly zero. My bet is an RF
problem of some sort.

And why would you consider some sort of "debounce" circuit? DO you have a
clue what a debounce circuit does?

Jim




wrote in message
oups.com...

If it was a broken/loose wire in the mic circuit I would think I would
not get a good sidetone in the headset. Have seriously cleaned both of
the mic jacks but now that you mention it I'll bet it could be the PTT
button (duh!). I probably need to make a debounce circuit or something
if possible. At least I could try taking a passenger to try the rear
xmit button. Thanks for the "wake up call".

Jim

Bought a homebuilt once. Both radio antennas had been installed the same
way - with the cardboard drilling pattern glued to the aluminum - then left
there, and the antenna installed over the top of it. No ground plane.

The SWR was OK (1.8:1), believe it or not. It worked out to 10 miles or
so - then things started getting weak and scratchy.

Even a fancy Anritsu test set I borrowed from work showed no great problem.

But when I removed those drill patterns, things started working fine.

And why would you consider some sort of "debounce" circuit? DO you have a
clue what a debounce circuit does?


Jim

A friend at work told me it is like an electronic switch that "locks on
and off" to prevent a poor or intermittant connection. Maybe I don't
have a correct clue about it but I have the utmost confidence someone
will straighten me out on the subject. Thats why I came here; to learn.

Jim