Transmittion Noise - High Power Settings

We recently upgraded the avionics and instrument panel, which was a great
improvement. However, I'm having a problem transmitting under high
power. I get a crackly static (during transmittion only) when over say
20" MP and 2500 RPM or 25" MP and 2200 RPM. At the start of the
attempted transmittion there is a lot of static and then if I hold the
transmit button down the static slowly dies away after about 5 to 10
seconds but the transmittion is still garbled (it is not sync'd with the
engine RPM - it is either there or it isn't). With less than those power
settings the transmittion is totally clear with no crackling. All during
my recent IFR training we never had the problem, however, my instructor
was good about taking it easy on the engine and other than climbing we
were always at 20" MP and 2400 RPM or less power. I'm thinking we would
have noticed the problem under higher power settings and the problem
would have been intermittent.

I had this problem once in a while even before we did the avionics
upgrade. It again started about 5 hours ago and is now a persistent
problem that I need to get fixed. Which leads me to believe that there
is something else going on besides anything with the avionics. A
vibration issue that adversely impacts something? My instructor and I
both didn't notice any unusually bad vibration at high power.

This is the troubleshooting I have done so far:

1. Determined the power setting where the problem starts to happen
(over say 20" MP and 2500 RPM or 25" MP and 2200 RPM).

2. Shut off each mag and the problem persists (eliminates the mags?)

3. Shut down the audio panel to fail safe problem persists
(eliminates audio panel?)

4. Problem exists on both COM1 and COM2 (eliminates the antennas?)

5. Swapped COM1 and COM2 in the trays and problem persists
(eliminates a bad radio?)

6. Tried transmitting from both the pilot and copilot positions and
problem persists (eliminates microphone wiring and headsets?).

7. Cleaned up and fixed up the P-Leads on both Mags (they were in bad
shape and now even though they are the old wires, the shield connection
is good) - still have the problem.

8. Tightened the engine-grounding strap that was loose at the
firewall bolt. No change in the problem.

9. I purchased two Mag filters, however with my mags I cannot find a
good place to mount the filters so I have not installed them. Seems like
a band-aid as the problem was intermittent and is now persistent and I
would rather find the problem and fix it.

Other things I have not tried but I'm guessing could be the culprit is
loose spark plug wires, generator or maybe the voltage regulator -
however 20" MP and 2500" there is no problem so I'm not leaning towards
the generator as the problem.

I really need to get this fixed as there is no way I can blast off IFR in
the fog climbing and talk to ATC.

Thanks for anything you can suggest. Sorry for the novel. I just wanted
to make sure you had all the details.

Paul Smedshammer
1967 Mooney M20F, N3506X

Equipment: PMA6000MC Audio Panel, Narco COM1 810+, Narco COM2 810,
Nav1/DME/RNAV KNS-80, Nav2 Narco 825. Not state of the art but it works
great - other than the transmittion problem!

Paul Smedshammer wrote:
We recently upgraded the avionics and instrument panel, which was a great
improvement. However, I'm having a problem transmitting under high
power. I get a crackly static (during transmittion only) when over say
20" MP and 2500 RPM or 25" MP and 2200 RPM. At the start of the
attempted transmittion there is a lot of static and then if I hold the
transmit button down the static slowly dies away after about 5 to 10
seconds but the transmittion is still garbled (it is not sync'd with the
engine RPM - it is either there or it isn't). With less than those power
settings the transmittion is totally clear with no crackling. All during
my recent IFR training we never had the problem, however, my instructor
was good about taking it easy on the engine and other than climbing we
were always at 20" MP and 2400 RPM or less power. I'm thinking we would
have noticed the problem under higher power settings and the problem
would have been intermittent.

I had this problem once in a while even before we did the avionics
upgrade. It again started about 5 hours ago and is now a persistent
problem that I need to get fixed. Which leads me to believe that there
is something else going on besides anything with the avionics. A
vibration issue that adversely impacts something? My instructor and I
both didn't notice any unusually bad vibration at high power.

This is the troubleshooting I have done so far:

1. Determined the power setting where the problem starts to happen
(over say 20" MP and 2500 RPM or 25" MP and 2200 RPM).

2. Shut off each mag and the problem persists (eliminates the mags?)

3. Shut down the audio panel to fail safe problem persists
(eliminates audio panel?)

4. Problem exists on both COM1 and COM2 (eliminates the antennas?)

5. Swapped COM1 and COM2 in the trays and problem persists
(eliminates a bad radio?)

6. Tried transmitting from both the pilot and copilot positions and
problem persists (eliminates microphone wiring and headsets?).

7. Cleaned up and fixed up the P-Leads on both Mags (they were in bad
shape and now even though they are the old wires, the shield connection
is good) - still have the problem.

8. Tightened the engine-grounding strap that was loose at the
firewall bolt. No change in the problem.

9. I purchased two Mag filters, however with my mags I cannot find a
good place to mount the filters so I have not installed them. Seems like
a band-aid as the problem was intermittent and is now persistent and I
would rather find the problem and fix it.

Other things I have not tried but I'm guessing could be the culprit is
loose spark plug wires, generator or maybe the voltage regulator -
however 20" MP and 2500" there is no problem so I'm not leaning towards
the generator as the problem.

I really need to get this fixed as there is no way I can blast off IFR in
the fog climbing and talk to ATC.

Thanks for anything you can suggest. Sorry for the novel. I just wanted
to make sure you had all the details.

Paul Smedshammer
1967 Mooney M20F, N3506X

Equipment: PMA6000MC Audio Panel, Narco COM1 810+, Narco COM2 810,
Nav1/DME/RNAV KNS-80, Nav2 Narco 825. Not state of the art but it works
great - other than the transmittion problem!


I'd suspect a drop of electrical system voltage or power
feed to the radio.

One possibility is that the engine mounts let the engine
move a little and there is a ground strap that quits working
(loose or broken).

In a similar way, there may be a cable bundle pulling
a flaky contact.

--

Tauno Voipio
tauno voipio (at) iki fi

Defective squelch? (More noise at high power). Some of the new audio
panels have auto squelch which may need to be adjusted.

Sorry, Paul, I just realized you have the PMA6000, which I also have,
which has manual squelch.

And the squelch does not, I think, affect transmission, only cockpit
conversation.

Look for a loose or corroded ground from the radio stack to the
fuselage. You might be able to simulate the problem by wiggling the
harnesses while on the ground with the engine shut down.

Craig C.

Paul Smedshammer writes:



8. Tightened the engine-grounding strap that was loose at the
firewall bolt. No change in the problem.

Remove, clean & reinstall all other ground cables.


--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

Paul Smedshammer wrote in
8.29:


We recently upgraded the avionics and instrument panel, which was a
great improvement. However, I'm having a problem transmitting under
high power. I get a crackly static (during transmittion only) when
over say 20" MP and 2500 RPM or 25" MP and 2200 RPM. At the start of
the attempted transmittion there is a lot of static and then if I hold
the transmit button down the static slowly dies away after about 5 to
10 seconds but the transmittion is still garbled (it is not sync'd
with the engine RPM - it is either there or it isn't). With less than
those power settings the transmittion is totally clear with no
crackling. All during my recent IFR training we never had the
problem, however, my instructor was good about taking it easy on the
engine and other than climbing we were always at 20" MP and 2400 RPM
or less power. I'm thinking we would have noticed the problem under
higher power settings and the problem would have been intermittent.


Problem solved! Thanks everybody who responded. After really trying
everything, I gave it one last try last night and took off. Same
problem. So, in an effort to try and figure it out, I was a little more
aggressive pushing in the radios trying to seat them deeper in their
trays. Well, nothing worked by just by chance my fingers hit the PMA6000
audio panel and the problem immediately went away! Taking my fingers
off, the problem came right back. It seems that even though the PMA6000
is seated all the way back in the tray and can't go any further (the
faceplate isn't touching the airframe it is only on the tray), it can
still move a little up and down in front. The higher power settings just
seem to cause a change in the vibration enough to cause the problem.
Just a very slight push with one finger on the front of the panel and the
problem goes away. Back on the ground I could replicate the problem by
pushing on the PMA faceplate softly and putting pressure up and down on
the faceplate. I will call PMA for suggestions on fixing or if it could
be something wrong in the unit.

This was a highly frustrating problem. We checked everything from the
battery to the engine mounts and everything in-between. It is so great
to finally find the problem and now we can fix it! Whew! Thanks again
everybody.

Paul Smedshammer