Tough noise problem in Arrow radios

karl mcgruber wrote:

Take out the ELT and see what happens. Sometimes they will cause
problems. Mine did.


Did it, no change. I strongly suspect that it is a new noise getting in
by way of a fault (perhaps in the ground system) that has always been
there. Looks like there are two choices: rewire the stack going by the
install manual for the KMA-24 to hopefully eliminate the entry point
into the audio system, or find the source of the noise and eliminate it.
Neither is particularly attractive, which is probably why the avionics
shops don't want to touch it.

Ray -

What type A/C and engine? If you have a belt driven alternator, drive
it with an hand held electric motor & see if you can get the noise.
Might it be a mechanical voltage regulator points chattering (which
they do all the time), but without any of the arc suppression ballast
resistors having continuity on the back side of the regulator, in
which case they could radiate a lot of noise.

A better description of the noise would really help. Can I assume it
is not magneto related? (i. e. have you tried operation on each
mag?)

Thanks for your response.

We flew it with the alternator field turned off, no change, which I
think rules out the charging system. I'll get him to do an in-flight
mag check when he gets it out of annual to see if it goes away on one
set or the other, good idea.

The noise is an impulse noise, could be ignition noise except it doesn't
seem to change at all with RPM. It does seem to be related to vibration
however, so I suppose a marginal ignition lead could do it. It comes
across as bursts of static. Not sure really how to describe it.


nrp wrote:

Ray -

What type A/C and engine? If you have a belt driven alternator, drive
it with an hand held electric motor & see if you can get the noise.
Might it be a mechanical voltage regulator points chattering (which
they do all the time), but without any of the arc suppression ballast
resistors having continuity on the back side of the regulator, in
which case they could radiate a lot of noise.

A better description of the noise would really help. Can I assume it
is not magneto related? (i. e. have you tried operation on each
mag?)

Ray Andraka wrote:

We flew it with the alternator field turned off, no change, which I
think rules out the charging system. I'll get him to do an in-flight
mag check when he gets it out of annual to see if it goes away on one
set or the other, good idea.

The noise is an impulse noise, could be ignition noise except it doesn't
seem to change at all with RPM. It does seem to be related to vibration
however, so I suppose a marginal ignition lead could do it. It comes
across as bursts of static. Not sure really how to describe it.
\

Simple things first. It seems to me that you have a vibration induced
bad connection somewhere. With engine off, master and avionics on, while
listening to the headphones:

Bash the panel alongside the COMs with a plastic screwdriver handle.

Thump the audio panel.

Thump the panel near where the mic/headphone jacks are mounted.

Reach up under the panel and flex the wiring bundles.

Twang the antennas.




Some likely non-vibration induced sources that you may not have thought of:

Disconnect the plug(s) at the rear of the turn coordinator or electric
gyro, or if you have pullable breakers, turn them off.

Do you have a motorized beacon? Turn it off.

Turn off the transponder.

MikeM wrote:

Turn off the transponder.

Turn off the avionics fan (if you have one)

OK, looks like a mystery solved. We found a power lead to the KMA-24
that was chafing on the back of one of the other trays, and had worn far
enough to intermittently short, but apparently not long enough to pop
the breaker. It did it enough to leave carbon on the wire insulation at
the site of contact. That was repaired, and seems to have fixed the
problem. Another one for the books I guess. Thanks to all who chimed in.