Cherokee Strobe Noise

I know that this has been beaten to death, and I have searched the
archives, but I'm trying to narrow my specific case down a little bit.

I have a Cherokee with factory-installed wing tip strobes. I have an
original (1977) King Silver Crown avionics package, including audio
panel, KX170Bs, etc. I also have a Sigtronics panel-mount 4 place
intercom that was installed at some point before I purchased the
plane... I believe it is a SPA-400.

Everything is fine, except that the wing tip strobes can be heard like a
siren in the headsets. The siren sound is also occasionally transmitted
over the radio when we make transmissions, because I have been told to
"check my ELT" after making transmissions. Here are my observations:

1 -- Noise is present with intercom on or off.

2 -- Noise is WORSE when any radio, but especially marker beacon, is
selected on the audio panel.

3 -- When all radios are unselected on the audio panel (i.e. nothing
selected to either speaker or headsets) noise goes from very noticeable
to barely noticeable.

4 -- Noise is much WORSE when things are "cold." After about 20 minutes
of flying time, the noise is still noticeable, but not nearly as bad as
it was when the flight began.

5 -- Unless I'm hearing things, which is possible, I can SOMETIMES hear
the noise through the headset with the strobes on but avionics bus OFF.

I am not sure whether the strobe power pack is crying out for help,
whether I have a ground problem elsewhere in the system, or whether I
have an improper installation of something (like the intercom), or
whether the audio panel is shot.

The strobes were factory-installed, so I assume that the installation of
the strobes is correct. The SPA-400 intercom was obviously not factory
installed, and I am not convinced that insulating washers were used on
the headset plugs, so I do need to check that out. I do know that
shielded cable was NOT used on the intercom installation.

Does anyone have any ideas? I've basically narrowed it down to three
things, in no particular order:

1 -- intercom
2 -- audio panel
3 -- strobe power pack


Thanks,
JKG

On Wed, 27 Oct 2004 00:43:50 GMT, Jonathan Goodish
wrote:

I know that this has been beaten to death, and I have searched the
archives, but I'm trying to narrow my specific case down a little bit.

I have a Cherokee with factory-installed wing tip strobes. I have an
original (1977) King Silver Crown avionics package, including audio
panel, KX170Bs, etc. I also have a Sigtronics panel-mount 4 place
intercom that was installed at some point before I purchased the
plane... I believe it is a SPA-400.

Everything is fine, except that the wing tip strobes can be heard like a
siren in the headsets. The siren sound is also occasionally transmitted
over the radio when we make transmissions, because I have been told to
"check my ELT" after making transmissions. Here are my observations:

1 -- Noise is present with intercom on or off.

Conclusion, it is not coming in within the intercom.

2 -- Noise is WORSE when any radio, but especially marker beacon, is
selected on the audio panel.

The noise is coupling into the marker beacon audio. Is the marker
beacon ground connection the same point as the strobe ground? Is the
marker beacon audio lead shielded wire?

3 -- When all radios are unselected on the audio panel (i.e. nothing
selected to either speaker or headsets) noise goes from very noticeable
to barely noticeable.

Sounds like a nosy ground problem.

4 -- Noise is much WORSE when things are "cold." After about 20 minutes
of flying time, the noise is still noticeable, but not nearly as bad as
it was when the flight began.

Sounds even more like a loose ground problem. Aluminum srinks more
with cold temperatures than the steel screw that holds the wire to the
airframe.

5 -- Unless I'm hearing things, which is possible, I can SOMETIMES hear
the noise through the headset with the strobes on but avionics bus OFF.

If the ground resistance were bad enough it would do this.

I am not sure whether the strobe power pack is crying out for help,
whether I have a ground problem elsewhere in the system, or whether I
have an improper installation of something (like the intercom), or
whether the audio panel is shot.

Find out where the ground of the strobe pack is connected to the
airframe and clean it.

The strobes were factory-installed, so I assume that the installation of
the strobes is correct.

Don't bet on this.

The SPA-400 intercom was obviously not factory
installed, and I am not convinced that insulating washers were used on
the headset plugs, so I do need to check that out.

Unless you have a large current through the airframe or have some poor
ground points insulating the jacks may be a wasted effort. If it does
work you need to look at the ground connections for the radios ,
intercom and audio panel because they will almost certainly be not
tight and will be high resistance.

I do know that shielded cable was NOT used on the intercom installation.

VERY VERY BAD!!! This is how the noise is coupling from the strobe
power leads on to the audio leads if they are in the same wire bundle.
ALL of the audio leads MUST be shielded wire.

Does anyone have any ideas? I've basically narrowed it down to three
things, in no particular order:

1 -- intercom
2 -- audio panel
3 -- strobe power pack

A big 1000uf cowpasture on the strobe power pack input power may help.

Thanks,
JKG

Find a avionics shop that knows what they are doing and have them fix
it. They are very hard to find and generally will NOT have the lowest
rates but they will generally save you money.

John

In article ,
(John_F) wrote:
2 -- Noise is WORSE when any radio, but especially marker beacon, is
selected on the audio panel.

The noise is coupling into the marker beacon audio. Is the marker
beacon ground connection the same point as the strobe ground? Is the
marker beacon audio lead shielded wire?

I don't know the answer to either question off-hand, but I do know that
the marker beacon receiver is on the shelf in the tail that is not too
far from the strobe power pack.



VERY VERY BAD!!! This is how the noise is coupling from the strobe
power leads on to the audio leads if they are in the same wire bundle.
ALL of the audio leads MUST be shielded wire.

Yes, I know that this is bad, but it probably could be worse (and
remember, I'm not responsible for the intercom installation). The leads
to and from the intercom don't appear to be bundled with the strobe
power leads, or even close to them. I am hoping that this isn't the big
problem, and I can let this ride until I have the interior replaced and
then replace the wiring at that point.



Find a avionics shop that knows what they are doing and have them fix
it. They are very hard to find and generally will NOT have the lowest
rates but they will generally save you money.

I don't want to give someone a blank check to troubleshoot a problem
without some idea of what it may be. In other words, I need to try to
troubleshoot as much as possible before engaging the avionics shop for
more in-depth troubleshooting. I do believe that I have a good avionics
shop in the local area, and they've done work for me before, but I
haven't classified this problem as more than a nuisance at this point.
However, it may be getting worse, and I certainly don't want it to reach
a point where it becomes a show-stopper.


JKG

On Wed, 27 Oct 2004 00:43:50 GMT, Jonathan Goodish
wrote:

Everything is fine, except that the wing tip strobes can be heard like a
siren in the headsets.

Sounds like a problem I have had with a Whelen power pack.

Can you hear the sound at the power pack? I could.

When I called Whelen, I was told that the problem was a bad capacitor.
They told me which capacitor to replace. I replaced it and it worked fine
for a few years, although the noise is just now starting to come back.

The power pack, however, is riveted closed. So I had to drill out the
rivets, replace the electrolytic capacitor, and put the box back together.
Took about 10 minutes on the phone with Whelen tech support; 30 minutes of
labor and a $2.30 capacitor. Needed some pop rivets, too, to put the box
back together.



Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

Jonathan,
Do you have a capacitor across your alternator output? If so that may be
bad as well. The last one about the capacitor in the unit sound likely
as well. The repair though sounds a little well not completely legal
unless they are an A&P and have the manual that says they can replace
the part.
Michelle

Jonathan Goodish wrote:

I know that this has been beaten to death, and I have searched the
archives, but I'm trying to narrow my specific case down a little bit.

I have a Cherokee with factory-installed wing tip strobes. I have an
original (1977) King Silver Crown avionics package, including audio
panel, KX170Bs, etc. I also have a Sigtronics panel-mount 4 place
intercom that was installed at some point before I purchased the
plane... I believe it is a SPA-400.

Everything is fine, except that the wing tip strobes can be heard like a
siren in the headsets. The siren sound is also occasionally transmitted
over the radio when we make transmissions, because I have been told to
"check my ELT" after making transmissions. Here are my observations:

1 -- Noise is present with intercom on or off.

2 -- Noise is WORSE when any radio, but especially marker beacon, is
selected on the audio panel.

3 -- When all radios are unselected on the audio panel (i.e. nothing
selected to either speaker or headsets) noise goes from very noticeable
to barely noticeable.

4 -- Noise is much WORSE when things are "cold." After about 20 minutes
of flying time, the noise is still noticeable, but not nearly as bad as
it was when the flight began.

5 -- Unless I'm hearing things, which is possible, I can SOMETIMES hear
the noise through the headset with the strobes on but avionics bus OFF.

I am not sure whether the strobe power pack is crying out for help,
whether I have a ground problem elsewhere in the system, or whether I
have an improper installation of something (like the intercom), or
whether the audio panel is shot.

The strobes were factory-installed, so I assume that the installation of
the strobes is correct. The SPA-400 intercom was obviously not factory
installed, and I am not convinced that insulating washers were used on
the headset plugs, so I do need to check that out. I do know that
shielded cable was NOT used on the intercom installation.

Does anyone have any ideas? I've basically narrowed it down to three
things, in no particular order:

1 -- intercom
2 -- audio panel
3 -- strobe power pack


Thanks,
JKG



--

Michelle P ATP-ASEL, CP-AMEL, and AMT-A&P

"Elisabeth" a Maule M-7-235B (no two are alike)

Volunteer Pilot, Angel Flight Mid-Atlantic

Volunteer Builder, Habitat for Humanity

On Wed, 27 Oct 2004 11:45:50 GMT, Michelle P
wrote:

Jonathan,
Do you have a capacitor across your alternator output? If so that may be
bad as well. The last one about the capacitor in the unit sound likely
as well. The repair though sounds a little well not completely legal
unless they are an A&P and have the manual that says they can replace
the part.
Michelle

The repair was performed under the supervision of an IA.


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

And did John mention that he feels it's a GROUND problem? Also, how
could you have the strobes installed at the factory without shielded
wire? Were they added at some point? If you're able, check all
grounds before spending money with a shop.

On Wed, 27 Oct 2004 02:20:37 GMT, (John_F) wrote:

On Wed, 27 Oct 2004 00:43:50 GMT, Jonathan Goodish
wrote:

I know that this has been beaten to death, and I have searched the
archives, but I'm trying to narrow my specific case down a little bit.

I have a Cherokee with factory-installed wing tip strobes. I have an
original (1977) King Silver Crown avionics package, including audio
panel, KX170Bs, etc. I also have a Sigtronics panel-mount 4 place
intercom that was installed at some point before I purchased the
plane... I believe it is a SPA-400.

Everything is fine, except that the wing tip strobes can be heard like a
siren in the headsets. The siren sound is also occasionally transmitted
over the radio when we make transmissions, because I have been told to
"check my ELT" after making transmissions. Here are my observations:

1 -- Noise is present with intercom on or off.

Conclusion, it is not coming in within the intercom.

2 -- Noise is WORSE when any radio, but especially marker beacon, is
selected on the audio panel.

The noise is coupling into the marker beacon audio. Is the marker
beacon ground connection the same point as the strobe ground? Is the
marker beacon audio lead shielded wire?

3 -- When all radios are unselected on the audio panel (i.e. nothing
selected to either speaker or headsets) noise goes from very noticeable
to barely noticeable.

Sounds like a nosy ground problem.

4 -- Noise is much WORSE when things are "cold." After about 20 minutes
of flying time, the noise is still noticeable, but not nearly as bad as
it was when the flight began.

Sounds even more like a loose ground problem. Aluminum srinks more
with cold temperatures than the steel screw that holds the wire to the
airframe.

5 -- Unless I'm hearing things, which is possible, I can SOMETIMES hear
the noise through the headset with the strobes on but avionics bus OFF.

If the ground resistance were bad enough it would do this.

I am not sure whether the strobe power pack is crying out for help,
whether I have a ground problem elsewhere in the system, or whether I
have an improper installation of something (like the intercom), or
whether the audio panel is shot.

Find out where the ground of the strobe pack is connected to the
airframe and clean it.

The strobes were factory-installed, so I assume that the installation of
the strobes is correct.

Don't bet on this.

The SPA-400 intercom was obviously not factory
installed, and I am not convinced that insulating washers were used on
the headset plugs, so I do need to check that out.

Unless you have a large current through the airframe or have some poor
ground points insulating the jacks may be a wasted effort. If it does
work you need to look at the ground connections for the radios ,
intercom and audio panel because they will almost certainly be not
tight and will be high resistance.

I do know that shielded cable was NOT used on the intercom installation.

VERY VERY BAD!!! This is how the noise is coupling from the strobe
power leads on to the audio leads if they are in the same wire bundle.
ALL of the audio leads MUST be shielded wire.

Does anyone have any ideas? I've basically narrowed it down to three
things, in no particular order:

1 -- intercom
2 -- audio panel
3 -- strobe power pack

A big 1000uf cowpasture on the strobe power pack input power may help.

Thanks,
JKG

Find a avionics shop that knows what they are doing and have them fix
it. They are very hard to find and generally will NOT have the lowest
rates but they will generally save you money.

John

Jonathan Goodish wrote:
: VERY VERY BAD!!! This is how the noise is coupling from the strobe
: power leads on to the audio leads if they are in the same wire bundle.
: ALL of the audio leads MUST be shielded wire.

: Yes, I know that this is bad, but it probably could be worse (and
: remember, I'm not responsible for the intercom installation). The leads
: to and from the intercom don't appear to be bundled with the strobe
: power leads, or even close to them. I am hoping that this isn't the big
: problem, and I can let this ride until I have the interior replaced and
: then replace the wiring at that point.
Not that big of a deal, actually. Shielding gets an over-used emphasis in
most installations. There is not that much coupling between adjacent wires at audio
frequencies (4kHz). Not that there isn't any, but for any signal it's small. For
any signal above mic level, it's fairly insignificant. Also, only the inductive
coupling has any important coupling for a wiring bundle with the types of impedances
used in a typicaly aircraft system, so only wires carrying current can couple noise.

: Find a avionics shop that knows what they are doing and have them fix
: it. They are very hard to find and generally will NOT have the lowest
: rates but they will generally save you money.

: I don't want to give someone a blank check to troubleshoot a problem
: without some idea of what it may be. In other words, I need to try to
: troubleshoot as much as possible before engaging the avionics shop for
: more in-depth troubleshooting. I do believe that I have a good avionics
: shop in the local area, and they've done work for me before, but I
: haven't classified this problem as more than a nuisance at this point.
: However, it may be getting worse, and I certainly don't want it to reach
: a point where it becomes a show-stopper.

Good idea. It will depend on how friendly your A&P is with you crawling under
the panel, removing things to get at wiring, and tracing the wiring itself. It's
probably about 95% certain that it's a ground loop issue... in particular with the MB
it sounds like. Since many installers will ground different avionics willy-nilly
throughout the airframe wherever it's convenient, you get ground currents causing
voltage drops everywhere and coupling into the audio system.

Since you said things change as they heat up, it might help a bit to try to
locate the grounds on all the audio equipment and clean them. Most likely, however,
it's due to things being grounded at different places.

You might be able to band-aid the specific problem of strobe noise by running
a dedicated ground wire to it from "some good ground." That "some good ground" sorta
depends, but good places to try for troubleshooting would be the battery negative, and
alternator casing. That doesn't fix the multipoint ground problem in the audio
system, but it might reduce the structural ground currents enough to reduce the noise.

Cheers,
-cory


--

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

I'd vote for adding filter capacitor(s) at the strobe power supply 12V
(or whatever) input, immediately adjacent to the power supply.
Something sound fishy inside that strobe power supply.

Where's Jim Weir when you need him?

In article ,
Ron Rosenfeld wrote:
Everything is fine, except that the wing tip strobes can be heard like a
siren in the headsets.

Sounds like a problem I have had with a Whelen power pack.

Can you hear the sound at the power pack? I could.

Yes, I can. I can hear it when I'm standing outside the airplane with
the strobes on -- it's that loud. Seems to quiet down a little when it
warms up.

I figured that, one way or the other, the big fix was going to be
pulling the power pack, but I wanted to make sure there wasn't something
else that I was overlooking.


JKG