Headset Noise

[Jonathan Goodish]

I have a Sigtronics SPA-400 intercom in my Cherokee. Everything works
fine, except that the right seat position has an annoying amount of
electrical noise in the headset. It is a high-pitch whine (alternator
noise), and the pitch seems to follow RPM. This is not noticable in the
pilot's headset, even with the headset volume at full. It is much less
noticable in the right seat headset with the headset volume turned all
the way down, but obviously this makes things harder to hear for the
right seat occupant (often another pilot).

In an attempt to rectify the problem, I replaced the intercom wiring
with shielded Tefzel cable, replaced all of the jacks, and isolated all
of the jacks from the panel with fiber shoulder washers and brought
everything back to a single point ground. That helped, but only slightly.

I'm trying to figure out what's causing it or, at least, how to stop it
on that particular jack. Obviously, the intercom wiring runs adjacent to
the electrical buses on that side of the panel, so I'm not sure what
role the buses may play, especially since I'm using shielded cable.

Does anyone have any ideas?


Thanks,
JKG

[Jay Honeck]

> I'm trying to figure out what's causing it or, at least, how to stop it
> on that particular jack. Obviously, the intercom wiring runs adjacent to
> the electrical buses on that side of the panel, so I'm not sure what
> role the buses may play, especially since I'm using shielded cable.

Well, if it makes you feel any better, the co-pilot's jack in our
Pathfinder has a bit of whine in it, too. None of the other jacks do.

It's not bothersome, and I always figured it was the proximity to the
electrical bus.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

[Don Tuite]

On 8 Apr 2006 20:18:37 -0700, "Jay Honeck" > wrote:

>> I'm trying to figure out what's causing it or, at least, how to stop it
>> on that particular jack. Obviously, the intercom wiring runs adjacent to
>> the electrical buses on that side of the panel, so I'm not sure what
>> role the buses may play, especially since I'm using shielded cable.
>
>Well, if it makes you feel any better, the co-pilot's jack in our
>Pathfinder has a bit of whine in it, too. None of the other jacks do.
>
>It's not bothersome, and I always figured it was the proximity to the
>electrical bus.

Well, I'm sure it would be contrary to the regs to solder a 0.1
microfarad capacitor across the earphone terminals on the right-seat
headset jack. It's even a bad idea to try it with some alligator
clips first to see if it works.

Don

[[email protected]]

What you describe is classical ground-loop troubles. Your first attempt to
fix the problem should have done it... insulating the jacks from the panel (mics in
particular). If that truly didn't fix it, you may need to look at other ground
locations. You did say you replaced/insulated *all* jacks, right?

You can try relocating the ground on the Sigtronics to another location (like
the same location as where the audio panel connects would be good). With a large
number of older avionics with a single-ended design, the noise could be coming from
other places.

As far as the *location* being the copilot, you could check to make sure that
when the plug is inserted into the jack that it doesn't touch anything when the prongs
spread. IIRC, the SPA-400 has pilot/copilot isolation, so the signal will have
different paths depending on which plug is connected... separate internal circuits.

Basically, troubleshooting ground loops sucks.

-Cory

Jonathan Goodish > wrote:
: I have a Sigtronics SPA-400 intercom in my Cherokee. Everything works
: fine, except that the right seat position has an annoying amount of
: electrical noise in the headset. It is a high-pitch whine (alternator
: noise), and the pitch seems to follow RPM. This is not noticable in the
: pilot's headset, even with the headset volume at full. It is much less
: noticable in the right seat headset with the headset volume turned all
: the way down, but obviously this makes things harder to hear for the
: right seat occupant (often another pilot).

: In an attempt to rectify the problem, I replaced the intercom wiring
: with shielded Tefzel cable, replaced all of the jacks, and isolated all
: of the jacks from the panel with fiber shoulder washers and brought
: everything back to a single point ground. That helped, but only slightly.

: I'm trying to figure out what's causing it or, at least, how to stop it
: on that particular jack. Obviously, the intercom wiring runs adjacent to
: the electrical buses on that side of the panel, so I'm not sure what
: role the buses may play, especially since I'm using shielded cable.

: Does anyone have any ideas?


: Thanks,
: JKG

--

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

[Jonathan Goodish]

In article . com>,
"Jay Honeck" > wrote:
> Well, if it makes you feel any better, the co-pilot's jack in our
> Pathfinder has a bit of whine in it, too. None of the other jacks do.
>
> It's not bothersome, and I always figured it was the proximity to the
> electrical bus.

On my installation, the whine is only slightly noticeable with the
headset volume at its lowest, but then I have to turn up the
radio/intercom volume to compensate, which distorts the audio. When I
turn the headset volume up to high, the whine drives me crazy after
about 15-20 minutes or so.

Of course, I would never experiment with a capacitor that may be in
violation of the regs, even if it would enhance comfort and safety.
That's just something that I don't do during the week.


JKG

[Jonathan Goodish]

In article >,
wrote:
> As far as the *location* being the copilot, you could check to make sure
> that
> when the plug is inserted into the jack that it doesn't touch anything when
> the prongs
> spread. IIRC, the SPA-400 has pilot/copilot isolation, so the signal will
> have
> different paths depending on which plug is connected... separate internal
> circuits.

I'm not convinced that it's a ground loop because the intercom system
has a single-point ground. I have the system grounded where at least
SOME other avionics are grounded, but not sure that all avionics are
grounded at that point.

Nothing is touching any metal or anything else in the panel--I've
already been down that road.

I'm wondering whether there's just enough noise around the bus area (and
the b+ in particular) to create a problem. The lighter receptacle is
located above the copilot jacks--not sure whether this is relevant to
the problem or not.

Also, I've noticed a crackle interference in BOTH headsets when the
landing light is turned on, but I suspect that the landing light leads
are brushing against the muffler shroud. I need to check this out, but
it's not related to the high-pitched whine as far as I can tell.




JKG

[nrp]

>I'm not convinced that it's a ground loop because the intercom system
has a single-point ground. I have the system grounded where at least
SOME other avionics are grounded, but not sure that all avionics are
grounded at that point>

Lift that single ground ground and verify with a continuity test that
with power off, the intercom system is now floating. Make sure there
there isn't some other sneak path to ground that you don't know about.

The frequency of the alternator whine isn't so high that it would be
capacitively coupling into all but high impedance circuits.

If it is noise is present on the B+ line, it should be present at both
headset locations, and a simple power supply filter should take care of
it.

I'd be suspicious of the ground/common connection between the
alternator and the battery. It should be done only with heavy wires,
and not involve the airframe to carry the alternator current.

Another ground loop vs B+ capacitive coupling check - does the whine
become worse as the alternator load is increased, say by turning on
pitot heat and the landing lights? If so, it is more likely a ground
problem.

[Jonathan Goodish]

In article . com>,
"nrp" > wrote:

> >I'm not convinced that it's a ground loop because the intercom system
> has a single-point ground. I have the system grounded where at least
> SOME other avionics are grounded, but not sure that all avionics are
> grounded at that point>
>
> Lift that single ground ground and verify with a continuity test that
> with power off, the intercom system is now floating. Make sure there
> there isn't some other sneak path to ground that you don't know about.

I am quite confident that there is no sneak path to ground from the
intercom system, because I had everything disconnected when I rewired
(including the wiring harness). All jacks are isolated with fiber
shoulder washers. If there is a sneak path, I'm not sure where it would
be unless the SPA-400 is grounding through the mounting screws or on/off
switch barrel, but in that case I would expect the whine to be
system-wide.



> Another ground loop vs B+ capacitive coupling check - does the whine
> become worse as the alternator load is increased, say by turning on
> pitot heat and the landing lights? If so, it is more likely a ground
> problem.

Well, the whine goes away completely when the alternator side of the
master switch is turned off. I would say that it gets slightly worse as
more load is placed on the bus (landing light and pitot heat, for
example.) It is virtually non-existent in the pilot side, and is very
noticeable on the copilot side.

Before I dug into the intercom wiring last year, I did clean up the
ground at the battery box and strobe power supply (strobe noise problem
mostly cured with new power supply.) However, I haven't cleaned
airframe to engine ground or any grounds under the panel (other than
intercom system.)



Thanks,
JKG

[Tauno Voipio]

Jonathan Goodish wrote:
> In article . com>,
> "nrp" > wrote:
>
>
>>>I'm not convinced that it's a ground loop because the intercom system
>>
>>has a single-point ground. I have the system grounded where at least
>>SOME other avionics are grounded, but not sure that all avionics are
>>grounded at that point>
>>
>>Lift that single ground ground and verify with a continuity test that
>>with power off, the intercom system is now floating. Make sure there
>>there isn't some other sneak path to ground that you don't know about.
>
>
> I am quite confident that there is no sneak path to ground from the
> intercom system, because I had everything disconnected when I rewired
> (including the wiring harness). All jacks are isolated with fiber
> shoulder washers. If there is a sneak path, I'm not sure where it would
> be unless the SPA-400 is grounding through the mounting screws or on/off
> switch barrel, but in that case I would expect the whine to be
> system-wide.
>
>
>
>
>>Another ground loop vs B+ capacitive coupling check - does the whine
>>become worse as the alternator load is increased, say by turning on
>>pitot heat and the landing lights? If so, it is more likely a ground
>>problem.
>
>
> Well, the whine goes away completely when the alternator side of the
> master switch is turned off. I would say that it gets slightly worse as
> more load is placed on the bus (landing light and pitot heat, for
> example.) It is virtually non-existent in the pilot side, and is very
> noticeable on the copilot side.
>
> Before I dug into the intercom wiring last year, I did clean up the
> ground at the battery box and strobe power supply (strobe noise problem
> mostly cured with new power supply.) However, I haven't cleaned
> airframe to engine ground or any grounds under the panel (other than
> intercom system.)

It still has the smell of a ground loop if the intercom
power supply filtering is OK.

Maybe a ground loop between the intercom and the radios
(audio panel)?

--

Tauno Voipio
tauno voipio (at) iki fi

[[email protected]]

Tauno Voipio > wrote:
: It still has the smell of a ground loop if the intercom
: power supply filtering is OK.

: Maybe a ground loop between the intercom and the radios
: (audio panel)?

Now you're talking. Like I said, I don't know the particulars on the SPA-400,
but many intercom models are simple single-ended... a ground is a ground is a ground.
The intercom *does* connect to the audio panel, which connects to the nav, com, dme,
adf, radio, anklebone, etc, etc.

I suspect if you *truly* were to disconnect absolutely everything that could
re-ground the intercom (and thus render most of the avionics stack useless), float the
case and all jacks, you won't hear any whine.

As far as the pilot/copilot side being worse, remember that only the bus
(+14V) goes to the battery on a dedicated line (I don't recall what plane you're
talking about... if it's composite, I'm wrong). The alternator current meanders
through the aircraft structure somehow and eventually makes it to the battery. The
structure could very well favor more of this current on the copilots side and generate
a few more mV of alternator-induced IR drop there.... or rather where "there" is the
distance between where the intercom and some other avionics is grounded.

-Cory


--

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

[Travis Marlatte]

Maybe I missed it earlier in the thread. But, why have you all ruled out the
possibility of a problem with the alternator?

Of course, there is always some ripple on the DC voltage and a grounding
problem will allow it to be more noticeable. But, it could be that the
grounding is just fine and the ripple got worse. Either a blown diode or a
bad phase.

Changes frequency with RPM? Alternator not regulator. Gets louder with load?
Alternator phase not diode.

I had a whine show up recently. It's was only bothersome with ANR headsets.
If you concentrated, you could also hear it with regular headsets. I checked
grounds and switches. Nothing made the least bit of difference.

Finally, used an oscilloscope to look at the alternator output. Nice
consistent ripple. No spikes. But, it was minimally .5 volt p-p growing to
1.5v p-p with load. Typical alternator output ripple should be more like
20mv to 50mv p-p.

They're rebuilding my alternator now. I'll let you know what happens in a
couple of days.

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

> wrote in message
...
> Tauno Voipio > wrote:
> : It still has the smell of a ground loop if the intercom
> : power supply filtering is OK.
>
> : Maybe a ground loop between the intercom and the radios
> : (audio panel)?
>
> Now you're talking. Like I said, I don't know the particulars on the
> SPA-400,
> but many intercom models are simple single-ended... a ground is a ground
> is a ground.
> The intercom *does* connect to the audio panel, which connects to the nav,
> com, dme,
> adf, radio, anklebone, etc, etc.
>
> I suspect if you *truly* were to disconnect absolutely everything that
> could
> re-ground the intercom (and thus render most of the avionics stack
> useless), float the
> case and all jacks, you won't hear any whine.
>
> As far as the pilot/copilot side being worse, remember that only the bus
> (+14V) goes to the battery on a dedicated line (I don't recall what plane
> you're
> talking about... if it's composite, I'm wrong). The alternator current
> meanders
> through the aircraft structure somehow and eventually makes it to the
> battery. The
> structure could very well favor more of this current on the copilots side
> and generate
> a few more mV of alternator-induced IR drop there.... or rather where
> "there" is the
> distance between where the intercom and some other avionics is grounded.
>
> -Cory
>
>
> --
>
> ************************************************** ***********************
> * Cory Papenfuss *
> * Electrical Engineering candidate Ph.D. graduate student *
> * Virginia Polytechnic Institute and State University *
> ************************************************** ***********************
>

[mikem]

Travis Marlatte wrote:
> . . .

> I had a whine show up recently. It's was only bothersome with ANR headsets.
> If you concentrated, you could also hear it with regular headsets. I checked
> grounds and switches. Nothing made the least bit of difference.
>
> Finally, used an oscilloscope to look at the alternator output. Nice
> consistent ripple. No spikes. But, it was minimally .5 volt p-p growing to
> 1.5v p-p with load. Typical alternator output ripple should be more like
> 20mv to 50mv p-p.

Travis, if you connected the alternator Field directly to a 2 to 5V DC
supply, connected a heavy load (like a couple of landing lights across
the alternator output), then you spun the alternator in a drill press
or lathe, while viewing the voltage across the load with a scope, you
would see about 4V p-p of ripple. Is that a reason to re-build the
alternator?

No!! It is actually the aircraft battery which "filters" the output of
the alternator, and removes the ripple. If you see ripple, it is
because there is some resistance in the path between the alternator
output and the battery posts, or a bum battery. It could be either in
the positive path or in the ground return. I predict that you will
still have your ripple when you get your freshly o/h alternator back...

Several hundred mV of ripple measured at the B terminal of an
alternator is perfectly normal...

MikeM

[mikem]

Jonathan,

read this
http://tinyurl.com/l4rkk

[nrp]

mikem wrote:
> Jonathan,
>
> read this
> http://tinyurl.com/l4rkk

Mike M is right. The ripple on the alternator output will be higher
rather than lower.

Did you measure that you indeed had intercom isolation with the ground
disconnected?

[Jonathan Goodish]

In article >,
Tauno Voipio > wrote:
> It still has the smell of a ground loop if the intercom
> power supply filtering is OK.
>
> Maybe a ground loop between the intercom and the radios
> (audio panel)?


My wife is summoning, but some quick additional info:

1--When the alt. side of the master is switched off, the noise
disappears completely.

2--The noise is equally noticeable in the right FRONT and right REAR.
It is not noticeable in the left FRONT and left REAR.

3--Gets slightly worse as electrical load is added.

4--Sidetone from the radios is not as loud from the copilot position.

5--There is occasional popping when the mic is keyed from the copilot
position (but never from the pilot position.) The popping seems most
frequent when the engine is advanced to move onto the runway for takeoff
(during which time the mic is often keyed to announce takeoff.) This
electrical popping noise is never heard from the pilot position, and may
or may not reappear for the copilot when operating the mic in flight.


Thanks
JKG

[soxinbox]

Have you checked to see that the mike wires are shielded. Ideally the mike
wires should be run in one shielded cable and the speaker wires run in
another shielded cable. the shields should be grounded only at the intercom.
The pilot and co-pilot wiring run in different areas so an unshielded pilot
headset wiring may be OK were as the co-pilot may require better shielding.

"Jonathan Goodish" > wrote in message
...
>I have a Sigtronics SPA-400 intercom in my Cherokee. Everything works
> fine, except that the right seat position has an annoying amount of
> electrical noise in the headset. It is a high-pitch whine (alternator
> noise), and the pitch seems to follow RPM. This is not noticable in the
> pilot's headset, even with the headset volume at full. It is much less
> noticable in the right seat headset with the headset volume turned all
> the way down, but obviously this makes things harder to hear for the
> right seat occupant (often another pilot).
>
> In an attempt to rectify the problem, I replaced the intercom wiring
> with shielded Tefzel cable, replaced all of the jacks, and isolated all
> of the jacks from the panel with fiber shoulder washers and brought
> everything back to a single point ground. That helped, but only slightly.
>
> I'm trying to figure out what's causing it or, at least, how to stop it
> on that particular jack. Obviously, the intercom wiring runs adjacent to
> the electrical buses on that side of the panel, so I'm not sure what
> role the buses may play, especially since I'm using shielded cable.
>
> Does anyone have any ideas?
>
>
> Thanks,
> JKG

[Jonathan Goodish]

In article om>,
"mikem" > wrote:

> Jonathan,
>
> read this
> http://tinyurl.com/l4rkk


Mike,

That's very helpful, but unless I'm missing something, I'm not sure that
it explains why I'm only having the whine problem on the right side of
the cabin (in both front and rear jacks). All jacks are isolated from
the airframe, and I'm running a common ground back to the intercom
ground. The braided shield is floating at the jack ends and grounded at
the intercom ground point. The whine on one side of the cabin would
make more sense to me if the jacks were grounded locally to the
airframe, but not sure how noise would enter (or exit) the system on one
side of the cabin with the intercom system on a single-point ground.

Outside of the stuff I've posted to the newsgroup, one other oddity:
when I activate the marker beacon output on the audio panel, the whine
is more noticeable.

Obviously, it sounds like there's a ground problem somewhere, but I
don't know where to start. I'm baffled as to why I'm only hearing the
problem on one side of the intercom system and not the other, with none
of the jacks or PTT switches grounded directly to the airframe. The
noise must be alternator-induced, because it is only present when the
alternator field is activated. If something was looping through the
audio panel or somewhere else, why would it only affect the jacks on one
side of the aircraft?



JKG

[mikem]

Jonathan Goodish wrote:

> That's very helpful, but unless I'm missing something, I'm not sure that
> it explains why I'm only having the whine problem on the right side of
> the cabin (in both front and rear jacks). All jacks are isolated from
> the airframe, and I'm running a common ground back to the intercom
> ground. The braided shield is floating at the jack ends and grounded at
> the intercom ground point. The whine on one side of the cabin would
> make more sense to me if the jacks were grounded locally to the
> airframe, but not sure how noise would enter (or exit) the system on one
> side of the cabin with the intercom system on a single-point ground.

I concur. I dont think it is a ground loop issue. I'm thinking a wiring
error, or short which occurs only when headphone plugs are pushed in,
or an internal fault inside the SPA400. Too bad you dont live close, I
would lend you mine to see if it made a difference.

> Outside of the stuff I've posted to the newsgroup, one other oddity:
> when I activate the marker beacon output on the audio panel, the whine
> is more noticeable.
>
> Obviously, it sounds like there's a ground problem somewhere, but I
> don't know where to start. I'm baffled as to why I'm only hearing the
> problem on one side of the intercom system and not the other, with none
> of the jacks or PTT switches grounded directly to the airframe. The
> noise must be alternator-induced, because it is only present when the
> alternator field is activated. If something was looping through the
> audio panel or somewhere else, why would it only affect the jacks on one
> side of the aircraft?

The SPA400 drives the pilot headphone jack differently than the other
three jacks. Is the problem only on the copilot's jack, or also on the
back-seat jacks?

I'm puzzled by the "crackle". The alternator whine caused by a ground
loop is usually musical, pitch varies with engine speed. Crackles are
usually vibration induced intermittent contacts such as an improperly
seated audio panel, or nav-com. These usually behave with the engine
off, but can be induced by bashing the panel with your hand. Crackle
could also mean loose electrical connections to the bus, unshielded
P-leads, noisy electric-powered gyro instruments, bad fuel senders,
vibrating point mechanical alternator regulator (hopefully yours is
solid-state) or a vibration-sensitive Over Voltage Protection Relay. If
you have a good DVM, connect it to main bus, and read the voltage prior
to engine start, during runup and flight to see if there are any
fluctuations which correlate with the crackle you are hearing...

[[email protected]]

Jonathan Goodish > wrote:
: That's very helpful, but unless I'm missing something, I'm not sure that
: it explains why I'm only having the whine problem on the right side of
: the cabin (in both front and rear jacks). All jacks are isolated from
: the airframe, and I'm running a common ground back to the intercom
: ground. The braided shield is floating at the jack ends and grounded at
: the intercom ground point. The whine on one side of the cabin would
: make more sense to me if the jacks were grounded locally to the
: airframe, but not sure how noise would enter (or exit) the system on one
: side of the cabin with the intercom system on a single-point ground.

The jacks may be a red herring. If they truly are isolated from the structure
and grounded only to the intercom, you're right... they *cannot* be causing a ground
loop problem. The shield could still be allowing some noise coupling, but I doubt it.

As far as the right side/left side stuff, remember that there's a *BIG*
current flowing out of the alternator (well, *into* the alternator's ground). That
current is just meandering through the structure somehow. It could certainly be more
preferential to one side or the other when you're talking a few miliohms total through
the structure.

: Outside of the stuff I've posted to the newsgroup, one other oddity:
: when I activate the marker beacon output on the audio panel, the whine
: is more noticeable.

Does the *audio panel* run to the same single-point ground as the intercom?
If they're spatially separated (especially if one is on port the other's on
starboard), it could do what you are describing. Of course *any* chunk of avionics
grounded elsewhere could be inducing the ground loop as well.

: Obviously, it sounds like there's a ground problem somewhere, but I
: don't know where to start. I'm baffled as to why I'm only hearing the
: problem on one side of the intercom system and not the other, with none
: of the jacks or PTT switches grounded directly to the airframe.

Do you have the schematics available for the intercom? I managed to find a
set for both of the intercoms I've had in my plane. They generally have different
circuitry for the pilot vs. copilot. That might allow for different coupling of the
noise.

The : noise must be alternator-induced, because it is only present when the
: alternator field is activated. If something was looping through the
: audio panel or somewhere else, why would it only affect the jacks on one
: side of the aircraft?

Again, different inputs. It *does* seem a bit odd, but I haven a hard time
believing that it's the shield. Again, you're *SURE* that the jacks are floating,
right? ... and they don't accidentally contact any chunk of airframe when a jack is
plugged in? For the longest time we didn't have side panels in our plane and I just
had the rear seat intercom jacks zip-tied to the side. Once in awhile they'd move and
rattle against the airframe and make a serious crackle racket in the intercom system.
Just a thought.

-Cory

--

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

[Jonathan Goodish]

In article . com>,
"mikem" > wrote:
> I concur. I dont think it is a ground loop issue. I'm thinking a wiring
> error, or short which occurs only when headphone plugs are pushed in,
> or an internal fault inside the SPA400. Too bad you dont live close, I
> would lend you mine to see if it made a difference.

Well, I rewired the intercom with shielded cable and was pretty darn
careful to follow the wiring schematic, and the system does function, so
I'm fairly confident that it is wired correctly. However, I do
understand that just about anything is possible.

Believe it or not, I actually sent the SPA-400 back to Sigtronics a few
months ago for them to bench test, and it came back fine. That's not to
say that they didn't miss something, as I'm not sure what testing
procedure is used.



> The SPA400 drives the pilot headphone jack differently than the other
> three jacks. Is the problem only on the copilot's jack, or also on the
> back-seat jacks?

The problem exists ONLY in the right side of the cabin (copilot and
right rear passenger.) The pilot side is fine (both front and rear.)
You can hear a very, very faint whine on the pilot's side if there is
radio silence, but it's almost unnoticeable even with the headset volume
set at max. Despite the whine on the copilot side of the cabin, voice
quality is still very good throughout the system.



> I'm puzzled by the "crackle". The alternator whine caused by a ground
> loop is usually musical, pitch varies with engine speed. Crackles are
> usually vibration induced intermittent contacts such as an improperly
> seated audio panel, or nav-com. These usually behave with the engine
> off, but can be induced by bashing the panel with your hand. Crackle
> could also mean loose electrical connections to the bus, unshielded
> P-leads, noisy electric-powered gyro instruments, bad fuel senders,

I think I have two incidents of "crackle." One is only present when the
landing light is turned on, and I suspect that one of the landing light
leads in the cowl is occasionally contacting the muffler shroud.

The other one, however, sounds like a "chop chop chop" which increases
in speed with engine RPM and is often (but not always) present when the
throttle is advanced when positioning onto the runway. It is only heard
when the copilot PTT is keyed. It is never heard when the pilot's PTT
is keyed.

As for the regulator, I'm not sure when solid-state regulators became
standard equipment on airplanes, but as far as I know, mine hasn't been
changed since the airplane was manufactured in 1977.



JKG

[Travis Marlatte]

Brought my plane home last night after having the alternator rebuilt.

As you can read in my earlier post below, I scoped out the voltage and
concluded that the alternator had blown a phase. If a diode had gone, I
expected to see ripple with injected spikes that the blown diode would have
rectified but didn't. Since I saw a steady ripple with no spike and at a
higher peak to peak level than I expected, I concluded that all the diodes
were rectifying the same and that the excess A/C component was because a
phase of the alternator had gone. This was my conclusion sitting the plane
with a scope and reinforced later by some web site that talks about trouble
shooting alternator waveform outputs.

Prior to having that alternator rebuilt, I had a whine that increased in
frequency with RPM and got louder with load. If I shut the battery off, the
noise was intolerable. Clearly, the alternator was creating an A/C component
that the battery could just not dampen.

Below, mikem suggests that the problem was more likely the battery or
connections. I believe that the battery does absorb some ripple but it
doesn't make sense to me that the alternator output would be so noisy that
you couldn't operate the plane with just alternator power.

By the way, before this, I had no idea that you could shut the battery off
and leave the alternator on. In 10 years of flying airplanes with split
master switches, I have never, ever, not once had the alternator on without
the battery. Trouble shooting scenarios always assume that if there is a
problem, it will be with the alternator and discuss either shutting the
alternator off or both. Some people start their plane with the alternator
off to reduce starting load and startup power spikes. I actually thought
that the switches were mechanically coupled so that it was either both or
just the battery. The first time I did shut the alternator off, my first
thought, "That's curious, I wonder if my switch is broken!"

Anyway, I picked up my plane last night. Started it without the alternator.
Cautiously turned the alternator on. No whine. Added load. No whine. Reved
it up a bit. No whine. Shut the battery off. No Whine.

The ticket says - "Rebuilt and tested." My mechanic says that they found a
bad diode. I don't really know if that was THE problem or just part of the
problem. Bottom line, removing and reinstalling a rebuilt alternator solved
my whine.

I continue to believe that the alternator should be creating a steady
voltage that should be usable without the dampening effects of the battery.
I understand that bad grounding connections, in particular, could impede a
radios ability to shed the A/C component but it should only have to shed
something like 50mv of ripple - not the .5 to 1.5v that I was seeing.

The bill was for 2.5 hours of labor and $273 for the rebuild. $450 total.
This was at Kenosha, WI.

mikem, are we really talking about the same kind of alternator? Are these
alternators typically 3-phase? With 3-phases and the typical diode pack, why
would you see 4v p-p? That seems excessive to me.

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


"mikem" > wrote in message
oups.com...
>
> Travis Marlatte wrote:
>> . . .
>
>> I had a whine show up recently. It's was only bothersome with ANR
>> headsets.
>> If you concentrated, you could also hear it with regular headsets. I
>> checked
>> grounds and switches. Nothing made the least bit of difference.
>>
>> Finally, used an oscilloscope to look at the alternator output. Nice
>> consistent ripple. No spikes. But, it was minimally .5 volt p-p growing
>> to
>> 1.5v p-p with load. Typical alternator output ripple should be more like
>> 20mv to 50mv p-p.
>
> Travis, if you connected the alternator Field directly to a 2 to 5V DC
> supply, connected a heavy load (like a couple of landing lights across
> the alternator output), then you spun the alternator in a drill press
> or lathe, while viewing the voltage across the load with a scope, you
> would see about 4V p-p of ripple. Is that a reason to re-build the
> alternator?
>
> No!! It is actually the aircraft battery which "filters" the output of
> the alternator, and removes the ripple. If you see ripple, it is
> because there is some resistance in the path between the alternator
> output and the battery posts, or a bum battery. It could be either in
> the positive path or in the ground return. I predict that you will
> still have your ripple when you get your freshly o/h alternator back...
>
> Several hundred mV of ripple measured at the B terminal of an
> alternator is perfectly normal...
>
> MikeM
>

[mikem]

Hi Travis,

First, Glad that you seem to have fixed your problem. Your alternator
must have had a bum diode, and my prediction was wrong.

Second, I want to point something out to you. Disconnecting the battery
while the alternator is spun-up (and maintaining field exitation) is
EXTREMELY DANGEROUS. Doing this could cause the bus voltage to spike
beyond 100+V, and destroy every piece of avionics in your panel. Rather
than trying to explain why here, do a Goggle search on the phrase
"ALTERNATOR LOAD DUMP".

Every aircraft I have seen is wired such that turning off the battery
master simultaneously breaks the path to the alternator field, thereby
shutting down the alternator. Some aircraft accomplish this with a
mechanical interlock on the switch (Cessna split master/ALT switch), or
by the way the ALT switch is wired. The fact that your Lake is NOT
wired this way seems to be a major screw up.

Disconnecting the battery in order to diagnose an alternator is NOT an
accepted diagnostic tool.

Third, to show the expected bus voltage under different conditions, I
did four different Spice simulations and put the results here:
http://tinyurl.com/lkzt4

Normal.jpg = 14V with Battery, loaded to 25A, showing typical
resistances. (50mv of ripple)

NoBattery.jpg = Same as above, but with no battery to act as a filter.
(2V of ripple)

BadDiode.jpg = same as Normal, except one diode is open. (400mv of
ripple)

BadDiodeNoBattery.jpg = same as above, but no battery to act as a
filter. (15V! of ripple)

Look at all four, and you can see the difference in the waveshape and
amount of ripple.

I also posted four more files based on the four simulations above,
which show the Spectra (FFT) of each case. Note that when a diode is
missing, the fundamental frequency is related to the rate of rotation,
while with six intact diodes, the fundamental frequency of the ripple
is six times the rotation rate.

MikeM

[David Lesher]

"mikem" > writes:


>First, Glad that you seem to have fixed your problem. Your alternator
>must have had a bum diode, and my prediction was wrong.

>Second, I want to point something out to you. Disconnecting the battery
>while the alternator is spun-up (and maintaining field exitation) is
>EXTREMELY DANGEROUS. Doing this could cause the bus voltage to spike
>beyond 100+V, and destroy every piece of avionics in your panel. Rather
>than trying to explain why here, do a Goggle search on the phrase
>"ALTERNATOR LOAD DUMP".

What he said (WHS). Although the figure I recall wasa mere 65 volts;
that from a discussion of such in a GE-MOV applications book many
years ago. Further, the resulting spikes is not just big (V) but
fat (long persistence) and thus high total energy.


>Every aircraft I have seen is wired such that turning off the battery
>master simultaneously breaks the path to the alternator field, thereby
>shutting down the alternator. Some aircraft accomplish this with a
>mechanical interlock on the switch (Cessna split master/ALT switch), or
>by the way the ALT switch is wired. The fact that your Lake is NOT
>wired this way seems to be a major screw up.

WHS!!!!

>Disconnecting the battery in order to diagnose an alternator is NOT an
>accepted diagnostic tool.

The ONLY good news is you'll blow up only the avionics; the magnetos
will not care and even an electric fuel pump may likely survive.

Do that stunt on modern car and you shall regret it...


>Third, to show the expected bus voltage under different conditions, I
>did four different Spice simulations and put the results here:
>http://tinyurl.com/lkzt4

Hmm, zip seen here, alas....
Is it working?


--
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

[mikem]

Try this:
http://home.utah.edu/~mgm17160/Alternator/

[David Lesher]

"Travis Marlatte" > writes:


>As you can read in my earlier post below, I scoped out the voltage and
>concluded that the alternator had blown a phase. If a diode had gone, I
>expected to see ripple with injected spikes that the blown diode would have
>rectified but didn't. Since I saw a steady ripple with no spike and at a
>higher peak to peak level than I expected, I concluded that all the diodes
>were rectifying the same and that the excess A/C component was because a
>phase of the alternator had gone. This was my conclusion sitting the plane
>with a scope and reinforced later by some web site that talks about trouble
>shooting alternator waveform outputs.

I do not recall; did you test this alternator at full load? That
will sometime expose bad diodes that look good at lower currents.
In days past, the gas station [remember THOSE?] had a volt/amp meter
with a carbon-pile load; tighten the pile until the belt is REALLY
singing and look at the scope.

>Below, mikem suggests that the problem was more likely the battery or
>connections. I believe that the battery does absorb some ripple but it
>doesn't make sense to me that the alternator output would be so noisy that
>you couldn't operate the plane with just alternator power.

See other post...

>The ticket says - "Rebuilt and tested." My mechanic says that they found a
>bad diode. I don't really know if that was THE problem or just part of the
>problem. Bottom line, removing and reinstalling a rebuilt alternator solved
>my whine.

I have to wonder if the rebuilding/reinstalling also solved a partial
ground issue somewhere....


>I continue to believe that the alternator should be creating a steady
>voltage that should be usable without the dampening effects of the battery.

Please stop believing that. Or believe it, but do not practice it.

>mikem, are we really talking about the same kind of alternator? Are these
>alternators typically 3-phase? With 3-phases and the typical diode pack, why
>would you see 4v p-p? That seems excessive to me.

Every auto alternator I have seen is 3-phase, Y wound. (But I've
heard tell of some that were delta..)

There are 6 main diodes, and 3 aux diodes.

(Note my Honda is a partial exception. For reasons I have yet to
grok; the center-point of its Y has 2 more main diodes, making 8
main. I can't wrap my head around what this accomplishes.)




--
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

[Jon Woellhaf]

Mike,

Thanks for publishing the alternator simulations. I found them very
interesting.

You said, "... with six intact diodes, the fundamental frequency of the
ripple is six times the rotation rate."

I read the fundamental ripple frequency as 3 kHz. If that's six times the
rotation rate, then the rotation rate must be 500 Hz. That's 30,000 rpm.
That doesn't seem reasonable. What am I missing?

Regards,

Jon

[mikem]

In all the years of chasing alternator whine in aircraft audio systems,
it sounded to me that it is at a couple of Khz.

Jon,

If the engine is turning 2600 RPM, that is 43 revs per sec. The pulleys
on the alternator have a ratio of about 4:1, so that would make the
alternator spin at about 173 revs per sec, or 173Hz. That would make
the alternator whine (assuming no bad diodes) of 1038Hz.

I should alter the sinsoidal current sources to have a freq of 173Hz,
instead of the 500Hz I used.

btw- for the electrical geeks out there, you are familiar with LTSpice,
arent you? If not, go look at www.linear.com and get a download. Best
Spice implemenation out there, and it is FREE!

[Jon Woellhaf]

Mike,

Thanks for the clarification and the link to LTSpice!

Jon

"mikem" > wrote in message
oups.com...
> In all the years of chasing alternator whine in aircraft audio systems,
> it sounded to me that it is at a couple of Khz.
>
> Jon,
>
> If the engine is turning 2600 RPM, that is 43 revs per sec. The pulleys
> on the alternator have a ratio of about 4:1, so that would make the
> alternator spin at about 173 revs per sec, or 173Hz. That would make
> the alternator whine (assuming no bad diodes) of 1038Hz.
>
> I should alter the sinsoidal current sources to have a freq of 173Hz,
> instead of the 500Hz I used.
>
> btw- for the electrical geeks out there, you are familiar with LTSpice,
> arent you? If not, go look at www.linear.com and get a download. Best
> Spice implemenation out there, and it is FREE!
>

[[email protected]]

mikem > wrote:
: Try this:
: http://home.utah.edu/~mgm17160/Alternator/

I see you didn't try to model the load dump transient... :) Trouble with that
one is that it's dependent on all sorts of other loads on the bus.

For those that didn't look up the potential damage of load dump transients,
the trouble is the tremendous amount of inductance in automotive claw-pole
alternators. Yes they're 3-phase synchronous machines, with six-pack diode rectifiers
to get "DC.".... very crappy 3-phase machines. When charging a battery that's sucking
down tens of amps, the backEMF necessary to overcome the AC voltage drop across the
(large) stator inductance can often measure 60-90V. If that battery load suddenly
goes away, all that magnetic energy stored in the inductor tries to keep the same
current flowing by increasing the output voltage. Then all your avionics go *poof*.

Now, if you've been flying for awhile and your battery is all charged up, it
will only be taking an amp or two. All other loads remaining constant, you could
disconnect the battery and run on just the alternator without hurting anything.
Trouble is, any other transient load shedding (gear motor, pitot heat, etc) will not
have the battery there to absorb it.

It seems like it might be useful as an emergency limp-home mode if the battery
somehow fails. I'm having a hard time envision a scenerio where it would be
safe/useful to do (other than a fried master relay).

-Cory

--

************************************************** ***********************
* Cory Papenfuss, Ph.D., PPSEL-IA *
* Electrical Engineering *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

[Travis Marlatte]

I can't get at the link but your textual description fits what I was seeing
and hearing. I'm a believer! I'm a believer! I will never shut the battery
off again!!

At the next annual, I will have my mechanic replace the master as well.

Without seeing the waveforms, it sounds like it would be very hard to
differentiate a bad connection from a bad diode problem. I apparently lucked
out in my ill-informed diagnosis.

Thanks for all you input.

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


"mikem" > wrote in message
ups.com...
> Hi Travis,
>
> First, Glad that you seem to have fixed your problem. Your alternator
> must have had a bum diode, and my prediction was wrong.
>
> Second, I want to point something out to you. Disconnecting the battery
> while the alternator is spun-up (and maintaining field exitation) is
> EXTREMELY DANGEROUS. Doing this could cause the bus voltage to spike
> beyond 100+V, and destroy every piece of avionics in your panel. Rather
> than trying to explain why here, do a Goggle search on the phrase
> "ALTERNATOR LOAD DUMP".
>
> Every aircraft I have seen is wired such that turning off the battery
> master simultaneously breaks the path to the alternator field, thereby
> shutting down the alternator. Some aircraft accomplish this with a
> mechanical interlock on the switch (Cessna split master/ALT switch), or
> by the way the ALT switch is wired. The fact that your Lake is NOT
> wired this way seems to be a major screw up.
>
> Disconnecting the battery in order to diagnose an alternator is NOT an
> accepted diagnostic tool.
>
> Third, to show the expected bus voltage under different conditions, I
> did four different Spice simulations and put the results here:
> http://tinyurl.com/lkzt4
>
> Normal.jpg = 14V with Battery, loaded to 25A, showing typical
> resistances. (50mv of ripple)
>
> NoBattery.jpg = Same as above, but with no battery to act as a filter.
> (2V of ripple)
>
> BadDiode.jpg = same as Normal, except one diode is open. (400mv of
> ripple)
>
> BadDiodeNoBattery.jpg = same as above, but no battery to act as a
> filter. (15V! of ripple)
>
> Look at all four, and you can see the difference in the waveshape and
> amount of ripple.
>
> I also posted four more files based on the four simulations above,
> which show the Spectra (FFT) of each case. Note that when a diode is
> missing, the fundamental frequency is related to the rate of rotation,
> while with six intact diodes, the fundamental frequency of the ripple
> is six times the rotation rate.
>
> MikeM
>
>

[mikem]

Travis Marlatte wrote:
> I can't get at the link but your textual description fits what I was seeing
> and hearing.

You missed the correction I posted: use

http://home.utah.edu/~mgm17160/Alternator/

[Travis Marlatte]

I had tried that link too. I couldn't get that either but your textual
description was enough for me. Thanks again.

--
-------------------------------
Travis
Lake N3094P
PWK
"mikem" > wrote in message
oups.com...
>
> Travis Marlatte wrote:
>> I can't get at the link but your textual description fits what I was
>> seeing
>> and hearing.
>
> You missed the correction I posted: use
>
> http://home.utah.edu/~mgm17160/Alternator/
>
>