UPDATE #2 -- Headset Noise

[Jonathan Goodish]

I flew the airplane today, and headset noise (alternator whine) has not
been completely eliminated by the resolution of the ground loop through
the intercom system, but it has been further reduced to a level where it
is just manageable. The "chop chop" or "flutter" electrical noise is
still occasionally present in the system when the copilot transmits, and
can be heard in the sidetone.

HOWEVER, I've now discovered another potential problem. In my quest to
inspect ground connections, I pulled the cover on the rear bulkhead to
look at the glideslope receiver, strobe power pack, and battery. The
master relay is next to the battery, and I nearly burned myself on it
after a 30 minute flight--it was that hot. I let it cool down, and ran
the airplane for another 5 minutes and it was almost too hot to touch.

Logic tells me that heat is created by electrical resistance, and I'm
guessing that there shouldn't be enough resistance in the master relay
to cause it to get too hot to touch. Is this a failure mode for these
relays? Could this be contributing in any way to my electrical noise
issues?


Thanks,
JKG

[Jim Logajan]

Jonathan Goodish > wrote:
> Logic tells me that heat is created by electrical resistance, and I'm
> guessing that there shouldn't be enough resistance in the master relay
> to cause it to get too hot to touch. Is this a failure mode for these
> relays?

Relays can fail such that they get hot. A relay that hot is probably a bad
relay. It needs to be fixed or replaced.

> Could this be contributing in any way to my electrical noise
> issues?

Possibly.

[mikem]

Jonathan Goodish wrote:

> HOWEVER, I've now discovered another potential problem. In my quest to
> inspect ground connections, I pulled the cover on the rear bulkhead to
> look at the glideslope receiver, strobe power pack, and battery. The
> master relay is next to the battery, and I nearly burned myself on it
> after a 30 minute flight--it was that hot. I let it cool down, and ran
> the airplane for another 5 minutes and it was almost too hot to touch.

Since the starter cranking current goes through the master relay, then
if the contacts in the master relay develop high resistance, because of
the voltage drop, you would not be able to start the engine. The coil
in the master relay should pull less than 2A, which would mean that the
coil in the relay is dissipating less than 20W. 20W of dissipation will
make the case feel too hot to touch...

Not all solenoid relays used in aircraft are interchangable. For
example, the one that is used as the master in my Cessna is rated for
continuous duty, and its coil pulls about an
Amp. The starter solenoid, however, is rated only for intermittent
duty, and has a much
higher coil current (about 3A), probably so it holds in stronger... I
have seem people mistakenly replace a continuous duty solenoid with one
that is intended to be used as a starting solenoid, and then they
wondered why it got so hot....

[mikem]

Jonathan Goodish wrote:
....I pulled the cover on the rear bulkhead to
> look at the glideslope receiver, strobe power pack, and battery.

On your alternator whine ground loop problem, check to see how the
audio gets out of the glideslope receiver to your audio panel. My
Skylane has a rear mounted battery and the Marker Beacon receiver is
mounted on an avionics shelf near the battery.

When I was chasing my ground loops, one of the biggest ones was that
the ARC marker beacon reciever had a case ground referenced audio
output, and because of the rear mounted battery, the alternator ripple
current flowed along the marker beacon's audio shield.

The Marker Beacon has an audio output transformer, but ARC chose to tie
one end of
the secondary to the MB's case. The solution was to cut the ground
strap, and tie that end of the secondary winding to an unsed pin on the
connector. That made a true MBhi, MBlow differential floating output,
which prevents the airframe ground voltage drop between the fiirewall
and the battery from coupling into the audio...

[David Lesher]

"mikem" > writes:


>Not all solenoid relays used in aircraft are interchangable. For
>example, the one that is used as the master in my Cessna is rated for
>continuous duty, and its coil pulls about an Amp. The starter solenoid,
>however, is rated only for intermittent duty, and has a much
>higher coil current (about 3A),

That was my thought as well -- it may be the wrong contactor.
Or, it could be bad connections at the, or maybe inside, the
contactor. Pull off the output connection, turn on the contactor
for 10 min.... can you touch it?

--
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]

Look at this diagram:
http://home.utah.edu/~mgm17160/Alternator/MBGroundLoop.jpg
That should make this clear.

[Jonathan Goodish]

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

> Jonathan Goodish wrote:
> ...I pulled the cover on the rear bulkhead to
> > look at the glideslope receiver, strobe power pack, and battery.
>
> On your alternator whine ground loop problem, check to see how the
> audio gets out of the glideslope receiver to your audio panel. My
> Skylane has a rear mounted battery and the Marker Beacon receiver is
> mounted on an avionics shelf near the battery.

I checked this yesterday, the case of the glideslope receiver has
continuity with ground when it is isolated from the airframe mounting
brackett and the antenna is disconnected.




JKG

[mikem]

Jonathan Goodish wrote:
> In article . com>,
> "mikem" > wrote:
>
> > Jonathan Goodish wrote:
> > ...I pulled the cover on the rear bulkhead to
> > > look at the glideslope receiver, strobe power pack, and battery.
> >
> > On your alternator whine ground loop problem, check to see how the
> > audio gets out of the glideslope receiver to your audio panel. My
> > Skylane has a rear mounted battery and the Marker Beacon receiver is
> > mounted on an avionics shelf near the battery.
>
> I checked this yesterday, the case of the glideslope receiver has
> continuity with ground when it is isolated from the airframe mounting
> brackett and the antenna is disconnected.

OK, then the wiring to the GS receiver could well be the source of your
ground loop, just like it was in my Skylane. Here is how you can find
out: Unplug the cable where it connects to the GS Rx. If there is a
ground strap coming out of the plug your just removed from the GS Rx,
disconnect it from ground. Insulate the cable end so it cannot touch
the airframe, and go fly the airplane.

If this reduces or eliminates your ground loop, then either modify the
GS Rx, or use a 1:1 500Ohm audio isolation transformer to break the
ground loop along the audio from the GSRx to the audio panel at the
front of the airplane. Write back if you have questions?

[mikem]

Jonathan Goodish wrote:
> I flew the airplane today,...The "chop chop" or "flutter" electrical noise is
> still occasionally present in the system when the copilot transmits, and
> can be heard in the sidetone.

The "chop chop" sounds like RF is getting into the mic audio line, and
is causing what hams call "motorboating" (The RF is rectified in the
intercom or audio panel input circuits, the resulting DC bias shift
causes the low-frequency oscilllation).

Solution is to prevent the RF entering the audio line. Put a 100pF RF
bypass capacitor at the Mic jack between either:
1). the Mic line terminal to the floating common terminal, or
2). " " " " to airframe ground near the mic jack.
Try 1) and 2) to see which accomplishes the desired result.

The question is why is RF floating around inside the cockpit. At other
things working properly, this is not supposed to happen. A bad Com
antenna (bad coax, oxidized BNC connection, bad RF ground between
antenna base and airframe) can put a high VSWR onto the Com's transmit
coax, and cause radiation off that coax so that it couples into other
nearby wiring. Fix the antenna, the VSWR returns to normal, and that
gets the RF out out your audio.