Piper Cherokee RFI Capacitor

Where is the RFI capacitor typically located in a 1971 Piper Cherokee 180?

On the firewall? In the cabin? Next to the battery?

Here's a link to the Cherokee electrical schematic.
http://www.needlescentered.com/cherokee_electrical.htm

Thanks,
Nathan

Nathan Young wrote:
Where is the RFI capacitor typically located in a 1971 Piper Cherokee 180?
On the firewall? In the cabin? Next to the battery?
Here's a link to the Cherokee electrical schematic.
http://www.needlescentered.com/cherokee_electrical.htm

Usually, it's bolted to the alternator (with it's wire connected directly
to the output terminal of the alternator).

--- Jay


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Nathan Young wrote:

Where is the RFI capacitor typically located in a 1971 Piper Cherokee 180?

On the firewall? In the cabin? Next to the battery?

Here's a link to the Cherokee electrical schematic.
http://www.needlescentered.com/cherokee_electrical.htm

Thanks,
Nathan
On the alternator.

What kind of RFI are you experiencing?

Things that cause RFI in GA airplanes:
Brush arcing in electric Turn Coord.
Brush arcing inside the alternator.
Brush noise in electric motor (such as old rotating beacons)
Bad shields on spark wires
Bad shields on P-leads
Bad shields on strobe wiring

MikeM

Your schematic shows it attached to the alternator.

Having said that, sometimes they are on the backside outside but if the
alternator has been replaced....might be inside.

Neal

On Mon, 12 Jan 2004 12:21:37 -0700, mikem
wrote:

Nathan Young wrote:

Where is the RFI capacitor typically located in a 1971 Piper Cherokee 180?

On the firewall? In the cabin? Next to the battery?

Here's a link to the Cherokee electrical schematic.
http://www.needlescentered.com/cherokee_electrical.htm

Thanks,
Nathan
On the alternator.

What kind of RFI are you experiencing?

Things that cause RFI in GA airplanes:
Brush arcing in electric Turn Coord.
Brush arcing inside the alternator.
Brush noise in electric motor (such as old rotating beacons)
Bad shields on spark wires
Bad shields on P-leads
Bad shields on strobe wiring

Thanks Mike,

Fortunately, I do not have any RFI problems with the plane. The
radios are clear, and the voltage is stable (ammeter is stable too).

However, I've been debugging a problem with my charging system, and
spent some time looking at the schematics. While doing this, I
realized had no idea where the RFI cap sits...

I checked this afternoon. It is mounted on the engine side of the
firewall. The model is a Hisonic RFI 70. It's probably as old as the
plane, so I wonder if it has any capacitance left, or is merely along
for the ride.

-Nathan

Nathan Young wrote:

I checked this afternoon. It is mounted on the engine side of the
firewall. The model is a Hisonic RFI 70. It's probably as old as the
plane, so I wonder if it has any capacitance left, or is merely along
for the ride.

-Nathan

If that Hisonic box is about 2" X 3" X 4", and has two terminals
(instead of one), it is actually a low pass filter containing series
rf chokes wound on ferrite as well as shunt capacitors. If it has just
one terminal, then it is likely just a shunt capacitor, where the
case is the other side...

In either case, the case has to be well grounded for it to have any
effect. The capacitor(s) in these things are high quality, and usually
do not dry out.

Mike M
Skylane '1MM
Pacer '00Z

On Tue, 13 Jan 2004 18:25:43 -0700, MikeM
wrote:

Nathan Young wrote:

I checked this afternoon. It is mounted on the engine side of the
firewall. The model is a Hisonic RFI 70. It's probably as old as the
plane, so I wonder if it has any capacitance left, or is merely along
for the ride.

-Nathan

If that Hisonic box is about 2" X 3" X 4", and has two terminals
(instead of one), it is actually a low pass filter containing series
rf chokes wound on ferrite as well as shunt capacitors. If it has just
one terminal, then it is likely just a shunt capacitor, where the
case is the other side...

In either case, the case has to be well grounded for it to have any
effect. The capacitor(s) in these things are high quality, and usually
do not dry out.

Thanks again Mike. The box is as you describe. A terminal on either
side, and presumably ground on the case...

-Nathan

Nathan Young wrote:

On Tue, 13 Jan 2004 18:25:43 -0700, MikeM
wrote:


Nathan Young wrote:


I checked this afternoon. It is mounted on the engine side of the
firewall. The model is a Hisonic RFI 70. It's probably as old as the
plane, so I wonder if it has any capacitance left, or is merely along
for the ride.

-Nathan

If that Hisonic box is about 2" X 3" X 4", and has two terminals
(instead of one), it is actually a low pass filter containing series
rf chokes wound on ferrite as well as shunt capacitors. If it has just
one terminal, then it is likely just a shunt capacitor, where the
case is the other side...

In either case, the case has to be well grounded for it to have any
effect. The capacitor(s) in these things are high quality, and usually
do not dry out.


Thanks again Mike. The box is as you describe. A terminal on either
side, and presumably ground on the case...

-Nathan

btw- I dont think that this big LPF box is standard equipt. It was
probably added by somebody (probably without logging the work) in a
misguided attempt to cure "alternator whine" in the radios. The Hisonic
filters are RF lowpass; they are NOT effective at audio frequencies and
do NOT filter alternator ripple at audio frequencies.

Brush arcing in an alternator causes RF noise (hash), and the shunt
hypass (oil filled tub) capacitor was usually installed at the factory
to suppress this RF at its source. The RF noise out of an unsuppressed
alternator will clobber ADF and Loran frequencies (100-500Khz), but will
rarely effect a VHF (120MHz) radio, so even if there is no bypass
capacitor on your alternator; you are not likely to hear hash in a VHF
radio.

Alternator whine (& Strobe squeal) is not caused by radiated RFI
travelling from the alternator to a VHF antenna. Rather, they are
usually caused by alternator ripple currents flowing along the airframe
coupling into the audio system. Said coupling can be fixed by correctly
wiring the audio system so as to eliminate multiple grounding points
(single point ground). Naive avionics techs and owners install the big
Hisonic box filters on their alternator output lead as a bandaid which
rarely works to eliminate alternator whine; however, they will improve
ADF and/or Loran reception.

MikeM
Pacer '00Z
Skylane '1MM