Car engine RF interference

[Dan Beck]

Hello all,

thanks to the advice of Tom Biasi and Bob Masta, I was able to tap into the
volume control of my vintage mono AM car radio, so that I could use my mp3
player headphone jack as a source. The referenced thread is linked, below.
I attenuated the stereo signals each with a 100 ohm resistor, summed them
into a single line, and soldered this line to the audio input to the trimmer
pot on the circuit board.

When the car is at rest, engine off, (ignition on) the mp3 player works
well. When I fire up the engine however, there is RF interference that
prevents me from using the mp3 player; the RF drowns out the music.
However, with the engine on the AM radio works great, without interference
at all.

The circuit is fused with a 2 amp slo blo fuse, and does not seem to blow
fuses ( I do not play the music very loud...). Would a 2 amp RF choke on
the power line to the radio be of any use for my problem? Are there any
other things I can try?

Thank you in advance for any and all information you can give me.

Regards,
Dan

http://groups.google.com/group/sci....345daaf07?hide_quotes=no#msg_2b5f57da7d1cffbf

 

[Jasen Betts]

Hello all,

thanks to the advice of Tom Biasi and Bob Masta, I was able to tap into the
volume control of my vintage mono AM car radio, so that I could use my mp3
player headphone jack as a source. The referenced thread is linked, below.
I attenuated the stereo signals each with a 100 ohm resistor, summed them
into a single line, and soldered this line to the audio input to the trimmer
pot on the circuit board.

When the car is at rest, engine off, (ignition on) the mp3 player works
well. When I fire up the engine however, there is RF interference that
prevents me from using the mp3 player; the RF drowns out the music.
However, with the engine on the AM radio works great, without interference
at all.

The circuit is fused with a 2 amp slo blo fuse, and does not seem to blow
fuses ( I do not play the music very loud...). Would a 2 amp RF choke on
the power line to the radio be of any use for my problem? Are there any
other things I can try?

Thank you in advance for any and all information you can give me.

Regards,
Dan

What tim said, and additionally the radio's signal ground may not be the negative
battery terminal however it will probably correspond to the quiet end of the
volume potentiometer

 

[Dan Beck]

If it doesn't happen when you use the radio it's very likely not radio
frequency interference -- if it were, it'd interfere with the radio. It's
most likely a baseband signal that's coupling directly into the audio,
either capacitively through the signal leads, because you've got a ground
loop, or because the power you're putting into the MP3 player is dirty.

Are you powering the MP3 player from the car? If you are, and you use the
player's battery, does the problem go away?

Are you connecting the audio cable shield to the radio's ground? Are you
connecting _any other_ grounds to the MP3 player (like, by powering it
from the car)?

Is the audio cable from the MP3 player well grounded?

Hi Tim (and Jasen),

thank you for your input thus far. Here is more detail.

I am not using car battery power to operate the mp3 player; I am using its
own battery.

The audio cable may not be well grounded, after all. I measure 2.1 ohms
resistance between the audio cable ground and the negative terminal of the
car battery. I measure 0.0 ohms for the radio case ground, to the negative
terminal of the car battery.

I soldered the ground wire of the audio cable to the potentiometer ground
wire connection at the circuit board in the radio. At the time I thought I
measured 0.0 ohms resistance between that site and the radio case; perhaps I
did not.

Is the audio cable shield the "tin foil" type material that was wrapping
around the three conductors in the audio cable? If so, it does not appear
to contact the audio cable ground wire in the splice that I made, but I
suppose it could be touching. Yes, I am a newbie at this...

This is a vintage car with the old fashioned plugs/points/condenser
(capacitor) ignition. I know there are tricks for suppression with these
components; perhaps I should consider them? Although, again because the
radio is NOT affected by interference it would make me think these
suppression maneuvers would be low yield...

Again, thank you for your time, and any further thoughts you can provide.

Regards,
Dan

 

[Jasen Betts]

Hi Tim (and Jasen),

thank you for your input thus far. Here is more detail.

I am not using car battery power to operate the mp3 player; I am using its
own battery.

The audio cable may not be well grounded, after all. I measure 2.1 ohms
resistance between the audio cable ground and the negative terminal of the
car battery. I measure 0.0 ohms for the radio case ground, to the negative
terminal of the car battery.

I soldered the ground wire of the audio cable to the potentiometer ground
wire connection at the circuit board in the radio. At the time I thought I
measured 0.0 ohms resistance between that site and the radio case; perhaps I
did not.


Is the audio cable shield the "tin foil" type material that was wrapping
around the three conductors in the audio cable? If so, it does not appear
to contact the audio cable ground wire in the splice that I made, but I
suppose it could be touching. Yes, I am a newbie at this...

it's the ground wire, the one that connects to the sleeve of the audio
plug,

This is a vintage car with the old fashioned plugs/points/condenser
(capacitor) ignition. I know there are tricks for suppression with these
components; perhaps I should consider them? Although, again because the
radio is NOT affected by interference it would make me think these
suppression maneuvers would be low yield...

if you connect short the contacts on the plug that goes into the mp3
player that should not cause noise with the engine going
(while the short circuit is in place)

if it does cause engine noise you've got atleast one of the wires hooked
up wrong, or maybe the cable is acting as a antenna ?

if it doesn't your mp3 player is picking up noise from the engine.

clean and/or replace the ignition system components.

 

[ehsjr]

Dan Beck wrote:

Is the audio cable shield the "tin foil" type material that was wrapping
around the three conductors in the audio cable? If so, it does not appear
to contact the audio cable ground wire in the splice that I made, but I
suppose it could be touching. Yes, I am a newbie at this...

Sounds like there's no braided shield on your audio cable.
If that's the case, do yourself a favor and get cable with
a braided shield. Something like CAT# MC-2 from allelectronics
http://www.allelectronics.com/

Also: what is the splice you mention? (You want to
avoid splices that break the shield.) Are you using a
switch as was recommended in the earlier thread? If so,
is it wired with shielded (braided shield) cable?

Ed

 

[Dan Beck]

Sounds like there's no braided shield on your audio cable.
If that's the case, do yourself a favor and get cable with
a braided shield. Something like CAT# MC-2 from allelectronics
http://www.allelectronics.com/

Also: what is the splice you mention? (You want to
avoid splices that break the shield.) Are you using a
switch as was recommended in the earlier thread? If so,
is it wired with shielded (braided shield) cable?

Ed

Hi Ed,

thank you for your input. I purchased a cable that would allow you to plug
headphone output of an mp3 player into an aux port on a radio/stereo
receiver. I cut off one end of the cable, and freed up the three conductor
wires, which all were wrapped around a foil material, which I would presume
to be shielding. To both the stereo channel input wires I added a 100 ohm
resistor, summed them together, and then soldered the resulting single wire
to the audio (AM) signal input to the trimmer potentiometer, on the circuit
board in the radio. The remaining ground wire from the cut cable I soldered
to the trimmer pot ground on the radio circuit board. Interestingly, there
is about a 2 ohm resistance between the pot 'ground' and the chassis ground.
The chassis ground measures 0.0 ohms to the battery negative terminal. The
'splices' I was referring to were the treatments described above.

The distributor wires in the car are silicone, RF suppression types, so
there is no more I can improve, there. I have read recipes about attaching
capacitors to the + side of the ignition coil, but I have not heard much
good about that, from car stereo buddies of mine.

Thank you in advance for your time, and any ideas you can give me.

Regards,
Dan

 

[ehsjr]

Hi Ed,

thank you for your input. I purchased a cable that would allow you to plug
headphone output of an mp3 player into an aux port on a radio/stereo
receiver. I cut off one end of the cable, and freed up the three conductor
wires, which all were wrapped around a foil material, which I would presume
to be shielding.

Ok, that "shielding" is poor, at best. Replace the cable with one
that uses braided shielding. You want the signal carrying conductors
inside the shield, not wrapped around it. Aside from that, it is
very difficult to get good electrical contact to the foil. You
want the shield to be electrically connected to chassis ground.

Ed