*repost* RF interference *please read*

[Lydokane]

This is a repost of a message that I posted through google... almost
two weeks ago. It finally got posted yesterday.

I have a hard drive based MP3 player. It is designed for automobiles
but with a transformer I can use it in the house or at work. The
problem is it puts out a large amount of RF interference. When I am
at home it isn't a big deal. When I am in the car or at work it
becomes a problem. The player literally wipes out the entire FM band.
Every station turns to static (actually you hear the wirring noise of
the hard drive). I am sure it is ‘airborne' interference. If I am in
the car and I pull up next to somebody, and their radio is loud
enough, I can hear my interference from their radio. Naturally, it
also affects my radio. At work it is even worse. I work in a
factory, which is mainly a steel, and cinder block building. With the
push of a button I can render every radio in the shop useless. I have
been told not to bring it back, unless I want to be flogged, tarred
and feathered.

What could be causing the interference? I know the unit has a regular
3.5" hard drive. I don't believe this could be the problem because we
have several (about 12 or 15) computers on the shop floor that all use
a 3.5" HDD. I know the unit has a eprom that is programmed to decode
the MP3 files and I think it has a small processor. It doesn't run
any kind of internal amp. Since it is designed for use in an
automobile it is drawing 13-14 V and 3amps.

What could I do to reduce/eliminate the interference?

Thanks,

 

[Glen]

On 6 Oct 2003 07:24:22 -0700, [email protected] (Lydokane) wrote:

Is the transformer grounded or is it 2-prong? Where does the ground
for the MP3 run to? If you don't have an earth ground reference, then
you're just asking for trouble with a stepper motor(hard drive).

Glen

 

[Lydokane]

On 6 Oct 2003 07:24:22 -0700, [email protected] (Lydokane) wrote:

Is the transformer grounded or is it 2-prong? Where does the ground
for the MP3 run to? If you don't have an earth ground reference, then
you're just asking for trouble with a stepper motor(hard drive).

Glen

Glen,
The MP3 player is a 2-prong (red & black). The transformer that I am
currently using is actually an old laptop transformer (Toshiba I
think). It outputs 15V @ 3A. It doesn't have a 3-prong grounded
plug. It is polarized. I also have another transformer that outputs
12V @ 3A (or 4A... I can't remember). This other transformer does
have a 3-prong plug. And I know it will work with the MP3 player
because I have used it in the past.

Any sugestions?

Thanks,

PR



** Original message **

 

[Lydokane]

** Original message snipped **

I'm assuming this thing doesn't have a transmitter inside it? You
don't play it through your FM radio by tuning to a specific frequency?
Do you plug something into the tape to get the music from the player
to the audio system in the car? How does the music get from the
portable drive to the speakers? or are you using head phones?

**snip & reply**

When I have the MP3 player in the car the sound comes out of the unit
through left & right RCA jacks to a externally mounted FM modulator.
I then tune the radio to either one of three Freqs. Two choices in the
high 80's and one in the low 90's. This isn't the problem though. If
I turn the power to the FM modulator off but leave the MP3 player on I
still can't get a radio station.
When at work I run the sound out of the headphone jack (1/8 inch) to a
set of amplified PC speakers.

**end reply**

Does it get power from the cigarette lighter socket? Does the power go
into the drive at the same connector at the factory and in the car?

**snip & reply**

When in the car the player is mounted into a docking bay that is wired
from the fuse panel. I wired it off the radio fuse because I knew
that when the car was turned off the player would also be turned off
(in case I forgot to turn it off). I do not believe that this is the
problem because if I remove the player from the docking bay in the car
and use the docking bay that I have set up to run from the transformer
and set the player near the car (20 feet or less), FM radio is knocked
out.

**end reply**

Does the interference vary in frequency with the speed of the engine?
You say it isn't a problem at home but only in the car/factory - is
that because it is less noticeable or less objectionable (but still
there) or gone entirely? That would be a clue.

**snip & reply**

With the engine? Do you mean the car's engine? If so, then the car
engine has no bearing on the problem as the problem repeats if the
engine is off but the key is turned to accessory. The reason that it
isn't a problem at home is because I/nobody listens to the radio at
home. If there is music playing it is coming from the player and
running from the afore mentioned RCA outs to the stereo receiver.
However, if I try to tune the radio at home the RFI is still present.

**end reply**

Changing the position of the player matter?

Try one of the inexpensive clamp on (clamp around ferrite core) line
filters from Radio Shack on the power to the HDD on the 12V to it.
(close to the drive- not the power source) You may have an internal
DC/DC convertor broadcasting RFI.

**snip & reply**

I have tried the Radio Shack ferrite core, but I had attached it to
the end of the power line coming in to the docking bay (as close to
the bay/player as I could get it). This didn't work. I will try to
mount the ferrite core to the 12V power line of the hard drive (if I
can remember what I did with it), but I don't think that this will
work either. In the past, while trying to track the problem down I
removed the hard drive from the unit and then hit the power and the
RFI was still there only it was 'white noise'.

**end reply**

If the case is metal try grounding it, to the body of the car.

**snip & reply**

The case is mostly metal. This is something that I will try.

**end reply**

If the RFI seems dependant on the speed of the drive it is the drive
motor circuit doing it (you may hear that as it spins up to speed as
an increase in frequency) The broadcast path can still be the power
leads so the clamp may work.

If the clamp on ferrite filter just attenuates the noise, don't
despair, rejoice - you're on the right track and that's a good start.
The next thing to try are two chokes and/or sheilded power wires and a
ground.

Shields on wires should be grounded at one place (END) only.

If it depends on the speed of the car's motor, that's a different
game, you may need one of the inductor/capacitor filters on the + lead
of the power to the unit. If that's the case - it works better if the
frame (metal mounting - not the power lead!) of the inductor is
grounded to the car body. That, of course, probably won't fix the
factory problem.

If it is transmitting through the FM radio, as a way to get from the
mp3 player to the car's speakers, that opens a whole additional
trouble shooting tree.

** end **

Thanks again for the replies. I hope that a bright and talented mind
in this newsgroup can help me so that I can once again be able to take
my music to work because music is a terrible thing to waste.

In an effort to encourage a solution to this problem I am offering a
reward to whomever solves this problem of mine. The reward is for
$1,000,000... which will of course be written on a post-dated check
that will be against an account that has been closed for some time
now; but since I found an old check book of mine, I would hate to
waste the last check in the book.

PR

 

[Anthony Fremont]

Lydokane wrote:

Sounds like it's putting out some RF around 10.7 Mhz. Consequently it
gets into every FM receiver around. I suggest you wrap the player in
aluminum foil with smallish ventilation holes. This should shield it
well enough to mitigate the problem.

michael

 

[Lydokane]

Mr Fremont's reply does lead to another idea. Take a short wave radio
and /or scanner and see where the thing is broadcasting. That doesn't
cure the problem, but may provide new approaches on how to proceed.


BTW have you tried running the output cabling through the ferrite
core?

Let me/us know how it goes since this is a rare problem you have.

I don't have a short wave but I do have a police scanner. I'll see
what I get sometime this weekend.

By 'output cabling', do you mean the RCA outputs that I run to a
stereo receiver? If this is what you are referring to then I have
tried attaching the ferrite cores to the RCA outs as well as the main
power (as stated earlier in this thread). I have also tried attaching
to both at the same time. With two separate ferrite cores (not one
ferrite core to both the power and RCA output).

Once again, thank you all for your patience and insightful ideas for
working with this problem. I suppose if all else fails I could make a
lead box for the damn thing. It wouldn't do much for the portability
of the unit but at least I would have my music.

PR