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Subject: RE: Recorder Hardware Question
From: VanBaren, Gerald (AGRE) (Gerald.VanBaren_at_smiths-aerospace.com)
Date: 2004-03-26


> -----Original Message-----
> From: rockbox-bounces_at_cool.haxx.se
> [mailto:rockbox-bounces_at_cool.haxx.se]On Behalf Of Ryan Press
> Sent: Friday, March 26, 2004 9:20 AM
> To: Rockbox development
> Subject: Re: Recorder Hardware Question
>
>
> Andreas Stemmer said:
> > I've got a broken recorder model here (old recorder 6GB). The broken
> > part is a small SMD part, but I don't know what it is. I
> marked it on
> > one of Phil's recorder photos from the rockbox homepage.
> >
> > http://home.vrweb.de/~andreas.stemmer/rec_iface_bottom.jpg
> >
> > The part is completely burnt, so there's no chance to
> identify it. If I
> > follow the traces, I can see that it was connected in
> series between the
> > center pin of the dc input and a zero ohm resistor which
> goes straightly
> > to the mc34063a dc/dc converter.
> > So what could it be? My first guess was that it was another zero ohm
> > resistor, but that doesn't make sense because there are no
> other traces
> > which have to be crossed. And we all know that the
> jukeboxes do not have
> > any protecting diode. Some kind of fuse? Any ideas?
> >
> > I repaired it by bridging the crater with a diode and the
> recorder works
> > well again, but I'm still wondering what it was.
> >
> > Andreas
>
>
> Hi Andreas,
>
> That component is a ferrite bead; an inductor of sorts. It
> is there to
> attenuate the high frequency noise that comes in on the power
> wire. This
> noise (if very bad) could cause the microprocessor to lock up
> or you may
> hear something in the headphone output.
>
> It is not really needed but here should be a suitable replacement:
> <http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=20989&Row=41105&Site=US>
>
> Ryan

Actually, ferrite beads (inductors) at power inputs are without any exception I know of there to prevent internal noise from leaking OUT (noise from the switching power supplies and microprocessors). This is necessary to pass FCC/VDE/CE interference requirements. It is non-essential as long as you are OK with the noise you may pick up on nearby radios and TVs (that was somewhat tongue-in-cheek -- you probably can find noise on a nearby shortwave AM radio, an may even be able to tell the difference in signal strength with and without the inductor, but it is highly unlikely that you will notice anything with normal use).

It is very odd that the smoke escaped, however.

For normal every day life, a diode is probably a more useful component in that spot.

gvb

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