Chris,

You are absolutely correct about getting turnings out.  It is difficult and
they tend to stand up and stick fingers very easily.  Otherwise it is quite
easy to turn the gap out without dissassembling and the wider gap gets
easier to clean.  I have also had some speakers with bolts or rivits from
front to back and these will sometimes break free after drilling out the
rivit and tapping a chisel between the magnet and pole piece.

My period is set at about 22 seconds so I think it must be electrical
problem involved.  What confuses me is the background noise and walk up test
don't seem much different than before when I could receive surface waves
easily.  I would expect the lower freqencies of these to look like a high
pass filtered FFT also.

Randy
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> | Message 3                                                           |
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> Subject: Re: Magnets
> From:    ChrisAtUpw@.......
> Date:    Tue, 29 May 2007 22:49:56 EDT
>
>
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> In a message dated 2007/05/30, rpratt@............. writes:
>
> > I have found the combination of teenage boys and subwoofers has created
an
> > abundant source of strong magnets for your type applicatiion.  We just
had a
> > junk day in town and speakers were plentiful.  If no junk day or
neighbor
> > boys then stop by an automotive sound shop and ask for some blown
speakers.  The
> > ring magnets are generally much larger ID than the visible gap so the
gap
> > can easily be cut out on a lathe or with a dremel tool and the post
turned to
> > fit your coil if need be.
>
> Hi Randy,
>
>        The problem that I have found with this approach is that most
speakers
> these days seem to use ferrite magnets and they are glued with epoxy to
both
> pole pieces. If you can get old speakers with cylindrical Alnico magnets,
> fine. Ferrite is rated for use up to about 250c and it looses it's
magnetism at
> 450 C. You need cook them to 150-180C, which is the temperature at which
epoxies
> loose strength and you can lever them apart.
>        If you have a lathe you may be able to turn out the flat top elect
> rode, but cleaning out the turnings out can be a problem, if you try to do
it with
> the mangets in place
>
>
> > I have been having trouble recording any surface waves lately.  I used
the
> > instrument several years ago and had no problem but now I do not seem to
get
> > low frequency.
>
>        I suggest that you measure the capacity of any old electrolytic
> coupliing capacitors and maybe replace them. PSN amplifiers use polyester
coupling,
> which should be reliable. Non polar electrolytic types are not expensive.
>
>        At what period is your sensor set? If you have a Lehman, but it is
> only set up for say 12 secs, it won't bring in 20 second waves very well.
They
> resetting to 25 secs?
>
>        Regards,
>
>        Chris Chapman
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