R.B. Annis makes nice ones... But here's something you might want to read
and then do some follow up research on the subject. This might also be a
good discussion for this forum. I have some insight...
_http://www.rbannis.com/products/magnet/5.html_
(http://www.rbannis.com/products/magnet/5.html)
I did not write this, but my research indicates that there is a lot of
support for this....
"First, what I'm about to write will be viewed by many as controversial, by
others as wrong, by still others as hopefully insightful. But I have no
doubt that, because what I'll say flies in the face of "common wisdom",
"standard practices" and even many maintenance manuals, there will be those that
just ignore all of this. That's fine, do what makes you feel good, but
there is at least some science behind my statements below.
Several decades ago I did some in-depth research into the effectiveness,
and necessity of demagnetizing recorder parts as part of routine
maintenance. I won't bore with details, but part of the research involved renting a
lab-grade hall-effect probe gaussmeter in order to measure DC magnetic
fields, analyzing their effect on the recording process, and their effect on
pre-recorded test signals. In addition to the gaussmeter, a swept spectrum
analyzer was used, as was a low distortion sine generator, and several strong p
ermanent magnets as DC field sources.
In short, what I found is this:
1. Operating a tape recorder does not cause anything in the tape path to
become magnetized. Not even a tiny bit.
2. All parts including heads have made from materials with a natural
neutral magnetic state, to which they tend to return even if deliberately
magnetized.
3. The magnetic flux required to affect a recorded tape is much higher
than anything anywhere around the tape recorder, except in the erase and
record head gaps. The effect of a steady-state magnetic field (a DC field) on
recorded tape is first noticed as partial erasure of high frequencies, and
that occurred with a field somewhere above 200 Gauss. Tape has very high
coercivity by design, meaning you need a high field strength to cross the
threshold above which the tape will become magnetized. That's what the bias
oscillator does. By being many times hotter than the actual audio signal on the
head, the bias forces the signal over the coercivity threshold into the
more "linear" range of the tape. But that means it's harder to erase tape
than you might think. Typical guide and head residuals were in the zero to 20
gauss range, far below tape coercivity levels.
4. Erase and record heads are self-demagnetized by the bias oscillator,
which provides an AC field in the gap that is thousands of times higher than
what a de-magnetizer can induce.
5. The field found on recorded tape is incapable of magnetizing anything.
It is minuscule, and when the tape moves, it becomes an AC field, which
would demagnetize if it had any effect.
6. The effect of a DC field near or in a record head produces two
measurable results: First, a significant elevation of even-order harmonic
distortion, and second, an elevation in low frequency noise (a sort of gravelly
sounding noise). But again, the field has to be fairly high for this to happen.
You need a spectrum analyzer to differentiate even harmonics from the
normal odd harmonics created in the recording process.
7. If tape machine parts become magnetized, there is a defect in
manufacture (the material doesn't have a natural, neutral magnetic state), or a
defect in the tape machine, either due to a failure or design flaw.
Asymmetrical bias waveforms will present a DC component, any DC on a head due to a
circuit defect (bad blocking cap, for example). Demagnetizing may temporarily
remove the residual, but if it creeps back, something is "broken", and needs
repair or replacement.
8. Lastly, casual use of a demagnetizing device, even if the classic
proper techniques are observed (slow removal, switching of when several feet
away), sometimes resulted in an increased magnetic state rather than a
decrease. There's no way to tell if you are demagnetizing, or magnetizing, without
further testing for the effects of magnetism. Once again, these are HF
erasure of a recorded tape, or increase in noise and even-order harmonic
distortion during record. Yes, you need instrumentation to do this.
During the research, I also found that attempts to deliberately magnetize
a tape head with a strong permanent magnet were successful, but over time,
the head returned to a neutral state by itself. It took 24 to 48 hours, but
then all the effects of magnetism vanished. The same was true of guide
parts, though some could not be magnetized being non-ferrous.
In summary, routine demagnetization is unnecessary, and could actually
make things worse. If you plan to demagnetize, you also need to be able to
measure the effects of residual magnetism to see if you've made it better or
worse, or if it was needed at all.
Have fun,
Jim
In a message dated 3/8/2013 7:09:57 P.M. Eastern Standard Time,
mellotronex@earthlink.net writes:
Frank.
Can you recommend a magnetometer?
This is the only one I can find.
_http://www.amazon.com/Pocket-Magnetometer-100-0-100-Gauss/dp/B0042U1AP8/ref
=sr_1_1?ie=UTF8&qid=1362787001&sr=8-1&keywords=Magnetometer_
(http://www.amazon.com/Pocket-Magnetometer-100-0-10
0-Gauss/dp/B0042U1AP8/ref=sr_1_1?ie=UTF8&qid=1362787001&sr=8-1&keywords=Magnetometer)
Thank you.
js
M400 #215
On Mar 8, 2013, at 4:28 PM, _lsf5275@aol.com_ (mailto:lsf5275@aol.com)
wrote:
Bernie, with a Han-D-Mag you can de-mag a headblock in a minute or two.
But that thing is not to be trifled with. It will do all kinds of cool things
but can really fuck up your shit. Get your tapes out of the room. Do all
the heads at once... Power on... slowly sweep from head to head... move the
HCM away and unplug.
The magnetometer is the thing. It lets you know for sure if or where you
have problems, and it lets you know if you solved them. No guessing.
Later you might want to get the Han-D-Mallet... for when nothing else is
working right.
In a message dated 3/8/2013 11:48:33 A.M. Eastern Standard Time,
_tron400@yahoo.com_ (mailto:tron400@yahoo.com) writes:
Thanks for those links, Frank.
Bernie
--- In _newmellotrongroup@yahoogroups.com_
(mailto:newmellotrongroup@yahoogroups.com) , lsf5275@... wrote:
>
> Han-d-mag
>
> __http://usrecordingmedia.com/handmagdebyr.html__
(http://usrecordingmedia.com/handmagdebyr.html_)
> (_http://usrecordingmedia.com/handmagdebyr.html_
(http://usrecordingmedia.com/handmagdebyr.html) )
>
> Getting a good magnetometer is a good idea. I find mine to be
indispensable
>
> __http://www.rbannis.com/products/handmags.html__
(http://www.rbannis.com/products/handmags.html_)
> (_http://www.rbannis.com/products/handmags.html_
(http://www.rbannis.com/products/handmags.html) )
>
>
> In a message dated 3/6/2013 12:33:34 P.M. Eastern Standard Time,
> tron400@... writes:
>
>
>
>
> Hey Frank, when I do a search on Man-D-Mag, I just get hits on Mad
> Magazine.
>
> Bernie
>
> --- In __newmellotrongroup@yahoogroups.com_
(mailto:_newmellotrongroup@yahoogroups.com) _
> (mailto:_newmellotrongroup@yahoogroups.com_
(mailto:newmellotrongroup@yahoogroups.com) ) , lsf5275@ wrote:
> >
> > Buy a Man-D-Mag and a humidifier.
> >
> >
> > In a message dated 3/6/2013 6:54:32 A.M. Eastern Standard Time,
> > tron400@ writes:
> >
> >
> >
> >
> > Well, not just M400's.
> >
> > Yesterday, after turning mine on and letting it warm up, I did the arm
> > across the keyboard trick to the end of the tapes and 3 of them didn't
> return.
> > I had to remove the keyboard to release them (loosening the pinch
roller
> > and pad didn't do it).
> >
> > I played with one of the keys until the tape wouldn't return, even
after
> > removing the keyboard. The tape had bunched up in front of the
capstan.
> When
> > I unbunched it, I found that a small tape loop had gotten in between
the
> > capstan and the long felt pad in front of the capstan. The tape
decided
> to
> > take this route instead of going into the spill box. Static? The room
is
> > pretty dry with forced air heat.
> >
> > I haven't ever demagged the heads or capstan. Does that sound like
what
> > needs to be done? I have an old fashioned head demagnetizer (this
kind:
> > ___http://www.splicit.com/images/Demagnetizer2.jpg___
(http://www.splicit.com/images/Demagnetizer2.jpg__)
> (_http://www.splicit.com/images/Demagnetizer2.jpg__
(http://www.splicit.com/images/Demagnetizer2.jpg_) )
> > (__http://www.splicit.com/images/Demagnetizer2.jpg__
(http://www.splicit.com/images/Demagnetizer2.jpg_)
> (_http://www.splicit.com/images/Demagnetizer2.jpg_
(http://www.splicit.com/images/Demagnetizer2.jpg) ) ) ), but I'm not sure it will work on the
Show quoted textHide quoted text
> capstan.
> > Suggestions?
> >
> > Bernie
> >
>
