The Mellotron Group

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 
> capstan. 
> >  Suggestions?
> > 
> > Bernie
>  >
>