Demagnetizing a tape recorder
Wolfgang Bleier, Austria
March 2007
Many questions and opinions concerning maintenance of reel to reel tape recorders circulate in the Web. Although quite important, one of the least-documented topics seems to be, how to correctly demagnetize the heads and the tape path of a tape recorder. Here is what I could gather about it.
The basic cause of magnetism is the spin and orbital motion of electrons. Charged particles in motion create magnetic moment. Thus, spin and orbital motion of electrons produce magnetism.
Ferromagnetism
Generally the magnetic moments of atoms are randomly oriented. But in ferromagnetic substances they become aligned in domains due to exchange interactions between neighbouring atoms. They become permanent magnets in a magnetic field. That is to say, that magnetism in our recorder‘s tape path not only arises from the quantum-mechanical spin and orbital motion of electrically charged particles, but it also remains in a tape head and other ferromagnetic parts of the tape path.
It's not a myth, it's real
Over the time, the magnetically charged audio tapes may leave residual magnetism in the metallic parts of the tape path that impairs the ability of subsequent tapes to record or playback accurately. Sooner or later the result may be increasing hiss, noise and distortion, which at worst can mean the loss of a recording. What in essence we do against it is to bring magnetic crystal structure in a ferromagnetic material from a more or less aligned into a random orientation. We do this with a degaussing coil that produces an alternating magnetic field, and by a procedure that first magnetizes the metallic part until it reaches saturation and then slowly decreases the amplitude to (almost) zero, at which point the magnetic particles remain in random orientation without magnetism.
Opinions are varying regarding the question, how quickly residual magnetism can build up in a tape head and other ferromagnetic parts in the tape path. Certainly this depends on several factors, including the tape speed, recording levels, the type of tape, the composition of the tape head, and whether a recorder is used rather for recording or for playback. Replay of ordinary recorded tapes at 7,5 ips (19 cm/sec) or less has rather little magnetic effect on the tape head especially on late models of "modern" reel to reel recorders, and demagnetizing after 30 to 40 operating hours should suffice. On the other hand, if a professional recorder is used as a studio recording work-horse, demagnetizing becomes necessary much more frequent, if not before every recording session depending on how much loss of recording quality you are willing to risk.
In the beginning I've demagnetized the tape path of my recorders approximately every three months (usually I replay tapes just for a couple of hours a week, and only once in a while I record a tape). Today, since I became quite familiar with the procedure, I demagnetize after approximately fifteen to twenty tapes have passed the heads. In any case I demagnetize the tape path before a recording session, if not recently done anyway.
How ?
First of all, use a strong demagnetizer. Since many years I am using a Han-D-Mag made in the United States by R.B. Annis Co., which I can highly recommend. The device is offered also for European 230V voltage. It outputs a magnetic field intensity of 350-400 oersted and is several times more efficient than tape deck brand demagnetizers (up to 100 oersted). The "Han-D-Mag" can also bulk erase tapes using the opposite side of the probe.
){kind=link}
Before the Han-D-Mag I've used a degausser that had to be energized by an on/off push button. This push button took too much of my attention to keep it safely pressed, while at the same time I had to concentrate on demagnetizing. Let go this push button could have converted the tape heads into permanent magnets. It was the only effective hand-held device I could find at that time, but made to demagnetize tools rather than tape heads. In any case the tip of the demagnetizer should be covered with heat shrink tubing or plastic electric tape so it will not get in touch with or scratch the delicate tape heads.
Before you energize the demagnetizer, remove everything that is at risk in magnetic fields (especially recorded tapes, bank- and credit cards) and switch off your recorder. I also unplug all cables which perhaps is a bit obsessive, but caution costs nothing but an extra moment or two.
Demagnetizing
Most of relevant literature that I could find in the Internet appeared pretty varying as to the procedure of demagnetizing. For quite some time I believed in a ritual to power the demagnetizer in well two meters distance from the recorder, and switch it off again in two meters distance after demagnetizing the tape path. What I did was kind of degaussing-voodoo until I have found an article by Jay McKnight of Magnetic Reference Laboratory in the US that is linked on the website of Richard Hess. In this article Jay McKnight explains in detail that switching on or off the demagnetizer in a distance of (at least) 7 cm from the heads is non-critical.
Well, there is no need go to such limits. Now I switch on the demagnetizer in a distance just as far as my arm can comfortably reach, then bring it slowly to the first part of the tape-path and slowly move on with smooth waving action in close distance from the first part of the tape path to the next, until the entire tape path is done. Then I very slowly remove the demagnetizer from the recorder (about 5 cm per second) and switch it off in the same distance where I have started.
Leaving the tape head to "cook" in the alternating demagnetizing field is of little value, so just continue moving the degausser. It is important to avoid jumpy movements, and to be very close to the heads in order that they get fully magnetized and reach saturation. Move the tip of the demagnetizer slowly and smoothly until all parts are done, and especially while moving away the demagnetizer from the recorder, which is the important final process of demagnetizing before you switch off the demagnetizer in a safe distance. In case that you make a quick movement, slowly move back to that place and redo.
If your demagnetizer has a switch or push button, make sure you’ll never switch it off nearby the heads, even though this thing can get quite hot. The collapsing magnetic field could leave you with a set of magnetized heads that will be difficult to demagnetize without a much more powerful degausser, or, as some say, at the worst case it could be the end of your tape recorder's heads!
And last but not least, don’t forget to demagnetize also your tools that you use for maintenance and repair of your recorder.
The above procedure preserves the clear sound of my Studer, Revox and Nagra reel to reel tape recorders as well as of my Studer cassette deck.
Cleaning
I clean the tape heads and other metallic parts with some Isopropyl alcohol ("90% Isopropanol") on a cotton bud always after around ten tapes have passed the tape path. Do not soak the cotton bud or make the head dripping wet, it should be rather damp. Do not just distribute the contamination, but rub firmly in only one direction until the cotton no longer shows contamination being removed. The pinch rollers I clean frequently using only a wet cloth, from time to time I use a special rubber cleaner and have no problem with that. To me nothing is worse than good but filthy audio gear, hence I made regular cleaning outside, and once in a while also inside of the recorder a routine.
For all the rest of care and repair I see Mr Anton Straka in Vienna, Austria ... he’s the uncontested expert in town!
