Tape Guide (Apr. 1980)

Precise Gausstimates

Q. I am puzzled and disappointed about the use of some pocket magnetometers I read about in Audio quite some time ago (early 1972). I purchased the R.B. Annis Co. Model 20 magnetometer which deflects full scale in a 10-Gauss field, having been led to expect residual magnetism on the order of 2 to 5 Gauss in the heads and guides of my tape deck. Unable to get any reading, I then purchased Annis' more sensitive Model 25, which deflects full scale in a 2-Gauss field.

Again needle deflection was minimal, about 0.2 Gauss using the clip-on probe, even when the deck had been used for 15 hours since the last demagnetization. Thinking that possibly mine is an exceptionally "clean" deck, I tried measuring the residual magnetism in a friend's deck (different make), but I got equally low readings.

I have reason to believe that both magnetometers are working properly, since the more sensitive model clearly registers the earth's magnetic field of about 0.2 Gauss horizontal and about 0.5 Gauss vertical; also, both meters indicate a field strength of about 0.2 Gauss at the tip of one of my screw drivers. I have been careful with both meters to ovoid overdriving them with any strong magnetic field.

As far as I can tell, there are no conditions under which my tape deck has residual magnetism of even 0.25 Gauss, which is less than that of the earth's field. Finally, I have never heard the effects of failure to demagnetize the heads. Thus, after years of scrupulous demagnetization every four hours, I am forced to the conclusion that demagnetization has been a waste of time. I will appreciate any enlightenment you can give me.

-Mark Cannelora; Santa Cruz, Calif.

A. I have not encountered a definitive study of the problem of magnetization in tape machines. The one who suggested that this may be a greater problem than most persons realize is Bert Whyte, Associate Editor of Audio.

What I know is principally what I have heard from others, including manufacturers of tape machines, principally home machines. It is quite possible that in better grade machines, such as those owned by you and your friend, adequate precautions have been taken against magnetization, and that machines of lesser quality face problems such as described by Mr. Whyte.

In my own case, my experience was much like yours--virtually no difference in magnetometer reading before and after magnetization; I used two different demagnetizers, one substantially more powerful than the other.

Yet manufacturers do constantly warn us (with an occasional exception) about the buildup of magnetism in the heads and guides. Should this happen, there is no question that the result would be an appreciable increase in noise and loss of high frequencies.

Therefore, the advice to demagnetize periodically seems to be a precaution against something that might happen but often doesn't. Since there is no harm in demagnetizing, nothing much seems lost by taking the precaution.

Incidentally, the usual prescription is to demagnetize after every eight, not four, hours of use.

Ferrite or Wrong?

Q. I own two tape decks, and in both cases the playback head wears rapidly. Is it possible to use ferrite heads as replacements? A local repair man told me that the switch to the ferrite heads could not be made. I am not convinced at all.

-Helmut Kranz; Philadelphia, Pa.

A. I know of no reason why you couldn't use long-life ferrite heads as a replacement. It would be necessary to obtain a head with the required mechanical features for mounting on your deck. In addition, the new record head must have electromagnetic properties similar to your old one to eliminate or reduce the need for changes in bias current and audio drive current.

You should be able to get additional useful information from a maker of replacement heads, such as Nortronics.

(Audio magazine, Apr. 1980; Herman Burstein )

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