Manufacturer's Specifications:
Form: In cassette shell.
Power Required: None.
Demagnetization: By permanent magnets.
Price: $19.95.
Company Address: P.O. Box 6021, Columbia, Mo. 65205.
Fig. 1--Output of D'Mag obtained from Nakamichi 582 deck in fast-wind/cue mode.
Horizontal scale: 0.2 S/division; vertical, 0.5 V/division.
At first glance, Discwasher's D'Mag is both interesting and puzzling. Certainly, it is handy to have a cassette deck demagnetizer in the form of a cassette, but the use of permanent magnets seemed wrong. Therefore, I removed the five shell screws carefully and lifted off the A side.
Resting on the B side were the two hubs, grooved on the outer diameter for a connecting rubber drive belt.
Resting against the outer edge was a round, flat, plastic container with small pinions on each face. The container was translucent enough to show a magnet structure with six outer poles.
The pinions were made to run within matching tracks in the A and B sides.
Teeth on the outer edge of the tracks made the magnet container spin at the same time that it was propelled along the track by the drive belt. The track is a large oval around the hubs, making the spinning magnets pass close to the locations for heads and capstans. The track is not a complete oval, for there is a break at the back of the shell. At the very end of the grooves, there are no teeth along the track edge, which prevents possible damage to pinion or track teeth at the end of the normal use cycle. That consists of inserting D'Mag, fast winding for 5 S, and then rewinding for the same length of time.
Basically, that is the time required to spin the magnet assembly from one end to the other at normal winding speeds. From this examination of the internal mechanism, we can understand how D'Mag can use permanent magnets to generate an alternating flux for demagnetization.
Travel along the track also provides the desirable characteristic of decaying flux at the end of the process. Figure 1 shows the output voltage from a Nakamichi 582 deck with the use of D'Mag for a 2-S period after sweep triggering, with the deck in "Cue" to increase the magnetic coupling from the rotating magnets to the heads. The alternating nature and the smooth decay of the waveform are obvious.
The playback head and the capstan shaft of an Aiwa AD-M700 deck were magnetized using a permanent magnet. D'Mag reduced the magnetization to satisfactory levels for both the head and the capstan, actually matching one battery-powered hand-held unit.
D'Mag was definitely better than a battery-powered, in-cassette unit for reducing play-head demagnetization, and the battery-powered demagnetizer had absolutely no detectable effect on the capstan magnetization. After a few uses of D'Mag, it was easy to tell when the whirling magnets had completed their trip along the track. I concluded it was important to make certain that winding times were sufficient to get from one end position to the other, to be ready for the next use. A change in the character of the sound indicated when that had occurred and that winding should be stopped.
For difficult demagnetization, a heavy-duty a.c.-operated unit is required. For day-in, day-out use by most audiophiles, the Discwasher D'Mag will be quite sufficient for demagnetizing the entire tape path, which other in-cassette units cannot do. D'Mag is recommended in particular for car stereo recorder/players where access is close to nonexistent.
The Discwasher device comes with a shielded storage clip which could prevent inadvertent erasures and perhaps help to keep the magnets at full strength.
--Howard A. Roberson
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(Adapted from Audio magazine, May 1984)
Also see:
Allsop 3 Cassette Deck Cleaning System (Equip. Profile, May 1984)
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