Figure 1: Inside the head assembly, an individual track

HEAD OF THE CLASS
Figure 1 is a single track from a Record/Playback head, emphasizing the gap, a.k.a. the “magnetic lens” that is quite literally focused when azimuth is adjusted. The gap width determines both the amount of over-bias for your machine as well as the efficiency of getting high-frequency signals to and from tape. From bottom to top, Fig. 2 shows a worn 2-track head face (to emphasize the uneven wear pattern). In the middle, the view shows how tape wears a groove into the head surface where oxide can easily collect and degrade performance. Relief slots minimize oxide collection. (Top image as well as Fig. 3d.) Figure 3 (highlighted in vivid blue-magenta on the left) clearly details how worn heads become flat and collect oxide.

RULE OF THUMB
Three of four possible mechanical head adjustments—Zenith (front-to-back tilt), Height and Wrap—are best left to a head-restoration company that uses precise optics to determine the optimum tape-to-head contact. Figure 4 is but one example of a head assembly layout, but don’t worry if your machine doesn’t match the picture. An Ampex MM1200, for example, has no azimuth, height or tilt adjustments.

Figure 2: Edge track performance can be improved by adding relief slots during the “relapping” process. The pink arrows on the face of the worn head indicate an uneven wear pattern.

Two methods can help confirm that contact area of the head has been optimally positioned. While playing a high-frequency (10 kHz or above), apply light pressure to the tape with your thumb (on a multitrack head, slide thumb from top to bottom as well as both sides of the crown). An output increase indicates work to be done. You can also “paint” the head’s crown from top to bottom with a Sharpie pen and then run some noncritical tape over the head until the ink wears off. Use a magnifier to see that the gap is centered and that the wear pattern is consistent from top to bottom.

Note: If you don’t positively know the location of the azimuth adjustment, don’t do it, okay?

AZIMUTH
For full-track (mono) heads, simply adjust for maximum output at each of the designated high frequencies on the test tape. For a multitrack machine, summing all channels is the best way to optimize azimuth for the entire stack. Using only two tracks—as you are forced to do on a stereo deck—can generate a “false positive” reading when summing the two channels.

To adjust azimuth without an oscilloscope using the 2-channel summing method, mechanical VU meters (referred to in this example as the Mix meter) are rather essential. Start with 1 kHz (on the test tape) and set levels, via console fader—one channel at a time—so that the Mix meter reads -6VU. When both faders are up, the Mix meter will read 0VU. Using the 8kHz tone, adjust azimuth for a maximum on the Mix meter. Then do the same with the 16kHz tone. Because the wavelength of the 16kHz tone is smaller, it is possible to misalign the head to a lesser peak, so be sure to check 8 kHz a second time.

An oscilloscope is a more precise tool for measuring azimuth. Connect two tracks to the X and Y—horizontal and vertical—inputs, respectively. Figure 5 shows a family of seven “Lissajous” patterns. A single channel would create either a vertical or horizontal line. Both channels, perfectly in phase, will create a line at a 45° angle (/). Either channel out-of-phase creates a 45° line in the opposite direction (\). Anything in between creates an ellipse, except a 90° phase error, which is a circle.

HIGH-FREQUENCY PLAYBACK LEVEL
High-frequency equalization is adjusted to 10 kHz, the location of which will vary depending on the type of test tape. Assuming the machine’s equalizers were correctly designed, test tapes that include tones up to 20 kHz can truly reveal the strengths and weaknesses

Figure 3: Tascam 80-8 half-inch 8-track heads details: a.) The “crown” or radius of the unworn section of the head. b.) The “schmutz,” oxide buildup from bad tape. c.) The “flat spot” caused by tape wear. d.) Relief slots improve the edge track’s immunity to oxide buildup.

of the playback system. In Part One of this article, the curves in Fig. 1 specifically detail one machine’s response at various speeds—30 ips (top), 15 ips (middle) and 7.5 ips (bottom)—although they are equally good for demonstrating the effects of head performance.

A head will eventually wear beyond its usable gap depth (see Fig. 1 for this article). As the gap widens, the frequency response will begin to look like the response at 7.5 ips instead of the ruler-flatness of 15 ips. Due to an uneven wear-pattern, a minor Zenith mis-adjustment can quickly reduce multitrack head life so that one track may reach the end of its Gap Depth before the others.

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Reprinted with permission from Magazine, September, 2000
© 2000, Intertec Publishing, A Primedia Company All Rights Reserved