|by John French
(reprinted from MIX Magazine, Vol. 10, No. 8)
Magnetic heads: ah yes, those small blocks of metal with a million wires sticking out the back are probably, inch for inch, the most expensive real estate on your tape recorder. Why are they so expensive? Why do they wear? How long do they last? And a question often asked, What are they?
Click on a diagram to see a larger, clearer version in a new window.
|Mechanical uniformity and operational consistency, both track-to-track (within the head), and head-to-head (for replacement compatibility), is extremely important. Precision tolerances must be maintained throughout the entire manufacturing and eventual refurbishing process. Because of the close relationship between core geometry, head contour, wavelength, and EQ design characteristics of the electronics, variations or alterations to the original head contour will usually result in changes to the low end response curve of the tape machine.
In both metal and ferrite head design, careful consideration is given to the choice of materials that comprise the critical tape contact area. Considering that this area of a metal head consists of laminated mu-metal cores, laminated shields, gap, epoxy and housing materials, equal hardness and abrasive resistance is of major importance to insure even wear characteristics.
To aid in even and consistent wear (edge-to-edge), all metal heads manufactured today incorporate edge relief slots. These machined slots are located outside the edge tracks and within the minimum width tolerances of magnetic tape specifications. In practical theory, magnetic heads with edge relief slots are far less susceptible to uneven edge wear problems caused by variations in tape widths and the razor sharp edges generated in the tape slitting processes. Edge relief slots are rarely used in ferrite head construction due to the long wear characteristics, hardness, and edge chipping susceptibility of the material.
As illustrated in diagram A, metal heads are constructed in two (mirror image) halves, each containing "C" laminated cores. The mating inner surfaces of the two halves must be lapped and polished flat to insure consistency in gap length, inductance and, particularly in multi-channel heads, gap scatter and phase. Great care is exercised in this process to not workharden or destroy the surface permeability of the cores within the mating surfaces. Poor handling at this stage can effectively make a head assembled with a 100 micro-inch gap perform electrically as if it had a 200 micro-inch or greater mechanical gap. In general, most quality heads manufactured today exhibit about 10 to 20 micro-inches of work hardening within their mating surfaces, and this figure is calculated into the size of gap spacer specified in the head design.
Assembly of the two halves, particularly core registration of indexing alignment (i.e. left of gap core alignment with right of gap core) is very important. While exact alignment is ideal, most quality heads are built to within a .001 index error. (Refer to diagram A).
As evidenced by diagram B, ferrite head design and construction techniques are totally different from that of metal heads. Due to its extreme hardness and composition, ferrite material is very brittle, and great care must be taken in the choice of mating materials, glass bonding, assembling, machining and the finishing processes so as not to incur stressing. Since ferrite does not flex, problems such as cracking and chipping are a result of material stressing and usually appear during manufacturing.
One of the most misunderstood terms is that of glass bonded ferrite. Glass bonding is a critical timing, temperature, and atmospheric control process that forms the gap and bonds the two "L" cores together. In this procedure, the two "L" cores are shimmed apart by the gap length desired. Temperature is brought up slowly to in excess of the melting point of the glass and held for about an hour. During this peak temperature period, liquid glass flows into the shimmed gap cavity area and bonding takes place. The entire process, including the cooling period, takes between six and eight hours.
Much like magnetic tape, many different compositions of ferrite material are used in a wide variety of magnetic head applications. Most of the heads manufactured for audio recording utilize a composition of manganese-zinc, which possesses the highest permeability and greatest stability characteristics.
Head manufacturing is a complex balance of machining delicate materials while keeping the alteration of their electrical properties to a minimum. Equipment manufacturers specify exacting electrical and mechanical parameters to which heads are produced. Finished tolerances in magnetic heads vary from manufacturer to manufacturer. However, in general, most fall within the following specifications:
It is important to note that as a head wears, the inductance drops proportionally. As a general rule, when a head is worn out (tip depth within .001 of going through) inductance will usually measure about 60 percent of the initial (new) reading. Since output is a function of the number of turns and efficiency of the core material, level will not drop as a result of the reduction of inductance.
Head wear is directly related to:
As a magnetic head wears in normal operation, certain conditions and minor problems develop that indicate a gradual deterioration in performance. On metal heads, the symptoms are a result of minor changes in contour and tape-to-head contact loss caused by wear. These include: signal amplitude instability, slight loss of high frequency, and unstable tracking or tape path.
|Usually by the time any of the above symptoms appear, others are present as well, and the result is a serious overall deterioration in quality. Since wide variations exist between tape machines, the materials used in head manufacturing, and of course, the quality of maintenance, an expected head life calculation based on hours is very difficult. The data accumulated in our studies over the years indicates that, in most cases, magnetic heads failing to meet operational spec exhibit wear pattern depths that are less than 50% of initial (new) tip depth. In other words, heads that no longer meet spec, can usually be reconditioned at least once and, in most cases have greater than 50% of usuable life remaining.
It should be noted that a ferrite record or playback head in serious need of refurbishing will show no obvious wear pattern, groove, or change in contour (as compared to metal heads.) Upon close visual inspection of the tape contact area, a matte or dull finish is noted as compared to the high polish non-contact area. Under a microscope, the dull finish reveals a surface deterioration and pitting at and around the gap area. This surface breakdown alters the surface gap length and definition (sharpness). The procedure of relapping, recontouring, reprofiling, refurbishing and so on, performed properly, restores a magnetic head to its original specifications. Strict adherence is paid to maintaining original contour, ramp angle, gap/crown centering, and end to end crown radius consistency. Zenith, or tilt 90 degree reference to mounting surface, and precision finishing is essential.
The two diagrams below illustrate the most common wear patterns found on metal heads. These same wear conditions are typical on everything up to two-inch multitrack head stacks. Assuming that the contour is consistent end to end, the widest wear area is always the deepest.
Most uneven wear conditions are the direct result of tape path problems and/or head misalignment. The four drawings below represent wear patterns caused by misalignment. As a general rule, metal heads will usually wear slightly deeper at the edges. This is due to tape tension and curl effect of magnetic tape (as shown on the top left drawing).
Click on either pair of drawings to see a larger, clearer version in a new window.
|There are many combinations that can include any or all of the above wear patterns within a single head. The longer a head is allowed to wear uneven, the more material or useable life is sacrificed in refurbishing to correct the condition. This is because the relapping procedure references to, and must maintain, the original contour and tip depth consistency (end to end). Usually an uneven amount of material must be removed to correct an uneven wear pattern, as illustrated in diagram C.
Since tip depth is directly proportional to dollars, removal of the absolute minimum amount of material in the refurbishing process is of major concern. For example, a $2500 multi-channel head with a .010 (ten thousands of an inch) tip depth means that every .001 (one thousandth) of depth is valued at $250.
Electrical and mechanical analysis, which consists of inductance, DC resistance, Q, measured gap depth, wear depth, initial contour and wear pattern is an important part of head refurbishing. A complete analysis must be performed prior to and after relapping. It is this information, coupled with documented wear characteristics, design data on the specific magnetic head, and tolerances which enables accurate performance and expected life estimates. Considering that the cost of refurbishing ranges from 5 to 25 percent of replacement, the savings are substantial.
An important part of a refurbishing program includes the precision head mounting and optical alignment of the entire head assembly. Since a large percentage of head problems stem from tape path and/or initial head misalignment, it is very important to start with a known reference when re-installing new or refurbished heads. Our procedure of Optical Alignment includes a thorough cleaning, wear analysis and any required repair, replacement and realignment of all tape path components on the head assembly. Fixed tape guides, roller guides, wear posts and flutter idlers should be cleaned, rotated to a fresh tape contact surface (if non-rotating), set to the exact OEM reference height and aligned for perpendicularity and parallelism. Magnetic heads are Installed and aligned for exact track placement (within .0005 to edge of tape location). Azimuth, zenith, and track placement is aligned with the use of high powered optical viewing and digital measuring equipment. Head wrap is set for gap centering on the scrub or wear pattern.
The reinstallation of a properly refurbished and optically aligned head assembly usually requires only a minor azimuth adjustment (for phase alignment) and the standard electrical adjustments. Tape tension must be checked and adjusted (reduced) to the minimum acceptable setting. The time saved in the studio for mechanical installation can be substantial.
Often times, the overall effects of normal head wear, and various minor misalignments within the tape path, result in the machine being only slightly out of alignment. Head wear in one form or another, is a gradual, long term, but continuous process, which in many cases goes unnoticed. The requirements and demands for higher quality analog recording grow each year. It is hoped that the information contained in this article will help in the understanding of magnetic heads and the benefits of a good maintenance program. •