by Howard A. Roberson
Just as with the purchase of a home, a car, or a major appliance, your responsibility as an owner of audio equipment does not end with your selection of components. To get the maximum pleasure from your system for the maximum number of years, you must periodically check its condition and perform routine maintenance tasks. The techniques and tools for proper care of open-reel recorder heads will be covered here, and much of this can be directly applied to cassette decks.
Keeping Heads Clean
Maintaining clean heads on a recorder demands that the entire tape path be kept clean. If you lake a good look at the route the tape follows from the supply reel all the war to the take-up reel, you'll find tape-tension/tape-break arms, guides and posts, the capstan, the pinch roller, perhaps a pressure pad, and a lamp detector assembly to sense tape run out. Any of these points is a potential source of scraping the tape surfaces or edges and a potential site for build-up of debris. Figure 1 is a close-up picture of a quarter-track head that is both dirty and worn.
Fig. 1--Worn and dirty quarter-track record head.
Fig. 2--Partial removal of Aud Vid Com "A" coating.
Fig. 3--Dirt removed by rotating a cotton swab.
Cleaning Products: Many different types of cleaners have appeared on the market, but most of the present formulations are based upon isopropyl alcohol or TCF (trichiorotrifluoroethane). The advantage of the alcohol is that it is inexpensive and readily available at a drug store. You must be certain, however, that you do not buy nabbing alcohol, which may have such harmful ingredients as lanolin added to the alcohol base. The desired product should be clearly labeled "91% isopropyl alcohol," as is Stericol from Berkeley Drug Co. Alcohol will probably not leave any residue on the heads, but it can be damaging to rubber pinch rollers over the course of time. There is no such disadvantage with TCF, which also will not attack plastics.
I evaluated tape-head cleaners made by Aud Vid Com, Chemtronics, Miller-Stephenson and Nortronics, as well as Stericol. Each product was rated for the ease with which it cleaned the deposits on the head and the extent to which it left the head completely clean. A few unnamed isopropyl cased cleaners did a good job of removing the tape residue, but they left behind a thin layer of something white and streaky. There was no similar deposit from Stericol, available at a fraction of the cost. TCF products, in spray cans from Chemtronics and Miller-Stephenson and in bottle/cans from Miller-Stephenson and Nortronics, all did the task in excellent fashion with no detectable residue.
Aud Vid Com is a two-fluid type of cleaner which the manufacturer claims will polish as well as clean heads. The "A" fluid is applied, allowed to dry, and then (Fig. 2) wiped off. The "B" fluid is used for the final conditioning. The two-step process made for good cleaning, but no better than any of the TCF cleaners, which involve only one step and cost less. Examination of both old and new heads under a microscope showed that Aud Vid Com did fill in some minor scratches in one or two cases, but in most cases there was no observable improvement.
There was little difference among the top cleaners, but my favorite was the Miller-Stephenson, followed very closely by Nortronics, and then by Chemtronics.
Tools and Techniques: I have a number of head-cleaning kits, and they do perform well. Those with angled handle fells are particularly good for recorders with recessed heads, such as the Revox A77. For most recorders, however, I use cotton swabs, the Chesebrough-Ponds Q-tips, which are well made and won't keep dropping fibers while you clean. Although I occasionally use a spray can, I really prefer to use the liquid TCF in a small, pump spray (empty Sound Guard) bottle so that I can spray the exact amount I want directly on the Q-tip.
Most manufactures of cleaners tell the user to "scrub” the head, but don't do that: You might scratch the head if hard particles are present. Let the cleaner do its work--loosening the debris for easy removal. Move the Q-tip across all parts of 'he head while rotating the swab (Fig. 3), as this will lift the dirt off the head and remove it. Many times it is best to use the first swab for rough cleaning of all the surfaces, including the guides and pinch roller. Use as many Q-tips as necessary to make certain that you are not re-depositing dirt already removed.
Demagnetization
Despite improved circuitry in recorders the past few years, it is still possible that there might be some residual magnetism which could be detrimental to an important recording.
Demagnetize after every 20 hours of use, before attempting any important recording, and before ever playing an alignment tape, if you wish to be absolutely safe. Specific recommendations by the recorder manufacturer should be followed faithfully.
Demagnetizers: Figure 4 shows two Nortronics demagnetizers with bent-tip rods, an old and inexpensive flat-pole piece unit from Lafayette, and the rugged Annis Handi-Mag. All but the Lafayette have the ends cowered with plastic, which is essential to ensure that the heads are not scratched during demagnetization. The Handi-Mag put out the highest flux level, making it my first choice, particularly when there are guides that would benefit from such attention. The large size of its pole pieces, however, prevent its use with recorders with limited head access. In such cases, the Nortronics units can induce more flux into the heads since direct contact can be made.
Fig. 4--Head demagnetizers.
Fig. 5---An assortment of tools for head alignment.
Demagnetizing Techniques: It is important to realize that demagnetizers will also work on VU meter pole pieces and alignment tapes, so don't be careless. The meters will not be damaged by the demagnetizer if used in normal fashion. Make it a practice to turn the unit on and off at least four feet from the recorder. After turn-on, bring the demagnetizer to the head (or guide) slowly, make gentle contact, and then move it slowly away. Do each element in turn, including separate track pole pieces if need be, always moving the demagnetizer smoothly. If you need to turn it off before finishing, trove it away first. If you accidentally turn it off at a short distance, do all the demagnetizing over-again referring to the manufacturer's instructions.
Head Alignment
For the serious audiophile or semiprofessional, head alignment can be a prime concern, particularly if the heads need to be replaced.
When all of the heads are in perfect alignment, their gaps are perpendicular to the reference mounting plane and the line of tape travel (azimuth), and the head faces are also perpendicular to the plane (zenith). Immediately, a reader may well wonder where that reference plane is on his recorder. Sometimes it is very obvious for there is a flat, metal plate providing support for everything involved in the tape path. With other recorders, there may be just a small plate supporting the head assemblies.
What we do know is that the playback head should be in alignment with a standard test tape, the record head should be in alignment with the playback head, and the erase read should match as well. We can all agree on the basic approach, but let's take a look at the tools and techniques.
Tools and Instruments: Figure 5 shows a collection of tools and other aids for head alignment. Going clockwise from the lower left, we see an Ivie IE-20B pink-noise generator, nail polish, a small machinist's square, Nortronics Magview, a flashlight, head cleaner, a Magnetic Reference Laboratory test tape, an Ivie IE-30A 1/3-octave RTA, a pull scale, eraser pencils, and, finally, a steel scale with graduations for each 1/100 inch.
There are other things hat will be used, of course, including an oscilloscope and audio monitor jeweler's loupes, to say nothing of a sine-wave source. If the entire head assembly is removable, some work might be most easily accomplished on a Pana-Vise surface plate. Various heights can be checked and transferred with a Nortronics PF-720 height gauge. Additional tools may be helpful at times, but it's time to discuss the procedure for doing the alignment, including what must be done if heads need replacing.
Checking Head Alignment: The first steps are to clean and demagnetize the tape path and to make any adjustments needed on tape tensions.
Look for any rough or sharp edges which might call for repair work before putting an alignment tape in your recorder. Clean any pressure pads with light brushing; do not use cleaner. Replace any pads that are hardened and/ or packed with oxide. The MRL swept sinusoid test tapes are my favorites for Iwo reasons: The swept-response that is shown or the scope can be used for both playback equalization and head azimuth adjustments, and their length of several minutes gives plenty of time for both of these tasks. Use the azimuth adjusting screw to get the maximum output at the highest frequency.
Alternatively, make the adjustment for exact (as possible) phase correspondence between tracks, using a two channel scope. (I am assuming the reader is most interested in two-channel stereo, with quarter-track recording format preferred.) After playback head alignment, the record head needs to be checked. I prefer to use a pink-noise source at -10 to -20 VU, with the playback fed to the 1/3 octave RTA. The azimuth peaking is done to get the maximum response from the 20-kHz filter, gently centering the adjustment between the initial fall-off points. The ETA display also allows making bias and record EQ adjustments at the same time for the best overall response. Discrete tones can be used, of course, with a low frequency for a rough azimuth setting, shifting to a higher frequency for better resolution. At the same time that all of the alignment checks are being made, observe the stability of the outputs as well as the amount of the high-frequency roll-off. What you see will indicate how soon you might have to replace one or more of the heads.
Replacement of Heads: When there is unacceptable high-frequency roll-off after alignment, and it can't be corrected with bias or EC adjustments, and/or the levels are bouncing up and down, it is tire lo think about changing the heads. Take a good look at each head face, aided by plenty of light and perhaps some sort of magnifier. See if there is a definite area that has worn away, perhaps on the order of 1/8 inch wide or more. If so, there will be little shoulders where the top and bottom edges of the tape would formally be.
These are points where the tape can be lifted away from the head gap(s) by the unworn shoulder when the tape shifts slightly up or down. Look at the wear areas of all heads very carefully.
The edge of the wear pattern should match the upper (or outer) end of the track 1 (left channel) pole pieces and their gaps.
Many times it makes sense lo replace all of the heads, even though one may show little wear: The advantage of replacing them all is that the tape path will then be determined by a smooth flow across the new head faces and not by the shoulders in a somewhat worn head. Before removing old heads, make notes on all wiring and its color coding. Carefully clean all terminals of the new heads using the pencil erasers. If possible, do not loosen any screws that affect the head height. Do note the location and purpose of each screw associated with the head assemblies and write down the number of turns given to any of the screws and determine their pitches (threads per inch).
When a new head is placed in its support carrier (or holder), make certain that it has the same in-out, left right position as before to ensure getting the correct wrap of the tape on the face. Return all screws to their original positions, adjusting the azimuth screws to make the gaps perpendicular, perhaps aided by a small square. Then, as shown in Fig. 6, use the square to make certain that the face surface is perpendicular to the plate-placing a white card behind the head will help you see exact verticality when the square is right up to the face. The Nortronics height gauge, preset to match the older heads, can verify that the height is still correct after any head tilting in zenith to match the square (Fig. 7).
Fig. 6--Using the machinist's square to make the play head face perpendicular
to the mounting plate.
Fig. 7--Using the Nortronics PF-720 height gouge.
The next step is to find out how the tape actually lies on the heads as far as height is concerned. First, record a high-level (+3 VU), low-frequency (400-Hz) tone on a bulk-erased (or new) tape on all four tracks at one end of the reel. Pull out a section that is recorded and spay it with Magview, oxide side, of course. When the fluid evaporates, there will be a pattern on the tape showing the location of the recorded tracks. If you have been careful, and maybe a little lucky, the pattern will look like Fig. 3, which shows three equal spaces among the four recorded tracks. If the spaces are quite different, you will need to move the record head:
Up, if the space in the center is larger, and down, if the space in the center is smaller. It is possible to measure the actual spaces directly with magnification and an excellent scale, but it is tricky.
Fig. 8--Results with Magview after record-head height adjustments.
Fig. 9--Intertrack spacing as a function of vertical head position.
Fortunately, there is a much easier method. Look to see what the ratio is between the outer spaces and the middle space, and then refer to Fig. 9 to determine how much the head should be shifted. For example, if the top (and bottom) space appears to be twice as wide as the middle space (TS/MS = 2), the head should be moved down 0.005 inch to put it at the correct height. If the height-positioning screws have 4-40 threads, a full turn would mean a shift of 0.025 inch. We want, Therefore, to turn the height-adjusting screws just one-fifth of a revolution.
Turn them exactly the same amount to keep the head face zenith setting correct: Recheck with Magview, and do any trimming required to adjust the height exactly.
Record high-level tones on each of the four tracks on bulk-erased tape. With both scope and audio monitors, check for proper erasing, one, track at a time. Check the separation between channels, but pay special attention to any crosstalk between tracks 2 and 3 (right channel with opposite play directions). Disconnect the source while these tests in playback are being made to prevent any source-to-tape leak through from appearing as a head height problem. If erasure is poor or if there is crosstalk, examine the erase and/or playback head heights with the tape running. Shift the head(s) involved to make the tape-edge/end-of-gap alignment more exact, and re-run tests.
When completed, perform playback and record alignments as discussed earlier, after demagnetizing the heads. Finally, apply a spot of nail polish to lock all azimuth adjusting screws in glace.
The instructions and guidelines described here should not involve an inordinate amount of your time. But by following these basic steps, you will prolong the useful life and fidelity of your open-reel recorders and cassette decks, and I hope that the information I've provided will serve you well.
(Source: Audio magazine, April 1982)
Also see:
Basics of Tape Performance (Sept. 1982)
Open Reel Recorders (The Mechanism Of Magnetic Tape Erasure; Focus On Head Demagnetization) (April 1981)
Performance of High Energy in Magnetic Materials in Audio Cassette Recording Tapes (Sept. 1978)