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By William B. Fraser
NOT LONG AGO, a customer brought me a tape recorder for repair. The complaint was poor and erratic sound.
The recorder was an expensive one and almost new. It didn't seem likely that the electronics were at fault. A check of the heads located the problem immediately. The playback head had been mounted incorrectly, so that the height was grossly out of adjustment. In the meantime, the machine had been operated enough so that an obvious tape groove was visible on the face of the head. Under these circumstances, it is usual to consider the head has been ruined. The conventional solution is to install a new head.
But a new head was expensive and not readily available. Further, the customer was anxious for speedy repairs. After some thought, I decided to attempt to salvage the head by re-facing it. I have not heard of this being done in a service shop (or elsewhere for that matter) and approached the job with some apprehension. Fortunately, one thing was going for me. The head was ruined anyway, so what was there to lose? The results of this first attempt were so satisfactory that subsequently I have refaced about a dozen other heads with 100% success. I have often wondered how long these reconditioned heads would last. So far it has been impossible to determine, simply because no customer has ever returned with a complaint. As a guess, perhaps the initial life expectancy of the head has been doubled.
Anything on the face of a tape recorder head which interferes with intimate contact between head and tape will cause unsatisfactory operation. A deep tape groove or a scratch or an erratic or incorrectly located wear pattern all cause the tape to lose contact with the head. It is these types of abnormalities which are considered in this article.
It takes me about one hour of grinding and polishing to reface even the worst head. Add to that the time to remove the head, then reinstall and align, and the entire job runs two to two and a half hours. This is a considerable saving to the customer over the cost of a new head. Occasionally, you will run into a head that is so severely worn that refacing should not be attempted. This is the time when you must depend upon your good judgment. In performance, a refaced head compares favorably with a new head in all important characteristics.
Here's how the job is done. First get the necessary materials. They are all inexpensive and readily available:
1. A wooden board on which to do the grinding and polishing. A small kitchen cutting board about 8 X 10 in. does nicely. It must have a smooth, flat surface.
2. One sheet wet sanding paper, 220 grit.
3. One sheet wet sanding paper, 400 grit.
4. One sheet wet sanding paper, 600 grit.
5. Jar of silver polishing cream, Wright's or equivalent.
6. Piece of soft flannel cloth about a foot square.
7. Magnifying glass, about 10 x.
Having assembled the equipment, you are ready to start. Remove the head from the machine and disassemble any mounts or shields which interfere with access to the face. Examine the wear pattern to make sure the head can be salvaged and to determine what you have to do. Let us assume the wear consists of a grove about three or four times the thickness of a 1 mil tape. This amount of wear will require a good deal of grinding, and so we start with the coarse (220) paper. The grinding should be done wet, that is with a small flow of water on the grit paper. Wet grinding produces a smoother result than dry grinding. A pressure of no more than six to eight ounces should be used, as heavier pressure will make deep gouges in the face of the head. If you're not sure what a 6 ounce pressure is, get out a pressure gauge and find out. It's important. Hold the head by its sides with the face against the grit paper. As you look downward toward the work, use a rotary motion of the hand, making circles about three or four inches in diameter. Simultaneously with this rotary motion, rock the head back and forth so the entire face is exposed to the grinding action. At all times keep an even pressure on the head. Occasionally, turn the head end for end to insure even grinding.
Examine the work frequently under the microscope. When the depth of the groove has been reduced to about half the thickness of a one mil tape, it is time to use the medium (400) paper.
Keep up the grinding until the groove is almost eliminated, then finish grinding with the fine (600) paper. Avoid grinding more than necessary, but be certain all traces of the groove have been eliminated.
The next step is to polish the face. Put aside all abrasive papers and wash the cutting board. Fold the flannel cloth double, moisten it, apply a small amount of polishing cream, and polish, using the same techniques as employed during grinding. Fifteen minutes of polishing usually will produce an excellent surface finish-provided your grinding was correctly done.
Let us retrace our steps for a moment to the point when we made the initial examination of the head to determine the depth and extent of the wear pattern. If the wear is light, the grinding should start with the medium or fine grit rather than the coarse grit. In case the coarse grit is employed late in the grinding process, deep scratches will be left in the face of the head. These tend to accumulate debris and interfere with head-to-tape contact.
Well, that completes the job as far as refacing is concerned. Of course, you must reassemble the equipment, align the heads, and test the equipment. As far as the head is concerned, you should achieve specified performance of the recorder.
(adapted from Audio magazine, Apr. 1972)
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