TAPE GUIDE (May 1986)

Intermittent Hum and Noise

Q. I have a factory-reconditioned tape deck about six years old. Initially I used it for about five hours, and every thing was fine. I left it alone for about one week. Then I played a tape I had made during the initial period, and heard strong noise and hum, with the music at very low level. I turned off the deck, cleaned the heads with a cassette cleaner, and demagnetized them with a cassette demagnetizer. I reinserted my tape, and everything was fine. About one week later I tried to use my deck, and again I got the hum and noise. This time I just cleaned the heads, and once more everything worked fine. What is going on?

-Leslie Page, Brooklyn, N.Y.

A. I can see no reason why dirty and/or magnetized heads would cause hum and noise and reduced sound level. I believe that cleaning and/or de magnetizing, followed by restoration of normal operation, was coincidence.

The problem may lie in a faulty component such as a resistor, capacitor, or transistor; the "shock excitation" of turning the deck off or on may be causing an intermittent condition to appear or disappear.

Intermittent problems such as this, which come and go almost at random, are very difficult to correct. The problem may fail to appear when the ser vice shop checks it, or may disappear, by sheer coincidence, when the repair shop does anything to cure it, relevant or not. It may be advisable to wait until the hum and noise problem appears more consistently before taking your deck in for repairs.

Mettle of Metal Tapes

Q. Can a metal tape recorded on a high-quality cassette deck confidently be played on other decks without losing high-frequency response? Is it possible to demagnetize the heads of less sophisticated units, such as portable players or car cassette players, so that metal tape will not undergo high-frequency loss? Also, can a metal tape be stored for an indefinite period under normal conditions without loss of quality?

-J. Gary Sparks, Indianapolis, Ind.

A. I doubt that your metal tapes will be harmed when you play them in relatively inexpensive cassette decks if you periodically demagnetize the players' heads-and even that should not be necessary often. So far as I know, metal tapes, like the other types, can be stored for long periods without loss of quality. However, it is a good idea to play these tapes every few months, or at least put them through fast wind and rewind to relieve stresses.

dbx I vs. dbx II

Q. What is the difference between dbx Type I and Type II noise-reduction systems?

-George Rauscher, Hepzibah, Ga.

A. Type I dbx is intended for professional applications; Type II is generally incorporated in cassette decks and outboard units for consumer use. Type I offers somewhat flatter frequency response than Type II. Circuitry is different in each type, so they are considered incompatible.

Shelf Life

Q. I am concerned with the long term use and storage of magnetic tapes, both used and virgin. Is there a "decay" factor? If a reel of tape is un played for, say, a year after recording, does it suffer any loss in quality? And is there some sort of shelf life for virgin tape?

-Ken W. Hind, Edmonton, Alberta, Canada

A. So far as I know, the shelf life of tape is unlimited, whether virgin or re corded, assuming temperature and humidity conditions are within the range called "normal." A recorded tape may acquire noticeable print-

through after storage. (The thinner the tape, the more likely this is to occur.) But most of this print-through occurs during the first hours or days of storage, so the length of time the tape is stored has less effect than the storage conditions. Print-through will be more severe under conditions of high temperature or in strong magnetic fields. it may possibly be reduced by putting the tape through fast-wind one or more times before playing it.

I gather from your mention of reels that you are using open-reel tape. If your tapes are recorded in one direction only, as quadraphonic, multi-track, and two-track stereo tapes are, you can minimize print-through problems by leaving the tape wound on the take up spool, and only rewinding it just before playing. This forces you to give the tape the recommended fast-wind before playing. It also renders less objectionable whatever print-through does occur, by making it appear as a natural-sounding post-echo rather than a pre-echo (something which cannot occur naturally).

Not rewinding immediately after play also leaves the tape evenly tensioned on the reel or hub, and usually more evenly wound than it would be after fast winding. This helps protect the tape from physical stretching and pre vents stray windings from poking out of the tape pack, where they can be damaged in handling.

Over time, a recorded tape might lose something like 1 or 2 dB of response at the very high end, although this effect tends to be minimal with modern tapes of good quality. Such tapes should be able to undergo at least 500 passes through a tape deck without appreciable deterioration of their magnetic and physical characteristics. Tapes of lesser quality are more likely to change shape or shed oxide during long-term storage.

Efficient Bulk Erasers

Q. I am becoming desperate to ac quire an efficient bulk eraser for my large collection of cassette and open-reel tapes. My present hand-held eraser, no matter how I use it, has given very poor results; instead of removing magnetic patterns it has introduced a lot of hum and noise. I would appreciate any suggestions.

-Richard Carle ton, Topeka, Kans.

A. Bulk erasers that perform effectively are not cheap. They may go for anywhere from about $75 to $200, and possibly even higher. Among companies that make them are R. B. Annis Manufacturing Co., 1101 North Delaware St., Indianapolis, Ind. 46202; Geneva (formerly Nortronics), 7255 Flying Cloud Dr., Eden Prairie, Minn. 55344; Robins Division of Benjamin Electro products, 232 Sherwood Ave., Farmingdale, N.Y. 11735, and TEAC, 7733 Telegraph Rd., Montebello, Cal. 90640.

Steady Hum

Q. My new cassette deck produces hum throughout a recording. It is particularly noticeable between selections and at low music levels. When I substitute my old, inexpensive deck for the new one, the problem disappears. I have already exchanged the new deck for another unit of the same model, but there is no improvement.

-Brian Rosenbaum, Chicago, Ill.

A. First determine whether the hum occurs in recording or playback. Record a cassette with your new deck; if the hum is present when you play it back on the new deck but disappears on the old, the problem is in the new deck's playback. If the hum is present in playback on both machines, the problem probably is caused by the new deck's recording. The deck may be to blame even though you say you exchanged it. This could be due to poor design, faulty parts or construction (such as internal layout of leads), etc. It may be necessary for you to make still another exchange, perhaps for another model or make.

It is also possible that the cables between your deck and your amplifier are not securely inserted into the jacks.

Try pinching the shells of the plugs to get a tighter fit, and twist each plug back and forth as you insert it into a jack. Another possibility, if your components are stacked atop each other or are in very close proximity, is that the playback head is picking up hum coming from the transformer of another component.

Adding HX Pro

Q. I would like to add Dolby HX Pro to my cassette deck for both playback and recording. Is there anyone who makes an add-on unit for this purpose? If not, can you suggest how I can accomplish this?

-James L. Dagley, Jr., Port Orange, Fla.

A. Dolby HX Pro is used only in re cording. It employs the high-frequency content of the audio signal as part of the bias in recording. As the high-frequency content increases, the amount of bias current drawn from the bias oscillator is decreased, and vice versa.

Thus the total bias-most from the oscillator and some from the audio signal-remains constant. This avoids over-biasing and the consequent loss of high frequencies. In other words, the deck provides greater headroom, i.e. protection against tape saturation by treble frequencies of high amplitude.

HX Pro is a quite sophisticated sys tem. It would require considerable technical knowledge and skill to incorporate it into an existing deck, and it is not possible, here, to tell you how to do this. Also, there is no such thing as an add-on HX Pro unit. If your deck gives you a high signal-to-noise ratio (which it should if it incorporates Dolby C NR), and if you believe you are running into tape saturation, back down on your recording level a few dB; the decline in S/N will probably be unnoticeable, or nearly so.

(Audio magazine, May 1986, HERMAN BURSTEIN)

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