TAPE GUIDE (Aug. 1985)

Residual Magnetism

Q. I have a magnetometer with a range of ±5 Gauss. Using the magnetometer on my tape deck, and allowing for the effect of the earth's magnetic field, I found that the field associated with the heads, guides, and capstans is negligible, probably due to my occasional use of a demagnetizer. It would be helpful to know what level of residual magnetism is considered tolerable in heads, guides, capstans, etc. with out appreciable effect on the tape.

I checked the magnetism on a friend's tape recorder, and found that stray magnetism from the unit's speaker, appearing approximately where the tape goes from the cassette guide to the cassette take-up reel, is 5 Gauss.

This seems high to me. Can you give me a reference to any article(s) on the effect of stray magnetic fields on re corded audio tape?

-H. B. Hoeper, Arlington, Va.

A. As a rough guess, a field of 1 or 2 Gauss might not be harmful. Five Gauss does seem high, but what counts is the distance between the tape and the field. A very slight separation can make a big difference in the degree of tape erasure by the field.

You may find helpful my article "Magnetic Shielding" in the April 1979 issue of Audio. Also of interest might be my articles "Focus on Head De magnetization" in the April 1981 issue and "Refocus on Demagnetization" in the September 1982 issue.

The Music Stops

Q. Three or four of my cassettes sometimes stop while playing. These are all of one brand, recorded on both sides, but played principally on one side and stored after fast-rewind.

These tapes have been played about 30 to 50 times each. I have frequently used the auto search feature of the deck, which puts the cassette into fast-wind. When the tape stops, it will not budge if I push the play, fast-forward or rewind pad. I take out the cassette and sometimes find that the tape is tightly wound or that about an inch of slack tape is hanging from the cassette. In the latter case, I wind the tape tight with a pencil. In either case, I then slap the cassette sharply on my thigh, reinsert it, and find that it plays. Examination of the cassette when it stops of itself shows that some edges of the tape protrude. Do you have any recommendations?

-Robert Olsen, Madison, Wisc.

A. Fast-forward or rewind puts the tape under a good deal more stress than normal operation. If the transport mechanism isn't operating properly or if the cassette isn't well-made (and making it well is rather an engineering feat), problems such as yours can arise. Hence, one gets the advice to avoid fast-wind when possible--for example, by storing a tape in the just-played or just-recorded condition.

Still, with a high-quality deck, such problems are abnormal, and a visit to a qualified repair shop is indicated if the deck indeed is at fault. Or the other hand, if the problem occurs with only one brand of cassette and with no others of high quality, the finger of blame points at the cassettes. And the simple solution is to avoid the brand and type in question.

Dropout Problem

Q. Lately I have been experiencing very severe treble dropouts at the beginning of some of my cassette tapes; these are high-quality tapes that I recently began using. I wonder if the cause is my tape deck or the brand of tape. Some of the tapes have worked fine, but others not.

-Philip Copeland; Natchez, Miss.

A. The problem could lie either in your tape or in your deck. But if it occurs only with some tapes and not, with others, one tends to suspect the tape. The tape pack on the hub tends to be out of round at its beginning, where the tape is joined to the hub, and this can cause problems. You might try skipping the first 15 S of the tape and confining your recordings to the remaining portion. It is also possible that the tape is skewing badly during the early moments, perhaps be cause of faulty construction of the cassette. Such skewing could also be a fault of the deck, such as improper tension, for example, or inadequate tape-to-head contact. The latter may be due to insufficient tension exerted by the reels or insufficient pressure exerted by the pressure pad.

It has frequently been noted here and elsewhere that some decks and tapes, even though individually excel lent, just aren't compatible with each other. This might be true in your case, requiring you to try another brand or type of tape.

Noise Reduction

Q. I listen to open-reel tapes, a few encoded with Dolby A or B, but most without Dolby because they were re corded in pre-Dolby times. This makes tape noise unbearable. Which Dolby system can I purchase that would be compatible with my tapes?

-Melvin A.

Petersen, Frederiksted, St. Croix, V.I.

A. For tapes without Dolby encoding, you would need a dynamic noise-reduction unit, which applies varying treble cut depending on the amount of high-frequency information in the audio signal. The less the high-frequency content, the greater the treble cut; thus, noise is reduced in the region where noise is most audible. (The February 1985 issue of Audio contains plans for building such a device, and lists a source for parts, kits or fully assembled units.)

Presumably, you already have de coders for proper playback of your Dolby A and Dolby B encoded tapes. I hope so, because stand-alone decoders may prove hard to get. Decoders and encode/decode units for the professional Dolby A system are probably available, but, since they're for professionals, even used ones won't be cheap. Dolby B decoders have apparently disappeared from the market, though you might find an old or used one after a long search. However, you can play tapes with Dolby B encoding pretty decently by turning your audio system's treble control down a bit.

Recording from Radio

Q. If you have a portable cassette receiver with no AUX (high-level) input jack, is there still a way that you can record directly from radio? I've noticed that if I try to record directly from the radio via a patch cord, inserting the cord's plug into the deck's microphone input jack, the sound is very distorted on playback. Is there an adaptor or some kind of device that would let me record through the microphone jack without distortion?

-Jimmy Edwards, Greenville, N.C.

A. If you really do mean a "portable cassette receiver," i.e., a combination radio and cassette recorder/player, I don't see the problem. Such units al most invariably provide for recording off the air without external wiring.

If what you have is actually a portable cassette recorder, with no built-in radio, then a simple solution is avail able. Cables are available with built-in attenuation networks to reduce the radio's output signal to the point where it will not overload the recorder's micro phone-input circuit. You may have to shop around a bit to find one, especially if you have a monophonic device on one end of the cable and a stereo phonic one on the other. Make sure, too, that you get the correct-size plug for each device; some plugs, such as the 2.5-mm micro and 3.5-mm mini phone plugs, are easily mistaken for one another.

You could also build your own resistive voltage-divider network, say 100 kilohms in series with a resistor of 10 kilohms, to reduce the signal, taking the signal for the mike input across the smaller resistor. If you still get distortion, try 5 kilohms or less for the smaller resistor; if the signal is too low to drive the tape deck adequately, try some thing higher than 10 kilohms--perhaps 20 kilohms.

There are safety considerations, too. If both the radio and recorder are battery-powered, no problems will arise. But if either or both are a.c.-powered, make sure their jacks are isolated from the a.c. line (as they will be, if these devices use transformer power sup plies), or else plug the devices into isolation transformers to prevent danger of electrical shock.

Snaps and Pops

Q. I am very pleased with my cassette deck's overall performance, but when I rewind and play or stop in mid-play and restart, I hear static in the form of snaps and pops. It sounds like the static you hear when pulling a new record from its inner jacket. I have tried both wand and cassette-type demagnetizers, and I regularly clean the heads and capstans, but to no avail. I use only high-grade cassettes.

-Pat Tufts; Westerville, Ohio

A. Static electricity is a mysterious form of annoyance for which I know of no sure cure. However, I can pass along two possible remedies which have worked in some cases. One is to give the cassette a sharp rap on a hard surface (not, of course, so hard as to damage either the cassette or the surface). Another is to disconnect all leads to the deck and then reconnect them. (Admittedly this second course sounds strange, but I have been assured that it has worked in some cases.)

(Source: Audio magazine, Aug. 1985, HERMAN BURSTEIN)

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