Tape Guide (Q and A) (Dec. 1973)

Demag After Playback

Q. I would like to know whether the heads on a tape deck require demagnetizing, even though I don't do any recording. And if they do, after how many hours of playback? Do playback heads ever wear out? How does one know when they need replacing?

-Jerry Meslin; Miami, Florida

A. The asymmetrical audio signal presented to the playback head in effect contains a d.c. component, which tends to magnetize the head. Therefore periodic demagnetization of the heads is ordinarily recommended, generally after about eight hours of use. At least one manufacturer, however, has recommended less frequent demagnetization.

Playback heads do wear out. How soon depends upon the quality of the head, the nature of tape tension (whether pressure pads are used), and quality of tape (how abrasive). Under the best of conditions, a conventional playback head may last 2,000 to 3,000 hours before its gap widens to the extent that treble response deteriorates significantly. Some recent heads claim a much longer life.

Amp Overload with Tape

Q. When I use my turntable as a source, the music sounds quite clear all the way to maximum volume. But when the same record has been recorded and is played back through my tape deck, the music becomes quite distorted at half volume. This is especially so in the lower and mid frequencies. Also, for the turntable volume to equal that of the deck at the 9 o'clock position, the volume control would have to be set about the 12 o'clock position. Is my tape deck faulty?

-Jeffrey Peace; Marysville, California

A. From your description, your tape deck produces considerably more signal than your turntable, inasmuch as the turntable produces as much volume at 12 o'clock of the volume control as the deck does at 9 o'clock.

Hence when you turn the volume up, it appears that you are overloading your audio system on the tape signal.

Offhand, it does not appear that there is anything wrong with your tape deck. On the other hand, there might be some very low-frequency wow produced by the deck which tends to overload your audio system, causing distortion. Or the deck might have excessive bass boost, again resulting in a tendency to overload at high volume.

See what happens if you adjust the output volume of your deck so that at 9 o'clock you obtain the same signal level on both tape and disc. If your problem doesn't then disappear, your deck may have a fault such as I have described.

Speed Conversion

Q. I have a model 7030 TEAC halftrack machine. It comes from the factor)) set for 7 1/2 and 15 ips, and I wish to convert to 3 3/4 and 7 1/2 ips. Other than changing the capstan drive shaft, will it be necessary to have a bias adjustment?

-Gus. W. Thomasson, San Luis Obispo, California

A. It will probably be necessary to change the bias. In order to obtain full treble response, manufacturers generally employ somewhat less bias than that which achieves minimum distortion.

In other words, at 7 1/2 ips the bias adjustment tends to represent a compromise between minimum distortion and extended treble. However, at 15 ips, such a compromise is little if at all necessary; bias can be adjusted for minimum distortion while maintaining treble response through the audio range. At 3 3/4 ips, it becomes all the more necessary to retreat from the bias producing minimum distortion, so that adequate treble response can be maintained. Consult the manufacturer for information on the specific changes in bias required in going from the faster speeds to the slower ones.

Let me add that in changing speeds, it will also be necessary to change record equalization, and it's probably desirable to change calibration of the record level indicator. And it will be necessary to change the playback equalization for the 3 3/4 ips speed (playback equalization is the same for 7 1/2 and 15 ips).

(Audio magazine, Dec. 1973; Herman Burstein)

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