wjm (01/03/83)
There are two items that should be included in user maintainence of tape decks
(the first one applies to both audio and video decks - the second to audio ones
only).
1. Cleaning - For best results, one should be sure that the tape heads and
tape path (the capstans, pinch rollers, and tape guides) and tape guides are
clean. Generally, there are only two safe things to use on tape decks - the
first is isopropyl (rubbing) alcohol, which you can get from your frendly
local drugstore. Try to get the kind which is either 91% by weight or 99%
by volume rather than the 70% variety (which often contains perfume as well
as H2O to make up the remaining 30%). The second is trichlorethene (better
known as Freon - the same stuff that makes your refrigerator and air conditioner
work) which is the active ingredient in the Nortronics tape head cleaner.
Do NOT use any other cleaner unless it is explicitly recommended by the maker
of your tape deck (many solvents, like lacquer thinner, can dissolve the
tape head mounting compound - requiring head replacement (big $$$)). The best
way to clean is with Q-tips dipped in alcohol or Freon (some of the head cleaning
tapes tend to be abrasive to the heads - although they are the only way to reach
the heads on some video decks). Some people feel that alcohol will eventually
degrade the rubber pinch rollers and prefer to clean with Freon, but there is
considerable disagreement about this. The important thing is to clean, and
clean often. Cleaning does no harm (as long as the proper cleaner is used) and
should be done every 10 hours or so.
2. Demagnetization. The second part of tape recorder maintainence is to
keep the playback head (and to a lesser extent the metal tape guides and capstans)
magnetically clean. Phil Karn is correct, modern tape decks all use high
frequency AC bias and bias is only applied to the record head during the
recording process. One pleasant side effect of this is that record heads (in
3 head machines) are self-demagnetizing, since the AC bias field removes any
residual magnetism from the head.
However, in the process of playing back tapes, a DC magnetic field can build
up on the playback head as part of the playback process and this must be removed
to prevent it from erasing the tapes. Since it is a weak field, it will not
totally erase the tapes, but will have the most effect on those signals which
are least strongly magnetized (low level) or cover the least area on the tape
(high frequency) causing a loss of highs and a degradation of the signal level
on the tape (effectively raising the tape noise). The demagetization process
removes this field by applying an AC field to the head which gradually decreases
in level to zero (by slowly removing the demagnetizer from the heads).
If done properly, demagnetization can do no harm and should be done when one
suspects that the heads have become magnetized or after every 40 or 50 hours of
use. It is important to gradually decrease the field to zero by slowly removing
the demagnetizer from the head and only turning it off when one is far away
(like the other side of the room) from the tape deck. Another reasonable
alternative is to use TDK's demagnetizer cassette, which works quite well and
is very convenient since it is automatic.
Bill Mitchell
Bell Laboratories - Whippany
(whuxk!wjm)prgclb (01/04/83)
Bill Mitchell had a minor error in his article on tape deck maintenance. He said that Freon is trichlorethylene (or vice-versa). Freon is a *fluorinated* hydrocarbon, whereas trichlorethylene is a *chlorinated* hydrocarbon. Freon, by the way, is a trademark (not sure what company -- I think DuPont) for a *family* of fluorinated hydrocarbons predominantly used in refrigeration, and until the recent ozone scare, as an aerosol propellant. Trichlorethylene, on the other hand, is a specific chlorinated hydrocarbon compound with two double-bonded carbons and three chlorine atoms. It, and its cousin perchlorethylene, is used as a dry-cleaning and grease-removing fluid. Available in hardware and paint stores under a number of trade names (e.g. carbo-chlor), it's great for cleaning, say, dirty bicycle chains, but I'm not so sure about using it on tape heads (unless, of course, your tape deck owner's manual says it's okay). Bill, don't take this as a put-down of your article -- it was complete and comprehensive. Just thought I'd take this chance to brush up on my organic chemistry . . . Carl Blesch Bell Labs - Naperville, Ill. IH 2A-159, (312) 979-3360 ihuxm!prgclb