Fostex 250 - Cleaning and Demagnetization; Cleaning Procedure; Demagnetization Process

being described. The ranges cited below are approximate.
With microphones, low impedance refers to. 5 0 to 250
ohms, whereas high impedance refers to 1 kohm or higher
actual source impedance.
With line level outputs on mixers and accessory equip¬
ment, tow impedance refers to actual source impedances
of 10 ohms to 600 ohms, whereas high impedance refers
to a 2 kohm or higher source impedance.
With line level inputs on mixers and accessory equip¬
ment, tow impedance refers to actual load impedances of
600 ohms to 3 kohms, whereas high impedance refers to
load impedances of 10 kohms to 100 kohms.
Speaker impedances typically vary from 3 to 16 ohms.
The actual source impedance of a power amplifier’s
speaker output is often in the order of 1/10 of an ohm or
less.
SECTION 10 CLEANING AND DEMAGNETIZATION
CLEANING
Magnetic recording tape, no matter how good in quality, will
always shed some of its oxide coating as it travels past the
tape guides and head assembly. Whenever a small particle of
oxide (or dust, dirt, hair, etc.) comes between the head and
the tape, audible performance can be degraded, especially at
high frequencies. Regular cleaning of the tape heads and
guides will avoid such losses and restore full fidelity.
If the oxide should be scraped off a portion of the tape back¬
ing, there will be a momentary loss of sound (a drop out).
Drop-outs are permanent flaws in the tape, and must be
avoided. Clean guides and heads are less apt to scratch the
tape. This is another reason for routine cleaning.
Standard tape head cleaning solvents are available from
most Fostex dealers. An ideal solvent consists of 100% pure
isopropyl alcohol. Rubbing alcohol should not be used; even
though it is isopropyl, it usually contains water and oils which
will leave an unwanted residue after cleaning.
CAUTION: Never use organic solvents such as methyl -
ethyl ketone (MEK), lacquer thinner, acetone, etc. These
can dissolve the materials that bind the heads together,
and, if spilled, can mar or deform plastic parts.
Wipe the heads, tape guides, pinch roller, and capstan with a
common cotton swab, moistened with the head cleaning fluid
or alcohol. Allow the parts to air dry before installing a
cassette. (The process should be repeated until the swab no
longer shows evidence of the reddish-brown oxide, and until
the heads are shiny and clean.)
NOTE: We do not recommend the use of so-called head
cleaning cassettes. These products typically use an
abrasive tape which can prematurely wear the heads,
and which does little to clean the capstan or pinch roller.
The exterior of the Model 250 can be wiped with a cloth that
is moistened with a weak detergent and water solution. Do
not use solvents, oils, waxes or spray-on cleaners.
DEMAGNETIZATION
When a recording is made, the record/play head applies a
powerful magnetic field to the tape. Iron oxide or similar
magnetic particles in the recording tape store a portion of
that magnetic field (this is the recording). It is a necessary law
of physics that, as the recorded tape is subsequently played,
some of the tape's magnetic field will be transferred to near¬
by metal parts. This residual magnetism is undesirable
because it, in turn, can partially erase a tape. The purpose of
demagnetization (degaussing) is to neutralize residual mag¬
netism in the vicinity of the tape path.
Demagnetizing (also called degaussing) is accomplished by
bringing a strong alternating magnetic field (created by a
demagnetizer) near the head area, then withdrawing the field
slowly. It is very important that this procedure be done
carefully, and that no tapes be within 6 feet (2 meters) of the
demagnetizer when it is operating. It is also important to turn
off the Model 250 during the time the demagnetizer is turned
on.
CAUTION: Demagnetizers are not dangerous devices,
per se, but if used improperly they can instantly erase a
valuable tape, and can permanently magnetize metal
parts—the opposite of the desired result. NEVER TURN
ON OR EVEN PLUG IN A DEMAGNETIZER UNLESS IT IS
AT LEAST 3 FEET (1 METER) AWAY FROM THE Model
250. ALSO, NEVER TURN THE DEMAGNETIZER OFF
UNTIL IT IS WITHDRAWN TO AN EQUAL DISTANCE OF
AT LEAST 3 FEET. It the demagnetizer should be turned
on or off nearby the Model 250, it may impart a magnetic
charge to the heads or tape guides which is too strong to
be removed by the same demagnetizer.
There is no rule as to how often demagnetization must be
done, but we recommend it be performed at least once for
every 10 hours of Model 250 record/play operation. We sug¬
gest observing the following procedure, step-by-step.
NOTE: Some demagnetizers do not have on/off switches,
and must be plugged in and unplugged. This design may
be safer because it avoids accidental switching when
near the recorder. In these instructions, we assume that
switchable demagnetizers are always turned on, and that
power is controlled by plugging in or unplugging the unit
from the AC mains.
1. Turn off the Model 250, and remove any cassette tapes to
a distance of at least 6 feet (2 meters).
2. Before plugging in the demagnetizer, hold it at least 3 feet
(1 meter) from the Model 250. Then plug it in.
3. Gradually move the demagnetizer toward the recorder until
the tip is about 1/8-inch (3 mm) from the heads.
4. Slowly pull the demagnetizer away from the heads to a
distance of about 3 inches (75 mm), then back to within 1/8
inch of nearby metal parts (tape guides, capstan, etc.) Con¬
tinue this process until the demagnetizer has been waved
near all parts in the tape path, but DO NOT TOUCH ANY OF
THE PARTS WITH THE DEMAGNETIZER PROBE.
5. Gradually withdraw the demagnetizer until it is at least
3 feet (1 meter) from the Model 250. Then unplug the
demagnetizer. This completes the demagnetization process.
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