Handbook for Sound Engineers

Magnetic Recording and Playback 1071

varies greatly with changes in the record head gap

length. Table 28-7 illustrates how this gap length affects

the recommended amount of overbias.

28.3.12 Problems with Older Tapes

The archives of American tape recordings contain tapes

that are up to 50 years old. Unfortunately, many of these

tapes have problems that could easily damage or destroy

their recordings. Some of these problems can be

corrected, but others are irreparable.

28.3.12.1 Adhesion and Peeling Oxide

Adhesion is the binding force that firmly holds the oxide

layer onto the plastic substrate. Two simple tests can be

used to evaluate the strength of the adhesion—the

Scotch tape test and the sharp edge test.

The Scotch tape test tries to rip the oxide from the

plastic substrate by brute force. Start with a strip of

Scotch Brand Magic Mending tape several inches long.

Adhere about 3 inches of the sticky tape to the oxide

surface of the recording tape. Rub the joint to assure

complete binding of the tapes. The test is to peel back

the sticky tape with a quick jerk parallel to the tape. If

the sticky tape comes off cleanly, the adhesion is good.

If the oxide layer delaminates and peels off with the

sticky tape, the adhesion is poor.

The second adhesion test utilizes a blunt edge to

create a very sharp bend in the tape. Find a sharp

perpendicular edge on a desk or a piece of plastic ruler

that has no rounding radius. Place the backside of the

tape sample against the sharp edge. Pull on the ends of

the tape to establish a firm tension in the tape. While

maintaining the tension, drag the tape over the edge in a

90 q bend. If the oxide does not loosen from the backing,

the tape passes the test. A poorly adhered tape may

suffer complete delamination of the oxide, with a solid

band of oxide peeling off and shooting away from the

backing.

28.3.12.2 Brittleness

The polyester base films and urethane binders of

modern tapes remain flexible under all normal circum-

stances. The base films and oxide layers of earlier tapes,

however, could become brittle. Plasticizers were

included in the binders and the acetate backing to

provide flexibility in the tape. Unfortunately, these plas-

ticizers can harden with age, causing the tape to become

brittle. Harsh storage conditions can accelerate the

breakdown of the plasticizers.

Brittleness cannot be reversed. The only remedy is to

use a tape transport that is extremely gentle on the tape.

Choose a transport with dynamic braking rather than

harsh mechanical brakes. A transport with constant tape

tension can be set for the lowest practical tape tension.

Some decks also feature gentle start capability that

ramps the capstan up to speed to smoothly accelerate

the tape rather than just slamming a pinch roller onto a

running capstan.

28.3.12.3 Splice Failures

In the early days, standard Scotch Brand cellophane

tape was the only splicing tape. Later on, splicing tapes

such as Scotch #41 and #620 were developed with

improved characteristics. Although these tapes were

fine for day-to-day operations, they have not survived

the test of 50 years of storage. For example, the adhe-

sive of both cellophane tape and splicing tape can ooze

out and stick to adjacent layers of tape. A common

remedy was to apply talcum powder to the sticky oozed

adhesive to avoid layer-to-layer adhesion.

With even more time the adhesive can dry out

completely, causing the splice to fail. In this case the

only remedy is to replace the splicing tape with new

tape. The newest splicing tapes, such as blue #67,

replace the original latex adhesives with synthetic adhe-

sives that do not ooze or dry out.

The tape operator must be watchful for two problems

created by bad splices. First is layer-to-layer adhesion

that can produce strong tugs that break older acetate

tapes. Second is tape separation at a failed splice. DO

NOT run the tape through the recorder at high speed if

you suspect either problem may exist. If the tape tugs or

separates at high speed, the loose end may be slapped

around and broken off before you can stop the spinning

reels.

If the tape is old, wind the tape slowly and carefully

to examine each splice. Re-splice all splices if there is

any hint that the splices may separate. Do not try to

remove any of the old splicing tape adhesive that has

Table 28-7. Short-Wavelength Dependence on
Record Gap Length

Record Gap Length 10 kHz Overbias @15 in/s

1.0 mil 1.0 dB

0.5 mil 2.5 dB

0.25 mil 3.0 dB