1074 Chapter 28
Since both types of sticky shed problems are treated
by baking, most people who bake tapes don’t know for
certain which problem they are treating, and if the
sticky shed is eliminated, they probably don’t care.
How long before a baked tape begins to again exhibit
sticky shed? Results will vary depending on the amount
of degradation, the tape type and specific batch, the
exact baking method, and the operating environment
after baking. Reports vary from days to years. Certainly
baking provides an adequate window for the tape to be
transferred to another medium.
Is there any degradation due to the baking process?
The most likely problem is print through caused by the
elevated temperature. Print through is a time-dependent
problem that peaks out at a maximum value after a long
time. Heat accelerates the printing. However, stored
tape probably has had enough time for the print through
to be near the maximum value before baking. As a
result, the additional print through caused by the baking
may be negligible. Follow the exercise process
described at the end of Section 28.3.12.4 to minimize
the print through before copying the tape.
How can sticky shed be avoided? The rate of hydro-
lysis depends on the storage conditions. Archival storage
at a temperature of 60qF (15°C) and relative humidity of
25% ± 5% is optimal, but few have the luxury of an
environmental chamber, so store the tapes in a cool, dry
location in the original package standing on edge.
Sticky shed may also produce layer-to-layer adhe-
sion. If you strongly suspect sticky shed, bake the tape
before trying any winding operations on a tape trans-
port. This will avoid the total loss of recorded segments
due to oxide being ripped from the tape’s plastic
backing during spooling.
28.3.12.6 Squealing Tape
One of the many ingredients in the coating recipe is a
small amount of lubricant. Obviously, the tape cannot
be too slippery or else the capstan couldn’t maintain
constant tape speed. Running the tape completely dry,
on the other hand, can produce an audible squeal. The
tape undergoes a “stick-slip” phenomenon on the
stationary guides and head, creating a jerky motion at a
high frequency. The irregular motion can even be
measured with a scrape flutter meter.
The squeal results from the loss or failure of the
original lubricant. The obvious solution is to replace the
lubricant. A can of 10-W30 motor oil isn’t appropriate,
but another common household lubricant, WD-40, is
recommended by Quantegy. Quantegy claims that
WD-40 is cheap, available, inert to all recorder compo-
nents, and a very good lubricant. Apply the oil sparingly
by lightly wetting a lintless rag or swab with the oil and
holding the applicator against the oxide side of the
moving tape at the first guide after the supply reel. A bit
of experimenting may be required to find the proper
amount of oil that is required to eliminate the squeal
without causing slippage and speed irregularities. When
you are finished using the tape, prepare the tape for
storage by passing the tape over a dry applicator in a
medium speed fast wind mode. (Use extreme caution on
tape transports that have elastomer coatings on the
capstan and/or timing rollers. Lightly lubricate the tape
while passing the tape directly from the supply reel to
the takeup reel, and then dry the tape with a second pass
over a dry applicator before threading the tape over the
elastomer components.)28.4 Analog Circuits and SystemsThe transport mechanism, heads, and tape should
combine to determine the basic performance limitations
of a tape recorder. The analog electronic circuits of the
recorder, on the other hand, should exceed the capabili-
ties of the heads and tape in all respects so that only the
heads and tape limit the quality of the final signal. In
practical terms, this means that the SNR, frequency
response, distortion, and head room of the electronics
are comfortably better than the heads and any tape,
including future improved tapes.
The block diagram of the signal electronics of a
typical professional audio recorder is shown in Fig.
28-36. In terms of actual hardware, approximately 75%
of the audio circuits of a modern professional audio
recorder are devoted to operator interfacing and
controls; the remaining 25% implement the basic func-
tions of erasing, recording, and playback. Since the
variation of features and technology used to implement
the interfacing and control functions is too broad to be
summarized herein, the following description covers
only the latter basic functions.28.4.1 Playback AmplifiersThe amount of electrical power that can be generated by
a magnetic tape passing over the face of a reproduce
head is exceedingly small. The output voltage from the
head for loud recorded passages will reach no more than
a few millivolts, with quiet passages dropping into the
microvolt region. This weak signal must be carefully
boosted without the introduction of additional noise to a