1094 Chapter 28
are ten times larger than the flutter ranges, are labeled
below the meter ranging pushbuttons.
The AM reading for 15 in/s (38 cm/s) operation is
typically 0.5% rms for a good roll of tape on a profes-
sional recorder. The texture of the demodulation prod-
ucts coming from the audio monitor should be a low
rumbling with only occasional moderate bursts. The
high-pass filter l should produce a uniform hiss.
Typical symptoms of bad rolls of tape include read-
ings that are approximately three times higher than the
normal readings or very large frequent bursts that drive
the meter pointer hard against the upper stop. Routine
studio tests of large quantities of tape stock over a
period of two years has shown that these easily spotted
characteristics are good indicators of defective tape.
Although amplitude variations are symptomatic of
bad tape, the tape transport and heads are also possible
sources. If the tape is not being held snugly against the
faces of the heads due to inadequate tape tension, the
tape may suffer irregular spacing loss. Other contribu-
tors are dirt on the heads or heads that have been worn
so flat that the gap is no longer pressed firmly against
the tape. Mechanical misalignments, such as a twisted
head or improperly positioned guides or scrape flutter
idlers, can also degrade the contact between the tape
and head.
Misadjustments of the bias amplitude or even-order
distortions of the bias or erase waveforms can also
produce excessive AM levels. Always verify that the
bias levels and tuning are correct before condemning
the tape.
A simple method of avoiding embarrassment when a
defective roll of tape is suspected is to recheck the
machine with a reference roll of the same type of tape
that is known to be good. If changing from the reference
roll to the suspect roll causes a large increase in AM
content, then the tape is the source of the problem.
Since none of the tape manufacturers supplies infor-
mation that is useful for specifying the AM performance
of a tape, the user must generate data by testing several
rolls of tape on machines. Once this process is begun,
subsequent additions to the database will provide even
more insight into the expected range of values.
28.8 Magnetic Head Troubleshooting and Main-
tenance
Troubleshooting any piece of complex equipment
requires a methodical search technique to isolate the
source of the problem quickly. The most productive
technique is to conduct a series of tests that subdivide
the faulty portion of the total system into smaller and
smaller parts until the fault source is finally isolated.
Applying this technique to a magnetic tape recorder
would lead to partitioning questions such as:- Is the problem associated with the tape drive, the
audio circuitry, or the control logic? - Does the fault occur during recording, playback,
and/or input monitoring? - Is the problem due to the recorder or the roll of
tape? - Is the problem similar at both tape speeds?
- Is the problem the same throughout the reel of tape?
- Does temperature or running time have an effect?
If the problem relates to the audio signal passing
through the recorder, a fundamental question that must
be answered is whether the problem is wave-
length-dependent or frequency-dependent. Wavelength
problems immediately isolate the problem to the inter-
face between the moving tape and the heads. Frequency
problems are often related to the audio circuits.
A very useful tool for separating wavelength prob-
lems from frequency problems is a simple device
known as a flux loop shown in Fig. 28-62. The flux
loop, which consists of nothing more than a few turns of
fine magnet wire driven with a constant current from an
audio oscillator, creates a magnetic field that simulates a
perfect lossless piece of tape. When the flux loop is
attached to the gap region of the playback head, the flux
from the loop excites the head much like the primary
winding on a transformer excites the secondary
winding. This direct excitation eliminates all the wave-
length effects associated with gap length, azimuth error,
and thickness loss. If the reproduce electronics perform
correctly when excited by the flux loop but still fail to
reproduce a known-good prerecorded test tape correctly,
the problem is a wavelength-dependent error at the
head-to-tape interface.
The playback response from a simple flux loop is by
no means flat. Since the dominant loss due to the
coating thickness is not present for flux loop excitation,
the high-frequency response with a flux loop will show
a pronounced rise that relates to the particular reproduce
equalization standard that is being utilized. NAB
low-frequency equalization will also produce a roll-off
below 50 Hz.
To simplify the measurement process, the oscillator
signal feeding the flux loop is usually preequalized to
accommodate these effects of the equalization standard.
Fig. 28-62 includes a simple circuit for correcting the
high end, with capacitor values for several common
equalizations. (The 600: impedance of the oscillator is