Handbook for Sound Engineers

1090 Chapter 28

difference between the tape tension on the playback
machine and the tension value used by the manufacturer
of the tape during the recording process. A correction
table is furnished with the tape for this purpose.
A more common speed test is to check speed unifor-
mity from beginning to end of a reel of a tape. The
following procedure outlines the general technique:

1. Using an oscillator that has been operating long
   enough to reach stable conditions, record a refer-
   ence tone in the range of 1–5 kHz at the head end of
   the tape.


2. Flip the reels so that the head end becomes the tail.


3. Using the console monitoring provisions of the
   console, mix the reproduced tone with the oscillator
   tone, listening for any major pitch differences. (If a
   significant error is detected, flip the reels again to
   verify that the oscillator has not shifted frequency.)

A more accurate version of this test is to use a
frequency counter to measure the frequency at both
ends of the reel. The speed error in percent is then
calculated as

. (28-12)

A speed error of 6% will yield a pitch change of one
half-tone step. Typical recorder specifications are in the
range of 0.1–0.5%. Machines with constant tape tension
will generally have the least error.
Possible causes of speed error include excessive
tension variations from beginning to end of the reel,
tape slippage due to a worn capstan surface or pinch
roller, inadequate pinch roller pressure, and unstable
capstan speed.
Assuming that tape tension has already been deter-
mined to be correct on both sides of the capstan, the
next test is to check pinch roller pressure. First, inspect
the pinch roller for glazing of the roller surface or
excessive wear. Fig. 28-57 shows roller wear patterns
that may reduce the traction between the tape and
capstan.

Next, a spring scale is coupled to the top (and the

bottom, if possible) of the pinch roller yoke or arm, as

shown in Fig. 28-58. The scale is pulled at right angles

to the support arm with just enough force to disengage

the roller from the capstan. The force reading at disen-

gagement should be compared with the recorder manu-

facturer’s recommended value.

For some transports the pinch roller force is set as a
fixed number of turns of a nut or screw. For this case the
roller linkage is first tightened to bring the roller into
light contact with the capstan, and then the recom-
mended clamping force is applied by tightening the
adjustment by the specified number of additional turns.
The surface of the capstan may become so highly
polished by the abrasive action of the tape that slippage
will persist for the correct values of tension and pinch
roller pressure. In this case the capstan must be resur-
faced by plating or sandblasting or both to restore the
required traction.
In very rare cases the capstan motor may actually
slow down due to excessive loading caused by bad
motor bearings or high tension. Bushing bearings,
which are used on many direct-drive ac synchronous
capstan motors and some capstan pinch rollers are an
especially noteworthy problem. Periodic lubrication of
these components is essential to maintain low-friction
operation. Although these components may appear to
spin freely when turned by hand in an unloaded state,
the friction can rise dramatically when the engagement
solenoid exerts several pounds of side load on the bear-
ings. The resulting drag and wear due to dry bearings
may produce substantial speed errors. One small drop of
oil can make all the difference in the world. To avoid
problems, follow the manufacturer’s recommended
lubrication schedule.
A simple strobe light, as shown in Fig. 28-59, can be
used to check the running speed of the flywheel or fan
on the shaft of the synchronous capstan motors.
Package the components inside a discarded plastic pen
housing with the tip of the bulb protruding. Hold the
light close enough to the rotating device to observe a
Figure 28-57. Pinch roller wear patterns. reflection. The reflected pattern must remain stationary

% speed error 2

head tail–

head+tail

----------------------------

©¹

= §·u100%

Figure 28-58. Pinch roller force measurements.

Scale

Pinch roller