Ampex 351 - Page 35

tow Slot ADJUSTMENT
HIGH SIDE ADJUSTMENT
6
Typical mechanical brake assembly as used by Ampex,
showing the two adjustment points.
Supply and Takeup Assemblies
When motors are used in the supply and takeup
assembly they are usually of the induction type, with
high resistance rotors. Reel motors must be as free
from cogging as possible, because cogging in the hold
back system has been responsible for many flutter
problems that have been blamed on the drive assem 
bly. It would be nice if we could discover a reel motor
whose torque would change with the tape diameter
on the reel, thus providing a constant tape tension
throughout the reel of tape. (Many constant tension
devices have been used in the past, but those designed
for audio equipment have not been too successful.)
A mpex is now using eddy current clutches on the
turntables of some of the latest recorders. These de 
vices provide completely cog-free operation (depen 
dent only on a well-filtered d-c supply voltage) and
thus result in improved flutter and wow. There are
no commutators or slip rings, therefore no replace 
ment problem, and no rf interference is generated.
Faster start times are realized because of the small
mass, and an associated low inertia, when compared
to the rotor of a conventional torque motor.
The brakes, generally associated with the turn 
table assemblies, can be either of the mechanical or
dynamic type. At A mpex , the feeling has always been
that mechanical brakes are superior. With mechani 
cal brakes, a self-limiting — or at least a non-energiz 
ing — configuration should be used. Energizing type
brakes that are not limiting will give quite different
braking forces as the coefficient of friction changes
with variations in temperature and humidity.
Another consideration in designing the brake
system is the differential. This differential, as applied
to magnetic tape recorders, means the difference in
braking force that exists between the two directions
of turntable rotation — with the greater force always
acting on the trailing turntable (in respect to tape
motion). The differential is expressed as a ratio.
hysteresis motor will sync a greater mass and thus
can handle a larger flywheel.
Drive Requirements
Designing a drive system usually entails a com 
promise between low flutter requirements and the
amount of money we can expect in return. There are
ways and means of producing transports that exhibit
extremely low flutter, the accomplishment, however,
is accompanied by a high price. These ultra precision
drives are usually employed only in certain instru 
mentation and data type recorders, with the cost pre 
cluding their use in other than very special appli 
cations.
CAPSTAN ASSEMBLY — First, the capstan shaft. A
small, round shaft seems quite simple and harmless,
but it can be a real troublemaker. It must be round
within small tolerances (0.2 mil) and mounted in its
bearing it must exhibit minimum “ run-out" (again,
0.2 mil) at the tape contact point. The shaft must be
corrosion resistant, and sufficiently hard to with 
stand wearing.
The diameter of the capstan should be large
enough to hold tape slippage and creep to a minimum,
with a compromise normally necessary between the
diameter and the speed of the shaft. For a given tape
speed an increase in diameter demands a decrease in
rotational speed, which in turn requires more fly 
wheel.
We generally will use as much flywheel as the
drive motor can handle while maintaining sync; this
is simply a matter of filtering any cogging of the
drive motor, or other irregularities. As the mass of the
flywheel increases, its efficiency in damping out high
frequency irregularities improves, but it might start
to accentuate low frequency disturbances. If this
occurs we must provide some damping arrangement
— for example, silicone coupling between the shaft
and flywheel.
DRIVE MOTOR — The drive motor must be of the
synchronous type in order to maintain the necessary
speed accuracy. Hysteresis synchronous motors are
usually employed rather than salient pole (reluc 
tance) types, although the latter is less expensive and
provides as good results insofar as flutter is con 
cerned. The reason for this preference is that the
For instance, to consider an exaggerated example,
if we were reproducing a sustained 1000-cycle tone
at a tape speed of 7 ’ /2 inches per second and that
speed suddenly dropped to 6 inches per second our
tone would be reduced to 800 cycles; then as normal
speed was again attained the tone would return to
1,000 cps.
Differentiating between flutter and wow has his 
torically been difficult, but speaking generally we can
consider that flutter consists of components about 6
or 7 cycles per second, with wow components falling
below that figure. (Normal flutter will extend to ap 
proximately 250 cps, but tape scrape flutter is usually
about 3500 cps.) Flutter and wow can result from
anything that affects tape motion, although the drive
system of a transport is most commonly blamed it is
not always at fault