Ampex 351 - Page 36

----- HEADS
7
* ----CAPSTAN
— C apstan drive motor
AREA MOST CRITICAL FOR
STIFFNESS AND FLATNESS
©T
Takeup Tension Arm
The main duty of the takeup tension arm is to
act as a tape storage loop and thus takeup any tape
slack that occurs during starting. It also usually incor 
porates a safety switch that automatically stops oper-
which is chosen to prevent excessive tape slack being
thrown in the stopping process from the normal or
fast winding modes of operation.
The most critical area of the transport for rigidity and
flatness is shoran by the shading.
Mounting Plates
Mounting plates should be sufficiently rigid to
maintain a natural resonance above 300 cps — or
notably higher than the 60 and 120 cps exciting fre 
quencies which emit from torque motors and drive
motors. This rigidity is most important in the area
surrounding the reel idler, heads and capstan; any
flexure in this area will cause flutter.
Of course, another reason for a rigid mounting
plate is to hold alignment between the various com 
ponents that control the tracking of the tape. This
is more important on '/z-inch tape or 1-inch tape than
it is with !Zi-inch.
Reel Idlers
The main purpose of the reel idler is to isolate the
heads from the disturbances originating in the supply
motor, by tape scraping against the reel flanges, by
splices as they leave the reel, or by tape layers slip 
ping as the reel unwinds. (This last effect may be
quite prevalent if tape is wound so fast that air is
trapped between the layers, thereby producing a very
loose pack )
While the reel idler minimizes such disturbances,
we must use care or we will create more flutter than
we eliminate. Reel idlers should have minimum run 
out, bearings must be selected for low noise and
smoothness of operation, and flywheels must be
dynamically balanced to close limits. And the diam 
eter of the idler and the tape wrap around it must
ensure positive coupling between the tape and the
idler. As with the capstan flywheel, a damping ar 
rangement might be necessary.
■REEL IDLER
0140
Back view of a typical professional tape transport.
Dashed line indicates heavy mounting casting em 
ployed in area where rigid construction is critical.
(Ampex Model 300.)
Tape Guiding
Next to flutter, our most difficult problem of tape
transport design is the tape guiding. Certain design
rules must be followed. All components in the tape
threading path must be kept in accurate alignment —
this means maintaining exacting tolerances on the ’
perpendicularity and flatness of all such components
(turntables, reel idlers, heads, capstans, etc.)
The capstan idler must hit the capstan squarely,
or the tape will be diverted up or down. Tape guides,
either rotary or fixed, should not be too small in diam 
eter, and guide widths must be held to close toler 
ances — normally not more than 2 mils over tape
width and preferably less. (Tape itself is slit to a tol 
erance of 0 to 6 mils under the nominal dimension.)
Tape guiding problems are multiplied when we
use thin base tapes. This is caused by the loss of stiff 
ness at the edge and because we must use lower ten 
sions with this type tape.
Incidently, if we have a well designed tape trans 
port that has received good maintenance and sud 
denly have tracking problems, we can suspect the
tape itself. A quick check on the tape is to stretch out
an approximate three foot length beside a straight 
edge. If it does not line up with the straightedge it
has been poorly slitted, or stored on a poorly wound
reel, and the best thing to do is dispose of it —
quickly!