IBM 727, Руководство по инструкции

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the coil is caused to move downward, called the "stop" position, both moving pulley
forks pivot away from their respective drive capstans.
4 .2 .1 Forw ard-Reverse Magnet Assembly
For only one moving pulley to cause driving at a time, it is necessary to bias the
linkage to the right or left. This is accomplished by using the forw ard-reverse mag­
net assembly.
When the forward magnet is energized, the linkage is biased to the right. When
the moving coil is then energized in the upward direction, the right moving pulley
makes contact with the right drive capstan but the left moving pulley is held away from
its drive capstan. If the moving coil is energized in the down direction, the left mov­
ing pulley is driven against the left stop capstan but the right one is held away from
its stop capstan.
The reverse happens when the reverse magnet, instead of the forward magnet, is
energized. With power on, the moving pulley linkage is always in one of four definite
positions: forward stop, reverse stop, forward start, reverse start. The normal
position is considered to be forward stop.
4. 3 SPLIT IDLERS— TAPE ALIGNMENT
To assist in maintaining proper alignment of the tape, the split idlers are used.
They are located just to the right and left of the head assembly on the upper head
plate. These split idlers keep the tape parallel to the main plate and a fixed distance
from it, regardless of variation of width of the tape. To avoid too complicated an
alignment system, it was preferred to maintain the alignment of one edge of the tape
only and let the other move freely. Figure 8 is a view of the split-idler assembly.
The front edge of the tape is held a fixed distance from the front plate by the stationary
portion of the idler. The rear portion of the idler is free to slide on the shaft and is
held in continual contact with the rear edge of the tape by spring tension. The entire
assembly, shaft and all, rotates within sleeve bearings in the associated housing.
When the tape expands in width, it spreads the split idler. The track farthest
away from the front edge of the tape is, therefore, most subject to misalignment when
the tape expands or contracts laterally between the time of recording and the time of
reading. If the distances of the two split idlers from the main plate are the same, a
minimum of skew can be expected. (The problem of skew is discussed in section 1 3 .3 .)
Measurements have shown that split idlers can be very effective in reducing skew to
a minimum. Also, alignment between machines can be very closely controlled.
4 .4 HEAD ASSEMBLY
The assembly generally referred to as the head assembly is made up of two v e r­
tical plates, the lower of which is stationary. The upper plate is built to move up and
down under power. On the lower plate are located the read-w rite head assembly,
the tape-break-light assembly, and the tape cleaner. On the upper plate are located
the erase head, load point, and end-of-tape p h oto-electric-cell assem blies, the split
idlers, and pressure pad.
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