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Performance Checkout
accuracy is not a factor in this test. Do not make measure-
ments during read-after write operation.
Note
The definitions apply for both forward and re-
verse directions. The capstan tachometer voltage
will reverse polarity when direction is changed;
however, there will be no observable difference
in the preamplifier output waveforms.
a. Start-Ramp Delay Time: The time (following a start
command) required for the capstan tachometer voltage to
reach a value that is greater than
a
Vdc but less than 3 per-
cent of the value reached at normal drive speed. Specifica-
tion is 0.5 ± 0.5 ms.
b. Start-Ramp 100 Percent Time: The time (follow-
ing a start command) required for the analog output of any
preamplifier track to first reach 100 percent of the peak-to-
peak (or O-to-peak) value established while reading an all
"1 's" tape under steady state drive conditions. This is a
function of tape speed; the following limits apply with the
capstan ramp adjustment being made during the forward
start mode only.
(1)
The 25 ips start-ramp 100 percent time
should be 14.5 ± 0.2 ms for forward ramp,
±0.3 ms for reverse ramp.
(2)
The 37.5 ips start-ramp 100 percent time
should be 9.5 ± 0.2 ms for forward ramp,
±0.3 ms for reverse ramp.
3-31.
START DISTANCE. Start distance is calculated
as follows. Measurement of the characteristics defined re-
quires that the tape unit be driven with commands that are
adjustable to periods greater than the specified start/stop
times and that there be a provision to synchronize an oscil-
loscope directly from these commands (both start and stop).
The tape to be read may be any previously recorded tape,
preferably at 800 cpi as this provides the best resolution
where measurements fram preamplifier outputs are required.
Wavelength accuracy is not a factor in this test. Do not
make measurements during the read-after-write operation.
Note
The definitions apply for both forward and re-
verse directions. The capstan tachometer voltage
will reverse polarity when direction is changed;
however, there will be no observable difference
in the preamplifier output waveforms.
a. Start-Ramp Time: This is the time required to ac-
celerate the tape from zero ips to the drive speed.
It
is de-
termined by subtracting the start delay time (measured)
from the start-ramp 100 percent time (also measured).
3-4
7970B/7970C
b. Start Balance Time: This is the time difference be-
tween the start-ramp 100 percent time and the specified
start time for the tape speed involved. This period of time
is allowed to provide a balance in tape distance as it offsets
the time delay at full drive speed represented by the stop-
ramp delay time.
c. One-half of the start-ramp time plus the start bal-
ance time (both in milliseconds) multiplied by the tape speed
in inches per second equals the start distance (in inches x
10- 3 ). Specifications 0.187 ± 0.020 inch.
3-32.
STOP MEASUREMENT. Stop measurements (both
forward and reverse) are made as follows:
a. Stop-Ramp Delay Time: The time (following a stop
command) required for the analog output of any preampli-
fier to fall to a peak-to-peak (or O-to-peak) value that is less
than 100 percent but greater than 97 percent of the value
established under steady-state drive conditions while read-
ing in all "l's" tape. Specification is 0.5 ± 0.5 ms.
b. Stop-Ramp 100 Percent Time: The time (follow-
ing a stop command) required for the capstan tachometer
voltage to reach a level between 3 percent of normal drive
speed value and
a
Vdc. Specification is a function of tape
speed and must not exceed a time that is at least 0.2 ms
less than the specified stop time for the tape speed involved.
3-33.
STOP DISTANCE. Stop distance is calculated as
follows:
a. Determine stop-ramp delay time.
b. Determine Stop-Ramp Time: This is the time re-
quired to decelerate the tape to zero ips from the drive
speed.
It
is determined by subtracting the stop-ramp delay
time (measured) from the stop-ramp 100 percent time (also
measured).
c. The stop-ramp delay time plus one-half the stop
ramp time (both in milliseconds) multiplied by the tape
speed in inches per second equals the stop distance (in
inches x
10-
3 ). Specification is 0.187 ±0.020 inch.
3-34.
INSTANTANEOUS SPEED VARIATION. Instan-
taneous speed variation is a measurement of the short term
departure of the tape velocity from its long term average
speed. Measurement is made as follows:
a. Write a length of all "l's" tape at 800 cpi and re-
wind as reqUired to be in position to reproduce this section
of tape.
b. Connect oscilloscope to any convenient preampli-
fier output, position so that waveform is centered on screen,
and adjust main sweep sync for positive slope and to trigger
at the zero crossover.
Part 2
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