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7970B/7970C
c. Use the delayed sweep feature to position the de-
layed sweep at the next zero axis crossover (this corresponds
to one bit-to-bit distance) with the delayed sweep auto trig-
gered from a main (delaying) sweep.
d. Adjust delayed sweep rate to permit good resolu-
tion of the time shift band at the zero axis crossover. During
this evaluation the vertical gain and the delaying sweep time
may be adjusted to optimize the resolution.
If
the correct
point is being observed, it will be the waveform that is com-
ing from top left of screen to bottom right with the slope
depending on the gain and delayed sweep speed.
e. Observe the jitter band under conditions in step
"d" in microseconds peak-to-peak time displacement and
multiply by tape speed to determine this distance in micro-
inches. This value must not exceed the following limits
which are based on ±3 percent instantaneous speed variation:
(1)
800 cpi must not exceed ±37.5 micro inches
(75 microinches peak-to-peak).
(2)
556 cpi must not exceed ±54 microinches
(108 microinches peak-to-peak).
(3)
200 cpi must not exceed ±150 microinches
(300 microinches peak-to-peak).
Note
The above values apply to both the forward and
reverse modes.
3-35.
DYNAMIC TAPE SKEW. Dynamic tape skew is
that variation in tape velocity which generates a differential
time position between the two outermost tracks on the tape.
Measurement is made as follows:
a. Write a length of all "l's" tape. Rewind as required
to reproduce this section of tape.
b. Use dual-trace oscilloscope and connect to pream-
plifier output of the two outside tracks (channels 4 and 5).
Use chopped mode (triggered by channel A) with the main
sweep synchronized for positive slope triggered at zero axis
crossover. (Use negative slope in reverse direction.)
c. Adjust both channel gains and positions to super-
impose the two waveforms (amplitude only; there will be
varying degrees of time difference due to static skew char-
acteristics ).
d. Use the delayed sweep feature to present the next
(one bit-to-bit distance later) zero axis crossover on the de-
layed sweep. The delayed sweep must be on internal sync,
triggered on the negative slope. (Use positive slope in reo
verse direction.) The earliest of the two tracks will sync the
delayed sweep and the other will arrive later and will have
jitter that represents the time differential in microseconds
Part 2
Performance Checkout
peak-to-peak. Again gains must be adjusted as high as pos-
sible (both the same) and the delayed sweep trigger set for
maximum stability of the stable waveform. There will be a
considerable dead zone in the delaying sweep adjustment.
Use the first operating position as the sweep is moved out
from minimum delay position. There is no significance to
the time differential between the stable and unstable wave-
forms. This does not represent static skew. The difference
is a function of oscilloscope gain and position settings.
e. Measure the peak-to-peak time band of the unstable
waveform as it crosses the zero axis. Convert this to micro-
inches peak-to-peak for the tape speed involved. Worst case
must not exceed ±50 microinches (100 micro inches peak-
to-peak) and applies to forward or reverse mode.
3-36.
FAST FORWARD/REVERSE START/STOP
CHARACTERISTIC. Fast forward and fast reverse charac-
teristics are measured by synchronizing an oscilloscope with
the appropriate drive command, and observing the dc out-
put of the tachometer using the TACH test point on the
capstan servo printed-circuit assembly.
CAUTION
Do not issue sequential fast drive commands
without allowing time to reach full speed or to
return to zero speed. Failure to observe this pre-
caution may cause excessive power dissipation
in the reel servo amplifier circuitry. Minimum
time between commands should be 1 second or
the sum of the start and stop times (whichever
is greater).
3-37.
Fast forward/reverse start or stop times are meas-
ured by observing the time (following a start or stop com-
mand) required for the tachometer output to either reach
its maximum value (for start time) or to fall to zero (for
stop time). Nominal times are from 400 to 700 milliseconds.
Typically, the value will be 600 milliseconds.
3-38.
Fast forward/reverse start or stop distances are de-
termined by the time required for the ramp to move from
one state to the other. This time is nominally 500 milli-
seconds. One-half of this time multiplied by 160 ips equals
the nominal start/stop distance of 40 inches.
3-39.
TRANSPORT FUNCTION, MOTION, AND
STATUS CHECKS.
3-40.
The following checks cover the 1/0 lines in the
status and motion command connector. Use appropriate
off-line test equipment to verify proper performance as in-
dicated.
3-41.
FUNCTION COMMANDS. Select, CS: The con-
trol and status PC assembly includes a jumper that can serve
to establish unit identification where the front panel select
3-5
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