On 09/06/2017 11:03 AM, Al Kossow via cctalk wrote:
On 9/5/17 11:09 PM, Chuck Guzis via cctalk wrote:
The read results have improved considerably.
tension arm servo acting reasonably?
what style arm sensor does your drive have?
OK, so one of the first checks is to put a scope on the tach
signal (should be a test point for that) and verify the
velocity is a perfect trapezoid. If there are regular
spikes in the trace, that could indicate a problem in the
tachometer. If there are bobbles in the trace at jerk
points, then adjusting the damping on the servo loop will help.
Then, you need a tape written at known density, and read
that, looking at a data track. The best is a tape with long
records of all ones, which puts transitions on all channels
(in 800 BPI NRZI mode). Adjust the right pot to set the
data rate to the calculated value for that drive's tape speed.
Then, move the scope to the skew test point. The drive will
have 9 FFs (one per track) and these go to a resistor
summing point. You should see nearly a square wave when
skew is perfect, but will get much more slope with little
steps on the flanks when skew is not good. The all-ones
tape is perfect for this adjustment. You will see the shape
of the signal fluctuate, as tape laying on the reel for some
time will stretch just a bit, and tend to "weave" across the
tape guides. There will be adjusting screws to set the head
skew to minimize this.
All the above applies mostly to 800 BPI NRZI. PE and GCR
are far less critical on this.
Jon
