On 09/06/2017 09:41 AM, Jon Elson via cctalk wrote: > 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?
The same as shown in the 7970B/C service manual--a semicircular slotted metal disk attached to the sensor arm intersecting a photocell/lamp box. Looking at the drive casting, it's marked as a Rev F DHB, if that means anything. Just judging by the CE notes scribbled on the inside of the door, I'm guessing that this is a late model. The write-enable sensor assembly is different from that pictured in the service manual. The sensor "finger" is attached to an L-shaped flat arm with two bolts for adjustment. I did replace the microswitch, as the lever arm had broken off at some point. > 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. Will do--I've mentioned in another message that I'm preparing a test tape using the drive, so I should get some inkling of timing and adjustments pretty quickly. --Chuck
