anderson@uwvax.UUCP (David P. Anderson) (09/30/84)
<> A while back I bought a second drive, Shugart mechanism, for my apple ][. It works OK, except that the first time it is accessed after power-up, it apparently seeks past track 0, making the nasty hammering sound. The original Apple drive doesn't do this. I am worried that the second drive is having alignment problems because it occasionally stops being able to read certain files. Has anyone else had this problem and/or learned how to solve it? Does the controller treat both drives equally? David Anderson (uwvax!anderson)
eraps1@nadc.ARPA (10/04/84)
> > A while back I bought a second drive, Shugart mechanism, for my apple ][. > It works OK, except that the first time it is accessed after power-up, > it apparently seeks past track 0, making the nasty hammering sound. > The original Apple drive doesn't do this. I am worried that the second > drive is having alignment problems because it occasionally stops being > able to read certain files. > > Has anyone else had this problem and/or learned how to solve it? > Does the controller treat both drives equally? > > David Anderson (uwvax!anderson) > David, Apples use a stripped down version of a typical disk drive. They do not have a track zero sensor (or an index pulse for that matter). When you first turn on your computer, it doesn't know what track the head is positioned on. To solve this problem, it steps the head 35 times. As you can see, this allows for the worst case solution (the head was on track 35 at power up). However, if the head was positioned somewhere in the middle of the disk, the head will be at zero after less than 35 steps. Since the computer has no way of finding this out, it steps the head the remaining number (of 35) times. The head on the disk drive is controlled with a vertical stepper motor, the motor has a plastic disk mounted upon it (level/horz). This plastic disk has a spiral grove carved in its top. The head has a feeler which presses down this grove, additionally, the head is constrained to moving accross the disk only (by mounting it on a track). Now, when the little plastic disk turns, the feeler is pulled along the direction of the track, and the head moves with it. When the head is positioned on track zero, it cannot move any farther, so neither can the feeler. This means that any additional stepping of the drive causes the feeler to 'jump a grove'. The clacking noise you hear is the feeler jumping the grove. This does not indicate a problem, in fact, the louder the noise, the stronger the spring in the feeler. I think that if you listen really closely, you will here a clcking noise on your old drive too, although, since the mechanism is slightly different, it may be quite (u-sci drive clack quieter due to the shape of the feeler and grove). Incidently, if you were to go buy a new 'Apple' drive, you would find that they are now the loudest of the loud. The second problem is probably due to one of two things; drive speed, or write current. These problems tend to be fairly easy to fix. The 1st possibility (speed) is caused by the tolerance of the read routine in the controller card (ie, the drive speed you read the disk with has to be close to the speed you wrote the disk with, or the space between bits will be read, instead of the bits actually stored on the disk). Note that this does not nessarily mean your drive is out of alignment, the drive that wrote the disk could be the culprit. The simplest way to check (and fix this if it is a problem) is to run one of the disk speed checker programs (these are found in many copy programs [under disk uilities], as was as being distributed with certain drives). To check and align the disk, 1) take off the cover (do not do this if it is still under warrenty) 2) boot up and run the disk check routine 3) turn the small multi-turn pot sticking out on the right side of the disk, in the very rear or the drive, until the speed reading on the screen holds to within +/- 5 (less if you can get it). 4) note that the speed is not expected to hold constant, and the slight variations in speed -5 <--> +5 are not a problem, just adjust it so that the number never exceeds these bounds. The second possibility is slightly more dangerous to fix if you don't know exactly what you are doing. To make the drive write the data a little more firmly on the disk, and to strengthen the signal upon read, you can tweak the little pot on the analog card (the big card mounted on horizontally over where the disk goes. Speed adjust TOP VIEW | ---read/write current adjust V | | | ------------------|---------------| | [| Drive ---------|-------- | | |<--- Card | V | | | | | [ ] | | | | | | | Front of Drive | | | | | <----- -------------- Analog | | | Disk goes in here Ribbon Cable Card --->| | | -------------- | | | | | ------------------ | | ----------------------------------| Do NOT turn the pot more than a 1/4 turn either direction, and remember where the pot was set originally, ie, when you are done, the pot should be no more than 1/4 off from before you did anything. If the problem gets worse, you turned the pot the wrong direction. (Note that turning the pot too far in the correct direction is just as bad as turning it the wrong direction) Finally, the controller card on the apple uses identical circuitry to control both drives (it treats them identically). Hope this helps, Rob Ginn (eraps1@nadc)