Saturday, October 28, 2017

Ethernet tool final testing, disk head repair on two Diablo drives


Finally back from my trip, which was extended a few days due to travel disruptions. We met on Friday at Marc's home and focused mostly on the disk drives, since we had two drives with crashes to repair.

The damaged heads are removed by unplugging their cables, loosening a cable clamp and loosening the allen head clamp screws that lock the head assembly in place. One clamp screw is common to both head assemblies and is normally torqued to 50 inch-ounces of force. Each head assembly also has a discrete clamp screw holding it in place, torqued to a lower force of 26 inch-ounces.

With the heads removed we could put them under a stereo microscope, examine the oxide stuck on them and begin cleaning. The normal method of cleaning is to use isopropyl alcohol (IPA) of at least 90% purity. 

IPA is hygroscopic and absorbs atmospheric water vapor, but we want a minimum of water in contact with the iron magnet poles on the head. We use IPA rated at 99.9% pure, so that even with absorption it will be well above the 90% target. 

Cotton swabs made to have minimum risk of detached cotton strands are used, since a strand of cotton is enough to cause a disk crash in a running drive. We scrubbed the heads quite a bit with the swabs and IPA, but were left with some oxide residue in the grooves in the head surface.

The head is a ceramic shape with a polished smooth surface shaped as an airfoil allowing it to fly some few microinches above the rotating disk surface. A cruciform shaped trench is formed in the ceramic to expose the iron poles from the read/write and erase coils.

The long trench, oriented at right angles to the disk center, gives a longer pole for the read/write coil. The shorter cross trench has a pair of pole ends from the erase head straddling the read/write pole. The pole of the erase coil is a U shape with the two top points of the U exposed in the trench and the rest extending below the head surface.

The trenches are not polished, thus the rough surface can accumulate oxide from a head crash and is harder to read and clean with a relatively huge cotton swab head. The degree of heat generated during the crash also determines the type of debris on the head. 

Disk platters are coated with a slurry of iron oxide powder and a binder - something like epoxy - which is spun onto the surface and baked dry. It is then polished smooth before being assembled into disk cartridges. 

Light crashes leave lighter brown material that is easier to clean, but a longer duration of rubbing raises the temperature, scorches the binder material of the oxide and makes for a dark and dense material. 

Marc has an ultrasonic cleaner which he put to use to work on the material in the trench we couldn't remove with swabs. The worst of the four heads (two per drive) needed about an hour of gentle ultrasound cleaning before it was 'like new'.

The heads were reinserted into the disk drives, the discrete clamp screws set to 26 inch-ounce of torque but the common clamp left loose. This will allow us to force the heads to slide inward towards the center of the disk platter as we attempt to align the drive. 

The side of each head assembly has a V notch cut in the metal. This allows a setscrew to be turned in the mounting block and as it extends, it forces the head assembly outward as the screw tip rides against one edge of the V towards it center.

Alignment makes use of a special CE disk cartridge which has a special pattern recorded on specific tracks. The drive comes in standard and high density versions; the Alto uses the high density type. Thus, we are interested in track 105 as the alignment standard.

Two stripes of data are recorded, centered on track 105, but spaced slightly to either side of the centerline. As the head position is advanced from below track 105 towards the center, the scope pattern will show peaks from the stripe that is closer to track 0 to be stronger than the offset pattern from the far stripe. 

When the two stripes are equidistant, the head is centered between them, over the location of track 105, and the scope shows twin peaks of equal amplitude. Thus the head assembly is advanced until we see that symmetric pattern on the oscilloscope.

We mounted the cartridge, spun up the drive a few times, finally allowing the heads to load. It looked good but after about a minute we started hearing an odd sound so we unloaded the heads. Visual inspection of both the heads and the cartridge surface showed no problems.

When we tried to spin up again, we heard some screeching sounds as rotation began. A known good cartridge also made the same sounds, with it and the CE cart now refusing to spin at all. We have a drive problem that we have to resolve before we can perform the alignment.


Ken worked on his ethernet tool. This consists of a Beaglebone board plus a PCB ken built to interface the signal voltages. He recently moved from the breadboard version used in development to a PCB (production) version.

He found some ringing in his signals outbound to the Alto which he addressed with the addition of a small capacitor, then moved on to testing and debugging all the functionality. The tool includes the IFS software developed by Living Computer Museum, providing a full suite of functionality equivalent to the bridge box they loaned us.

During long operations, typically a copydisk from a physical cartidge in the Diablo drive to a virtual cartridge in the tool, the Alto and tool would get out of sync. It appears to be an issue with collisions particularly with breath of life packets occurring very close to a copydisk packet.

The rate of errors is low but not zero, thus Ken will continue to probe this until he resolves the issue completely. Otherwise, the tool is working well. It connects directly to the multipin ethernet connector on the rear of an Alto, thus we have no need for the outboard ethernet transceiver or any 3Mbit cabling.

The tool allows routing, interfacing to modern 10/100Mbit ethernet networks and all the other functionality provided in the IFS code base. We are returning the LCM bridge box to Al Kossow at this point.


Marc has a number of old Macintoshes and drives, starting with the original Mac, then moving to the  512K, plus and others. He did some work to set up disk images on 400K and 800K floppies as well as with a hard disk, taking advantage of Appletalk and Ethernet networking as appropriate between the various machines.

I received the last of the incandescent lamp types needed for the HP 7970B and 7970E tape drives. I will bring one over to Marc to repair his 7970B drive, in a future session.

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