A 1970s disk drive that wouldn't seek: getting our Xerox Alto running again

Our vintage Xerox Alto has been running reliably for months, but a couple weeks ago the disk drive malfunctioned and the heads stopped moving. With a drive that wouldn't seek, our Alto wouldn't work.3 After extensive debugging and studying the drive's complex head movement control system, we discovered that the problem had a trivial fix. This blog post discusses our adventures debugging the Alto's Diablo hard drive and how we got it to work again.

The Alto was a revolutionary computer designed at Xerox PARC in 1973 to investigate personal computing. It introduced the GUI, high-resolution bitmapped displays, Ethernet, the optical mouse and laser printers to the world. The Alto I've been restoring came from YCombinator; the restoration team includes Marc Verdiell (curiousmarc on YouTube), Carl Claunch and Luca Severini.

The Xerox Alto's disk drive is the unit below the keyboard. The cabinet under the drive holds the computer itself.

For storage, the Alto used a removable 14" hard disk cartridge that held just 2.5 megabytes. (A user might have multiple cartridges for different purposes, similar to floppies a decade later.) This model 2315 cartridge was invented by IBM in 1964 and became an industry standard, used in minicomputers by HP, DEC, Wang and many other companies. The photo below shows how a disk cartridge is inserted into the Diablo drive. (The drive has been pulled out from the cabinet and its cover removed to show its internal mechanisms.)

A disk cartridge is inserted into the Alto's drive. The drive has been pulled out and the cover removed, revealing its internals.

Each disk cartridge contains a single platter. The drive has two heads, one for each side of the platter, and the heads seek (move back and forth) in unison. Each side of the disk contains 203 tracks at a density of 100 tracks per inch (.254mm spacing), so the heads need to be positioned with very high accuracy. The heads float 70 microinches (1.8 µm) above the disk surface on a cushion of air, so any contamination on the disk surface can cause a head crash, causing the head to contact the surface and scrape up the oxide layer.

Opening a disk cartridge reveals the single hard disk platter. The disk isn't scratched; it's just the lighting.

Our disastrous adventure started when we tried to help out another Alto owner whose disk drive suffered a head crash.1 (Because of the problems this drive has caused, it will be called the cursed drive, although diabolical fits too.) Replacing the heads in the cursed drive should have taken an hour or so, but became much more complex. I'll describe the full saga of the cursed drive in another post, but to make a long story short we installed new heads that immediately crashed so badly that the head arms were physically bent. After installing another set of heads and fixing various other issues the cursed drive finally seemed to work, so we connected it to our Alto.2 Boot almost worked, except any disk in the cursed drive got hopelessly corrupted. To make things worse, our previously-working drive started seeking erratically and then stopped seeking entirely. We suspected an electrical problem with the cursed drive had damaged the Alto's interface board or the good drive's circuitry. This was rather distressing since now we couldn't use our Alto.3

At this point, I should explain a bit about the Diablo drive and the complex mechanism it uses for seeking. The seek circuitry has two purposes. First, when the Alto wants to read from a particular track, the drive must seek, moving the heads to the desired track as fast as possible. Then, the heads must be held perfectly steady over the track. (Keep in mind that a track is only 0.007 inches (.18mm) wide.) Instead of a stepper motor, the drive moves the heads with a DC motor controlled as a servo. To make seeks faster, the motor runs at four different speeds, accelerating quickly and then slowing as it approaches the desired track. Once the head reaches the desired track, the servo mechanism constantly adjusts the head positioner motor to keep the head centered over the track.

The Diablo drive's circuitry pulls up for repair. The drive has three circuit boards on the left and three on the right.

The seek logic is implemented by the three circuit boards on the right.4 These boards mostly use simple DTL (Diode Transistor Logic) gates, integrated circuits from the 1960s that predated TTL. The innermost board receives the desired track number from the Alto. The next board computes the difference between the drive's current track and the desired track and determines how fast to move the head. Finally, the rightmost board is the analog board that drives the head positioner motor as well as processing head position signals from the transducer. 5 In a modern system, the seek logic could be compactly implemented with a microcontroller. But in the 1970s, controlling the heads took three boards full of integrated circuits.

The photo below shows the disk heads and the head position transducer, a key component of the seek circuitry.6 The heads are in the foreground, two barely-visible white ceramic circles on flat metal arms. The head positioner motor (hidden underneath) moves the heads in and out to the appropriate track. The head position transducer, the green disk in the photo below, provides electronic feedback on the head position. The yellow pointer and the scale on the transducer show the track number visually.

The green head positioner transducer provides feedback to the head servo mechanism. The pointer and dial indicate what track the heads are on.

The transducer generates two "quadrature" signals 90° apart, with one pulse per track.7 The disk drive counts these pulses to determine the current track number. By looking at the phase of the two signals, the drive can determine the direction of head movement.8 The video below shows the two transducer outputs displayed in X-Y mode on an oscilloscope. As the head is (manually) moved, the dot rotates 360° on the screen for each track. The direction of rotation indicates which way the head is moving. When the head is aligned over the track, the dot is at the top of the screen. Thus, the transducer outputs show the direction of head motion, the number of tracks moved, and alignment over the track.

Getting back to our disk drive that had problems seeking, we did some testing and determined that seeking had totally failed. The drive did not seek when requested by the Alto or Carl's FPGA-based disk controller. The drive didn't return the head to track 0 when the disk was unloaded. It didn't even hold the head in place over a track. This let us know that the problem was not with the signals from the Alto but something inside the drive.

We figured the complex seek control circuitry must have malfunctioned, so our strategy was to swap the three seek board with boards from a spare drive. Then we could replace boards individually until we found which board had the problem. Much to our surprise, the problem still remained even after we swapped the boards.

An oscilloscope trace shows signals in the malfunctioning disk drive. The motor control signal (yellow) causes the motor to be driven with +15V and -15V (pink), but nothing shows up in the current-sensing resistor (green). Xerox Alto oscilloscope-bad.jpg

Next, we checked out the drive's seek signals with an oscilloscope (above). We found that the seek circuitry was generating a motor control signal (yellow) and the motor driver board was sending +15V or -15V to the head positioner motor (pink). Although these signals weren't really what we expected to see, with full voltage to the motor, the heads should have been moving back and forth rapidly instead of remaining stationary. Also, nothing was showing up across the current-sensing resistor (green).9

The head-seek motor is driven through a large current-sensing resistor (left). (A disk cable or terminator is attached to the connector on the right.)

Although the seek circuitry was complex, the actual motor wiring was fairly simple. The motor received up to +/- 15V from a driver board, and was connected to ground through a large (10W) 0.2Ω current sensing resistor (above). A bypass capacitor across the motor (below) filtered out noise. We suspected a failure of the current-sensing resistor, the bypass capacitor, or the motor itself, so we tested these components. A multimeter verified the resistor hadn't burnt out. A LCR meter showed the capacitor had the right capacitance. We powered the motor directly from a power supply and the heads moved back and forth smoothly. This was a puzzle: all the components tested fine and we had measured voltage from the motor driver board, so why was nothing moving?

The head positioning motor moves the heads back and forth. Drive wires (yellow) are bolted to the motor. A bypass capacitor (black) is connected across the motor.

At this point, Carl noticed that one of the wires on the motor was loose. He tightened the nut and the seek problems were immediately solved. After all our investigation, the problem with our drive was simply a loose wire that prevented power from getting to the motor. Vibration must have slowly loosened the nut until the drive quit working. Apparently it was just coincidence that the problem happened when we had the cursed drive connected.

Conclusion

It was a bit anticlimactic to find a simple loose wire after all our investigation of the seek circuitry. But we were happy to have our drive back in operation, so we could use our Alto again. We still have to diagnose the problem with the cursed drive, but hopefully we're getting closer; I plan to write another blog post once we get that problem solved.

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Notes and references

1. Since the head flies at high speed above the disk surface, any particles on the disk can cause the head to crash into the disk surface, scratching the disk and clogging up the head with oxide. Usually the heads can be removed and cleaned. After reinstalling the heads, they need to be realigned with a special alignment pack so they are properly positioned over the tracks. ↩

2. The disk drives have two connectors on the back, so multiple drives can be daisy-chained together. This lets you have a two-drive Alto configuration, for instance. A terminator is connected to the last disk in the chain. Thus, the Alto was connected to the working drive in the Alto cabinet, which was then connected to the cursed drive. ↩

3. Our Alto wasn't totally dead without a disk drive, since we could boot over Ethernet using my Ethernet gateway. However, without a working disk drive the Alto was very limited. ↩

4. The three boards on the left of the drive aren't relevant for this repair, but I'll describe them for completeness. The leftmost board (J10) has the analog read/write circuitry that drives the heads. (You can see a wire from the upper left corner of the board going to the heads.) The next board (J9) controls the spindle drive motor, lowers the heads onto the disk after loading, and detects sector marks. The inner board on the left (J8) counts the sectors on the disk. It also generates the 5V supply and has an oscillator to drive the head position transducer. ↩

5. The Disk drive maintenance manual includes schematics and a detailed description of the drive's operation. ↩

6. Modern disk drives position the heads based on a servo track written on the disk, a technology developed in 1971 that provided better positioning accuracy. The Diablo drive on the other hand, used older technology where position feedback was part of the drive. ↩

7. The head position rotary transducer uses a special transformer to generate the position signals. A 50 kHz carrier signal is fed into the transducer. This signal is modulated based on the head position to yield two signals, the quadrature signals 90° out of phase. The transducer has two parts: a rotary member that receives the carrier signal, and a stationary member that provides the two output signals. I haven't disassembled the transducer, but based on similar rotary transducers, I believe the transducer is built from zig-zag windings etched into circular printed circuit boards in the transducer. The zig-zags are closely spaced around the transducer disk, with their spacing matching one track's rotation of the transducer disk. The two output windings have the same spacing, but are offset one quarter of a zig-zag, i.e. 90°. As the transducer rotates, the input winding will line up alternately in phase and opposite phase with the output winding, yielding a positive and then negative output, once per track. The other output winding behaves similarly, but 90° out of phase. ↩

8. A mechanical mouse uses a similar quadrature technique to determine the direction of motion. A mechanical mouse typically uses optical encoders rather than the disk drive's transformer encoders. ↩

9. We used an oscilloscope to examine the seek circuitry on a working drive, and found very complex, almost chaotic signals showing the constant adjustments of the servo circuitry to keep the head aligned.

   

   A working disk drive shows the complex signals in the servo mechanism. The input signal (blue) triggers variations in the motor control signal (yellow). The motor voltage (pink) is constantly adjusted so the motor current (green) tracks the control signal.

   The signals from the transducer are processed, combined, and differentiated to generate spikes (blue) as the head moves. This input is filtered to form the motor control signal (yellow). The voltage driving the head motor (pink) is constantly adjusted so the velocity signal (green, from the current-sense resistor) is proportional to the control signal (yellow). ↩