Involved in the heartbreaking job of dismantling a venerable Philips P|3435 hard disk drive, we couldn't do so without studying and documenting its workings.

I thought I'd put down these few historical notes not too long after taking it apart. It's surprising that I couldn't find anything about it on the Web, which further encouraged me to write a few words and share the memory.

1 Overview

We're talking about a huge, 90 kg hard disk drive of about 1.5×1×1 m. It bears a hinged cover with a porthole over the location with hard disk platter cases can be glimpsed at. Hard disks platters offer a capacity of the order of 5 MB.

The front side of the drive features a panel with a button and lights. Underneath the panel comes a filter behind which an air pipe leads to the disk platter cradle. The rear side of the drive holds a door leading to the inside of the drive case, which contains I/O cables, and provides power switches.

2 History

According to a label indicating when one of the filters has last been changed, and assuming it has never actually been changed – because that's just something one never bothers doing, I would guess the drive has been assembled well before 1984, although some components date back to 1975 (I'm thinking in particular about a large, nice, tar-coated transformer). A system backup hard disk last updated in 1980 has been loaded in.

The drive was for some time used to bit-dump music recordings with an interface to wire it up to an Apple II. We're talking here about massive connectors with a wealth of pins, as can be seen in the rear side on the bottom of the lot. It's not been its only job of course as it's been running for over 13 000 hours according to the hour counter.

3 Startup

A power plug comes out of the rear of the drive but nothing happened when plugging it before flipping the toggle switch and the main rotary switch. A strong fan sound making me think of a reactor filled the room when it started, lights flickering to life on the front panel.

4 Front Panel

The front panel is made of 6 lights:

1. The Start/Stop light is particular in that it's also a button. It lights up when pushed if the hard disk is ready to spin and can be switched back off by pressing it again in order to stop the disk.
2. The Power light goes on when the drive is powered on.
3. The Ready light lights up when a disk has been mounted and is ready to be driven. It actually takes some time before it lights up. Maybe it has something to do with a goal temperature to be reached, in which what must be heat probe diodes at the bottom of the hard disk platter cradle must be involved.
4. The Cartr Exch light is turned on when the drive is ready for mounting a disk or removing it. Again, it takes some time when calling Stop before Cart Exch lights up because the hard disk needs to spin down. Only then can you effectively open both latches in order to unmount a platter.
5. The Fault light croaks on when an error has occurred. It has happened once when the engine popped out its belt and timed out spinning free.
6. I've never seen the 1 light turned on. Presumably it blinks cheerfully when performing I/Os.

5 Operation

Once the drive is turned on and Cartr Exch lights up, a disk platter can be mounted, unmounted or started. It might be that the drive is even clever enough to remember the last state it was in during its last run and for instance start spinning the platter at once if that's what it had last been doing. That might very well be, since the right latch won't unlock unless the drive is up.

A disk platter comes and is used in a plastic container. It has a handle on the top. There's a lock slider on the handle which must be pulled before lifting the handle in order to mount the platter. The handle must of course be lowered again before operation. Likewise, in order to be able to remove the platter container, the lock slider must be pulled, then the handle be lifted before the whole container can be unmounted.

When a disk platter is mounted, its cover must sit on top of it to trigger small switches up the latches, otherwise the drive won't leave the Cartr Exch state.

6 Dismantling

The general impression one gets when dismantling a Philips P|3435 is the clever design overall, although you can tell different teams have been in charge of different component sets because while some are very ingenious, others are occasionally quite clumsy (e.g. parts apparently uselessly hiding screws).

Removing the first cover reveals a good deal of the key components of the drive:

1. The main logic is made of a set of PCBs inserted onto a motherboard. Although the boards are the only machine-assembled components, several human-soldered tweaks and adjustments have been added. It's worth noting that the labels on the lining don't give any obvious hint as to in which order the boards must be plugged in. The whole PCB box can be turned by 90° to reveal a larger label describing them. For instance, some are in charge of generating sines and cosines. One of the whole set of PCBs bears six LEDs which seem to be permanently switched on.

2. The head engine contains four strong magnets on the sides and two above, thus surrounding a coil through which the shaft of the heads can move back and forth. There are two heads reading two disk platters, one above the other. A little ruler indicates what the position of the heads is, in number of tracks. A lens is also used to determine the zero position. Wires are wrapped up in springs to ensure that they don't fold too sharply.

   A clever setup has been devised to keep the heads aligned with the tracks even when the disk platter undergoes heat expansion: an iron shaft mounted on two flexible plates has a thinner span on which a heat probe is mounted, thus allowing for small shifts as the disk platter expands and shrinks.

   The head engine, heads and the spindle are all attached on a high precision structure which makes for the largest fraction of the whole machine's weight. The bolts keeping the different components of the structure in place are virtually impossible to unscrew.
3. The drive actually comes with a built-in disk platter, presumably of 5 MB too, hidden beneath the removable disk platter cradle. This is why there are two heads. Something I should've tried was to Start the spinning without any removable disk platter in: supposedly it would have worked too.
4. Head pre-amplifier.
5. Power supply.

6. The engine is in charge of spinning the spindle by way of a belt. The belt made a rubbing noise and tended to let go.