The Collection

Date of introduction: 1970

Type: Two Disk Drives per cabinet using removable disk packs

Capacity per drive:

640 six-bit characters per sector

14 sectors per track

20 tracks per cylinder

200 cylinders per disk drive

This results in 35,840,000 characters (corresponds to 26.68MBytes)

Disk pack: CDC 871 or equivalent

Rotational speed: 2400RPM

Positioning technique: Hydraulic actuator mechanism

Positioning time (average): 75ms

Power requirements: 208V, three-phase, 7A per phase

Dimensions: Height: 170cm (67"), Width: 86cm (34"), Depth: 101cm (40")

Weight: 498,6kg (1100pounds)

S/N: 3180

Story:

This Control Data disk drive cabinet designated as the "841 Multiple Disk Drive" (MDD) was found within the large haul of computer artefacts discovered in Dortmund in spring 2012 which were the remainings of the former Computer Museum Aachen (Germany). Actually, there were two of these disk drive cabinets there.
The story about saving the remainings to avoid them to be scrapped this still needs to be written from my perspective. The team from the Robotron Museum described their side of the aventure that is nice to read (link to be added).
So what is so fascinating and wonderful about this disk drive type from the late 1960's / early 1970's?
The Control Data 841 MDD is the only disk drive made by CDC with the approach of a disk head positioner using a hydraulically driven moving mechanism! IBM also followed this approach with their xxx disk drive. However, both companies dropped this approach for later series, as the positioning accuracy could not be increased any further in order to obtain a higher storage density per disk storage area. However, these two disk drive types represent an interesting approach within the history of disk drive development that was dropped and replaced by electromagnetic positioning mechanisms still used nowadays.
Being aware of that technology approach, I absolutely wanted to rescue this drive, altough there neither was the corresponding disk drive controller, nor did I have a CDC omputer that made use of these.
Above all, at this point in time, I did not have any more space to store these massive disk drive after I already had safed a truck-load and a van-load with two large CDC tape drives, two other CDC disk drives, and Telefunken and Honeywell cabinets.
Therefore, I dediced to dismantle one complete cabinet by removing all the infrastructure for at least on disk drive and removed the logic boxes of both cabinets to have spares because they seemed to have different configurations of logic boards. The idea was to maybe one day mount back everything into another cabinet and be able to at least make the hydraulic positioner work again for demonstration purposes. I actually wanted to remove a second disk drive, but another young collector was so fascinated about it that I helped him to remove one for him, rather than for me. I have no idea though, what he did with that drive afterwards.

There were no documents except for some sheets about the factory quality control testing and configuration with the cabinets. On bitsavers.org, there is so far only a CDC brochure provided about these drives. However, some research revealed that this disk drive type was OEM'ed and used by Xerox for their Sigma computer systems. They rebranded these drives as 7242 drives and at first sight, they seem to correspond to the 841 CDC MDD. Thankfully, the Xerox documents are available on bitsavers.org.

There is very little information about the CDC "Multiple Disk Drive" systems to be found in the internet so far, and I am uncertain, if other systems are in museums or private collectors' hands. If you have bits, pieces, documents, and/or a complete cabinet/installation, please contact my via my email address provided on this website! I'd love to get in contact with you to share experiences and information about it.

The first set of pictures shown in the "pictures" section were taken in the old hangar where the two cabinets were exposed to climate changes and humitiy over several years. They were very dusty and showed signs of corrosion - particularly the disk packs found within the drives.

Pictures of the Control Data 841 Multiple Disk Drive:


	The nice "Control Data" logo on top of the cover panel. "Multiple Disk Drive" referred to the modular approach to connect up to four of these disk drive cabinets to a 3553-2 mass storage controller to obtain a mass storage subsystem, as advertised by Control Data. In fact, 216 MBytes of total disk storage was a huge amount of memory in the early 1970s.
This picture depicts the right side of the cabinet.
	Right side, lower half: On the bottom are located the 3-phase power controller and the air blower that distributes air to the two disk drives via large hoses. On top of it is located the lower of the two disk drives.
Right side, upper half: On the top can be seen the monstrous power supply box required for the two disk drive and the electronics. The circuit boards are housed in a small cabinet to the right of the power supply. Below the power supply and the disk drive logic can be seen the second, upper disk drive.
	The left side of the cabinet: From top to down: power supply and disk drive electronics, upper disk drive, lower disk drive and the finally on the botto the air blower with a distributor to which two white large tubes are connected.
Close-up of the power supply / disk drive logic section. Cabling is well arranged and everything looks tidied up.
	On top of the disk drives. When I first opened them, disk packs were still inside and unfortunately completely corroded. They are unusable. This is the first disk drive design, I saw in my life, that doesn't require to close the disk pack chamber to avoid contamination during operation! The datacenter rooms really had to provide a very clean environment!
Twenty disk surfaces per disk pack are writeable and readable by each drive. That requires twenty disk surface read/write heads!
	The logic box of the 841 disk drive. The two cabinets that were to be scrapped, if not rescued, had different logic card configurations. Thus, I rescued both logic boxes to make sure they are safe, to have spares, and to figure out later, what configuration is required to make one drive maybe operable one day.
The back of a disk drive seen from the left side of the cabinet. Visible are the large, white and flexible air tube for air circulation to prevent particle contamination at potential damage between disk surfaces and disk heads. On the left is also visible the hydraulic actuator with tubing made from what seems to be copper.
	Similar view as in picture above, but now, the motor can be seen that drives the disk drive belt with 2400rpm.
Back side view of a disk drive. On top is the logic box with three fans mounted on its bottom side to cool the logic boards.
	Close-up of the back side of a drive. The cable slap used to align the cabling when the drive is taken out from its position to load a disk pack can been nicely seen in this picture. An oil tube connecting the oil pump with the disk head actuator is also well visible.
Similar view as above, but a bit more to the left: The drive belt motor on the left, oil pump in the middle, and disk head actuator to the right.
	Back-side view of the cabinet where both drives are visible (actuators, ventilation tubing).
Picture taken from another angle at the back side of the cabinet.
	Three-phase power controller at the bottom of the cabinet.
Label of the CDC disk drives. "Multiple Disk Drive", Modell: BM101B, P/N: 40068241, S/N: 3180