CDC 841 Multiple
                  Disk DriveDate 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" was found within the large haul of computer artefacts discovered in Dortmund in spring 2012 that were the remainings of the Computer Museum Aachen (Germany). Actually, there were two of these 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 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 rapidly again as the positioning accuracy could not be increased any further in order to obtain a higher storage density per disk surface. 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, I did not have any space anymore to store these massive disk drive after I already had safed two large CDC tape drives, two other CDC disk drives, and Telefunken and Honeywell cabinets.
Therefore, I dediced to dismanted on complete cabinet by removing all the infrastructure for at least on disk drive and removed the logic boxes of both cabinets to have spares and because they seemed to have different configurations of logic boards. My 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 removed 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 not sure, 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 addThe ress provided by 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. 



More pictures of the Control Data 841 Multiple Disk Drive:




841 logo
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 mass storage subsystem, as advertised by Control Data. In fact, 216MBytes of total disk storage was a huge amount of memory in the early 1970s.
This picture depicts the right side of the cabinet.
841 right side
841 right side,
                      lower half
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.
841 right
                        side, upper half
841 left side,
                      upper half
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.
841 left side,
                        upper half
841 disk drive
On 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!
841 disk
                        heads
841 disk drive,
                      logic box
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 dis heads. On the left is also visible the hydraulic actuator with tubing made from what seems to be copper.
841 disk
                        drive, back side
841 disk drive,
                      logic box
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.
841 disk
                        drive, back side
841 disk drive,
                      logic box
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.
841 disk
                        drive, back side
841 disk drive,
                      logic box
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.
841 disk
                        drive, back side
841 disk drive,
                      logic box
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
841 disk
                        drive, back side