Control Data 841 disk drive's 3-phase power supply resurrection

Jon Elson elson at
Sun Feb 18 13:10:21 CST 2018

On 02/18/2018 11:34 AM, P Gebhardt via cctalk wrote:
> Hello list,
> currently, I am in the process of trying to bring back to life a disk drive installation from Control Data known as "841 Multiple Disk Drive" ( MDD ). From the early '70s. It uses hydraulic disk head actuators! Pictures of the subsystem are here:
> I started with the power supply. Most of the electrolytic capacitors need to be reformed which is being done.
> As far as I know, some computer installations used 400Hz 3-phase back in the days. Does anybody know, if that is the case for this type of drive systems? I couldn't find any indication so far, except for the input filter that supports up to 400Hz (written on it).
> I've quite some experience with old linear power supplies, but never worked with three-phase supplies, yet.
> Has anybody experience with this? Anything particular to be considered?
> There is an operator's manual, but there don't seem to be manuals or schematics about this type of CDC drive nor on bitsavers, neither elsewhere on the net. How could help me in pointing out where to get these?
> A lot of questions, I know.... :)
If the motors ran off 400 Hz, they would have to have LOTS 
of poles to keep speed within reason.  A two-pole motor on 
400 Hz would run close to 24000 RPM.  My guess is they did 
not run the spindles and blowers off 400 Hz, as a momentary 
dip would not cause the pack to lose much RPM.  The pack has 
a huge rotational inertia.

The electronics COULD be run from 400 Hz.  If a linear 
supply, then the transformers would seem to be insanely 
small for the power delivered.  That's your first clue.  It 
was quite common for CDC and other systems to run the CPU 
and closely-coupled components off 400 Hz created by rotary 
converters.  (The IBM 360 was one system that didn't, they 
used electronic converters to produce single-phase 2.5 KHz 


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