LGP-30 Memory Drum Update

Paul Koning paulkoning at comcast.net
Tue Jan 3 19:22:46 CST 2017

> On Jan 3, 2017, at 6:07 PM, Chuck Guzis <cclist at sydex.com> wrote:
> On 01/03/2017 01:42 PM, Mark Linimon wrote:
>> Very ambitious.  IIRC for the G-15 there is/are (one? two?) timing
>> track(s) which were written at the factory.  Of course whatever that
>> machine was, has not survived.  I remember reading that if you lost
>> that timing track, your machine was toast.
> Maybe.  ISTR that wehn CE's were working on the CDC STAR station drums,
> they routinely rewrote timing tracks.  I recall that their only tools
> were a scope and a Halliburton CE box.  Apparently, the goal was write a
> track such that there was no detectable "splice" in the beginning/end of
> the track.  Pretty much a cut-and-try operation.

The fixed head disk I saw rebuilt in the field was a DEC RF11 -- it had basically its entire guts replaced from spare parts.  That included a new platter, so its timing track had to be written.  The timing track writer box had a knob on it to tweak the clock frequency, and it worked just as you describe: keep hitting the "write track" button and tweak the knob until the track gap checker circuit said the gap size was within the required range.

The key questions for reconstructing such a device is what the modulation scheme is, and the pulse pattern.  There might be marker pulses for sector start, for example, or that might just be derived from a counter in the controller.


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