CDC 6600 display character generation

[Rick Bensene]

Speaking of CDC 6x00/Cyber 70-series consoles...

I had a bit of a  scary but memorable experience of sitting at the console of a Cyber 73, many years ago.

My job as a systems operator basically involved watching the console for magtape mount/dismount requests, printer service requests (e.g., out of paper), as well as in general monitoring the loading of the system, and assuring that all was running smoothly. 

The machine was running the KRONOS timesharing OS, with MODCOMP front-end communications processors providing serial terminal services.  There were two MODCOMP machines (not sure of the model, but would recognize them) that provided literally 100's of serial ports for terminal access through, in the earlier days, various port-selector equipment, and later, through a Sytek LAN-based terminal server networked together with thin coax strung around the buildings.

Anyway, I was sitting at the console one morning, and noted that very suddenly, the left tube's image coalesced into a single vertical line centered in the tube.  At the same time, I heard a quiet cracking noise coming out of the area of the console where the CRTs and final drive circuity was located.   The cracking noise very quickly increased in intensity, and then the vertical line of the left display (the right display stayed normal), suddenly collapsed into very bright dot in the center of the screen, then all at once, there was a loud BANG, the dot on the screen faded away quickly, the right screen went blank, and I was greeted by a shower of sparks and molten metal that spit out the slot underneath the tubes where the deadstart switch was located, and smoke coming out of the cooling slots in the cabinet.   Then, there was a clunk (someone throwing the power switch for the console), and everything settled down fairly quickly, other than there was a lot of stinky smoke in the air.

This whole sequence of events occurred in about 2 seconds.  I had just enough time to push my feet against the electronics bay underneath the console, and shove myself away from the shower of sparks.  I ended up with a small chunk of molten metal that landed on my left  leg and burnt through the fabric of my slacks and burned my skin pretty good.   There was a lot of acrid smoke that also came out of the console for a little while after the power was off.  We were concerned that there was a fire in there, but as it turned out, fortunately, there wasn't.      Other than the burn on my leg and a little hole in my slacks, I was unscathed.   But, I was a bit stunned by what happened, and it took me a few moments to realize what had happened.

The other people  in the data center reacted quickly.  One that was dismounting a tape on one of the drives that were situated behind the console by about 10 feet, ran to the master power switch on the console and shut it off.   

Another ran to me, and was checking me out to make sure I was OK.  

One of the other folks started up a magic program on the Cyber  from a Tektronix 4023 terminal in the data center that effectively provided the same displays as the Cyber console, except only one of the displays could be viewed at a time.  The display was updated using the addressable cursor of the 4023 terminal, though it wasn't nearly as "real-time" as the actual console displays.
Commands could also be given in the same form as they could be keyed on the console keyboard.    This served as the alternate console while the main console was dead.   

The smoke detection system in the data center triggered the fire suppression system (Halon in those days), and the klaxons went off indicating we had  30 (maybe it was 45, can't remember for sure) seconds to get out of the room before the Halon dumped and all the oxygen was flushed from the space.   There was an abort switch on the back wall of the data center, and one of the other folks ran and hit the abort to keep the Halon from dumping (which was rather expensive to recharge).  The fire department showed up almost immediately, because they were A) located at one corner of our business campus, and B) their station was tied into the fire systems in the data center, and were notified when the smoke sensors triggered.  They came and checked everything out to make sure no lingering hot spots could spark fire.  

After the ruckus settled down, I resumed monitoring the system with the 4023 terminal, and operations proceeded normally.  The outage of the console had no effect on the operation of the Cyber -- everything ran along just fine during the chaos.  End users generally didn't even know it happened.

We had onsite CDC service engineers, and they responded immediately.   It turned out that one of the big driver  tubes had failed in such a way it shorted, and that caused a cascade failure that eventually took out the other tube, and caused some pretty severe stress in other components in the high voltage power supply that ended up drastically overheating, resulting in the shower of sparks and some hot metal.   

One of the CRTs (the left one) had to be replaced.  When the beam went to dead center, it burnt the phosphor out at that point.
Both of the big final-stage driver tubes had to be replaced (even though only one had failed, they apparently had to be replaced in pairs).   Some smaller vacuum tubes, wiring harnesses, circuit boards and IIRC, a transformer that were cooked also had to be replaced.  Also, a few of the circuit modules in the base cabinet of the display were replaced.

They had the console back up and running smoothly in a little over 4 hours.   Amazingly, their local parts depot had everything they needed to repair the console in stock.  One engineer ran into town to pick up the parts while the other tore into the console, and by the time the guy arrived with the parts, everything was stripped down, cleaned up, and ready for the new parts to go it.

The little "fireworks" display I was front and center to witness will be something that I'll never forget.  

-Rick
--
Rick Bensene
The Old Calculator Museum
http://oldcalculatormuseum.com