CDC 6600 display character generation

[Paul Koning]

> On Jun 6, 2018, at 8:01 PM, Rick Bensene via cctalk <cctalk at classiccmp.org> wrote:
> 
> 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.
> 
> ...
> 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), ...
> 
> 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.

That's really weird.  Here's why.  The DD60 only has a single set of X/Y drive chains.  It's all differential, so there are four of everything, ending up at the pair of X and pair of Y plates of the CRTs.  The X/Y amplifiers connect to both tubes.  The reason you see two separate displays is that the tubes have separate unblank circuits.  So the same character waveforms go to both, but at any given time only one of the two tubes has its beam turned on.  Also, focus and astigmatism controls are separate for the two.  See the schematics.

I can try to explain what happened in front of you by broken wires or things like that, but it sure is hard to figure how that would cause fried tubes (other than the CRT, of course).

	paul