CDC 6600 display character generation

[Rick Bensene]

Paul wrote, concerning my "fireworks" show on the DD60 console of
Tektronix' Cyber 73 system:

>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).

This happened probably somewhere between 35 and 40 years ago.  I'm going
from memory, and, as we know, memory over that long can be sketchy, so
it's possible that I some bits were dropped or flipped over the ages.
   
Believe me, the console did spit out a lot of sparks and little blobs of
molten metal, along with quite a bit of smoke.  I still have a small
scar on my left leg where one of the little blobs of metal burned me
after burning through the material of my slacks.

I remember keenly one of the CDC field guys saying that one of the big
driver tubes had shorted.  There were quite a slew of other parts
(including some smaller vacuum tubes) that ended up being replaced, as
well as the left CRT tube which had a phosphor burn right in the center.


What I observed, I remember quite clearly, although a lot happened in a
short time, and it's entirely possible that the right tube may have also
had something weird going on while I was watching what was going on with
the left tube.  I happened to be looking at the display up on the left
tube when the failure occurred, so my attention wasn't directly on the
right tube.   I remember glimpsing quickly at the right tube noting that
its display had disappeared as I was pushing the wheeled chair away from
the console with my feet.

Whatever the cause of the failure, it was something that surprised the
CDC guys.   Maybe the shorted tube was an artifact of the failure, and
not the cause...hard to remember exactly.  But, I do know that two of
those big ceramic and metal tubes were replaced, as well as the left
CRT, and a whole slew of other parts.   And I do clearly remember them
saying that the driver tubes had to be replaced in pairs.

Anyway, all told, it was an interesting adventure.   I loved those
days...I have a photo of myself sitting at the console of that machine
back in the day...probably some of the happiest times in my work career
were those days at Tektronix.

-Rick