CDC 6600 display character generation

[Paul Koning]

> On Jun 7, 2018, at 2:50 AM, Rick Bensene via cctalk <cctalk at classiccmp.org> wrote:
> 
> ...
> 
> 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 definitely believe that.  There's quite a lot of stored energy in that device: 2 kV power supplies with a pair of 4 uF filter capacitors, if things short out you will definitely get some excitement.

> 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.

> ...
> 
> 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.

If the final drive tube shorts plate to grid, you'd get 2 kV working its way back into the previous stage.  Those tubes are only rated for a few hundred volts on the plate.  Not to mention that 2 kV feeding back into the previous stage power supply would be a problem also.

As for replacing in pairs, it's a differential signal path and it would not be surprising if they required matched pairs.

	paul