HP9836C colour alignment (grey scale tracking)

[Tony Duell]

> 
> The thing I've learned is that the controls usually interact - so if 
> you don't do things in the 'right' sequence, you can wander so far away 
> from 'good' settings that you'll never get back.  Or so I've heard.  
> ;-)  -- Ian 

They do, and HP docuemnt the fact.

There are basically 7 adjustments -- gain and cutoff for the 3 video 
amplifiers and the 1st anode voltage ('screen grid' across the Pond, in 
the UK we call everything after the cathode and control grid an 'anode', 
in the States, you have the cathode, several grids and one (final) 
anode).

I can't remember the exact procedure, but it's basically to determine the 
cut-off voltagees for the 3 electron guns, set the A1 voltage so that lowest 
cut-off gun is just on the point of cut-off, then for each electron gun to 
determine the cut-off voltage for a totally black (level 0) and just on 
(level 1) raster, and set the cutt-off preset to the average of those 2. 
Then display a level 15 raster, adjust the gain for right intensity, and 
then repeat the cuttoff and gain adjustments for that amplifier because 
they interact. And then do it for the other 2 amplifiers.

Perhaps I should explain that there are 3 4-bit DACs inside the computer, 
one for each primary colour and that a 'level n' raster is when <n> is 
fed to that DAC for the entire screen.

-tony