HP9836C colour alignment (grey scale tracking)

Tony Duell ard at p850ug1.demon.co.uk
Fri Sep 26 15:36:02 CDT 2008

> Tony Duell wrote:
> >> Tony Duell wrote:
> >>>>> Has anyone ever done this setup and can offer any tips?
> >>>> Sounds like you can hardly make it worse.
> >>> Well, that's the problem. According to one of the HP manuals, 
> >>> misadjustment can cause damage. Of course that damage might not be too 
> >>> serious so it's probably worth giving it a go.
> >> I'm guessing that they mean "driving the tube so hard the people across 
> >> the road complain" kind of damage.  Certainly I'd replace the two 
> > 
> > Yes, that was my thought. Excessive beam current could damage the CRT, 
> > the flyback transformer or bits on the video board. But I wonder just 
> > what 'excessive' is. I suspect I would notice it was far too bright long 
> > before anything was damaged.
> I suspect that you'd see noticeable flyback lines well before you got it 
> too "hot".

I would hope so. And this is an HP machine, from the 'good' days of HP, 
so it's likely to use somewhat overrated components.

I acutally wonder if adjusting for a certain maximum intensity is the 
best thing to do to safeguard the eleectronics. If the phosphors have 
aged, it may bt that I have to turn up the gain so far to get that 
intensity that the beam current is high enough to damage the flyback, or 
the video output transistor, or....

I am wondering about measuring the average beam current when displaying 
the level 15 raster of each primary colour and making sure it's 
reasonable. Beam current is near enough the final anode current, so 
measuring that would do. But that would mean having a microammeter 
floating at 25kV wrt earth. What fun...

> > Equally, I don;t know if the CRT has lost emission over time, it may well
> > be that I could safely turn things up a bit. 
> Fond though I am of spending my pocket money on expensive new toys, I'd 
> trust the Mk. 1 Eyeball on this job.

Well, the thing is that no way could I afford a new photometer, or even a 
'properly' recalibrated one. So even if I found one, I'd have the problem 
of knowing if it was anywhere near calibration. For most measuring 
instruiments I can think of 'sub^n standanrds' which would at least 
verify the readings were sensible (e.g. checking a voltmeter agains one 
of the voltage reference ICs -- it's not a traceable standard, but it'll 
at least show if the meter is wildly out). But I can't think of such a 
'standard' for luminous intensity.

Also, the 9836 has 4 bits/pixel in the the graphics memory, these are fed 
to a 16*12 bit RAM (colour lookup table), and then to 3 4 bit DACs. So 
there are a maximum of 16 colours o nthe screen at any time. OK, this is 
better than most of my other machines, but I am hardly likely to use it 
for displaying photographs or anything like that. No, the use of colour 
is for things lijke 'The red grpah is the I-V characterising with a bias 
of -2V, the blue graph is the same with a bias of -6V'. Which means the 
exact colours are not that important. HP make some comment about setting 
the grey scale carefully to avoid customer complaints that 2 9836Cs don't 
show the same colours, but I don't have 2 of them :-).

So provided I am not going to damage anything, presumably by excessive 
beam current, I don't think it's going to be that critcial _for me_

So yse, I guess I do it by eye, possibly also checking the beam current.


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