I'll write this as a suppliment to the existing adjusting-your-monitor
file. In particular, all of the warnings and cautions about working
inside a monitor with its dangerous voltages apply.

The first thing to do before any critical adjustments are made should
be to check purity and convergence.  Purity is checked by putting up 
full-screens of Red, Blue, and Green -- a problem will manifest itself
as blotches of the wrong color usually around the edges.  The "90% green"
pattern in the SGI confidence test will due for a start, but I like to
check the other to channels if possible.  Convergence is checked by
putting up a white on black cross-hatch pattern (there is one labeled
"convergence" in the SGI confidence test) and looking at the lines to
see that all three colors are on top of each other everywhere.

If problems with either purity or convergence are observed the first
step should be to throughly degauss the monitor.  Degaussing coils
designed for the purpose are available any TV-radio supply store.
If that fails to solve the problem its probably best to try and obtain
the service manual for the particular monitor as procedures vary considerbly.
Small problems near the edges of the monitor are almost always fixed by
the application of small magnets (such at the magnetic-rubber sheet
stuff you often see refergerator magnets made of). to the CRT tube.
If you problem is a small one you can try this approach.

Properly adjusting the color balance of a monitor requires an instrument
designed for that purpose.  We use a Minolta CA-100.  Brightness,
contrast focus and others can usually be adjusted by eye.

Almost all monitors will provide some means for adjusting both
the gain and offset of the video reaching the CRT.  The front
panel "brightness" control is an offset adjustment which effects
all three colors (hopefully equally).  The front panel "contrast"
adjustment is a gain adjustment for all three channels.  In addition
there are usually controls labeled "sub-contrast" and "sub-brightness"
in the back.  The exact effect of these varies from one model of
monitor to another but they are usually a coarse adjusment to
set the operating range of the front panel controls.
In any case the offset control (brighness) is used to set the darks
to the correct level and the gain control is used to set the brights
to the correct level.  The controls nearly always interact, and sometimes
in surprising ways.

If adjusting by eye the brightness should be adjusted such that black
just dissappears from view.  Contrast should be adjusted to a reasonable
viewing brightness.

Since there are always controls for gain and offset of all three channels
together (brightness and contrast), many monitors will lack such controls 
on one of the three colors individually -- usually green.  This means you 
need to set the green up using contrast and brightness and then the seperate 
blue and red controls to balance blue and red with respect to green.	  
The graphics monitors shipped with SGI systems I've seen so far have had
adjustments for all three colors, but you never know what they will do
next.

Adjusting color-balance by eye is at best extemely difficult -- however
it can help if your monitor is totally out of wack.  One of the SGI
confidence tests features a grayscale ramp which can be used pretty well
to get a rough balance.

At the time I first wrote this the Hitachi HI2073 was the most common
SGI monitor.  Since that time I've seen may newer models of monitor
but the principals are largely the same.  I've added editorial remarks
about other monitors in [].

Here is the 2073 proceedure in tack:

First some theory on the HI2073:

The "Drive" controls are simply gain controls on the R, G, and B inputs.  The
"Bias" controls set the DC operating level of the main video amps this
adjustment is not very critial.  The "Cutoff" controls adjust the clamp level
(black) of the final cathode drive, this clamp level is also controlled by the
front panel "Brightness" control and the "Sub-Bright" control.  The
"Background" controls adjust how much the "Brightness" and "Sub-Bright"
controls effect the particular cathode driver.  The "Sub-bright" control only
effects the brighter extreme of the "Brightness" control.  The "Sub-Contrast"
control serves mainly to keep the range of the front panel Contrast control
reasonably linear.

[on some monitors there controls labeled "bias" which are really Cutoff
controls as described above.  The bottom line is that the all have some
sort of gain and offset control on at least two of the three colors.]

I have programmed my own test patterns for setting up the monitors.  My
patterns are an approximately 3" square of either 100% white or 1% gray on a
18% gray background.  Those percentages are in terms of actual screen
brightness.  The correct voltages are 46% for the 18% gray and 12% for the 1%
gray.  (This is calculated from a gamma of 2.2 which seems pretty close to what
most monitor tubes are.)

The reason for using a small square in a large gray field is to minimize 
the effect large changes in total brightness has on the clamp point of
the various video amps and on the High-Voltage regulation.

Here is the procedure I now use to adjust the 2073 series Hitachi monitors:

1)  center all of the "Drive", "Background", and "Cutoff" adjustments.  Center
the "Sub-Bright" control, turn the Brightness and Contrast fully clockwise.
Turn the "Sub-Contrast" fully clockwise.

[most newer monitors will only have a single "bias" or "cutoff" adjustment
you may want to try just using the existing settings as a starting point.]

2)  Adjust the screen control until the black portions of the raster just 
disappear.  [You can think of the screen as an extremely coarse "brightness"
adjustment -- use it with caution I'm not sure what ill effects will come
from the screen voltage being different from it's design value.]

3)  Adjust the focus.  There is a cross-hatch pattern in the SGI confidence
tests which is good for this.

[Screen and Focus are generally located on high-voltage transformer --
 be careful, use a plastic screwdriver!]

4)  Put up the 100% white patch pattern.

5)  As the Bias adjustment for each color is turned clockwise you will
find a point where that color begins to smear.  Turn it back counterclockwise
until the smearing goes away.  I use a 'scope on the cathode drive output
and adjust this pot for the fastest rising (white to black) transitions
on cathode driver.  I don't know if this is really the optimum adjustment for
this but it seems to work.  The Hitachi manual is silent on the proper
adjustment.

[Once again newer monitors usually don't have this adjustment]

6)  Using the Minolta meter to monitor the screen brightness, turn the 
"Sub-Contrast" control counter-clockwise until the brightness just starts
to drop.

7)  Adjust the R, G, and B "Drive" controls for proper full white level
and color balance.  (I use the minolta color analyzer mode and set full white
as 25 foot-lamberts and 6500 degrees Kelvin).  If I can't reach 100% I 
use the brightest available balanced level.

[many of the newer monitors come adjusted much "bluer" than 6500 degrees
I'm told there are good technical reasons for this, but I'm not sure I
believe them.  In any case the choice of a white point will depend
on the user's application]

8)  Put up the 1% brightness patch

9)  Adjust the R, G, and B "Cutoff" levels for proper white balance and
1% brightness (0.25 foot-lamberts normally).

10) Put up the 100% brightness patch

11) Repeat steps 7-10 until no further adjustment is needed.

12) Turn the "brightness" control (front panel) fully counter-clockwise.

13) Adjust the R and B "Cutoff" controls so that the white patch is color
balanced.

14) Turn the "brightness" control fully clockwise.

15) Adjust the R and B "Background" controls so that the white patch is 
color balanced.

16) Repeat steps 7-15 until no further adjustment is needed.

[Newers monitors don't require anything like steps 12-16]
[note that the CM2086 is now an old monitor, although there are probably
many still in use]

I have looked at the schematics for the newer CM2086 monitors and note
that it is quite a bit different.  I have not yet worked out a good 
procedure for these but I can tell you something about the function
of the adjustments.

First of all there are both "Gain" and "Drive" adjustments.
Both of these adjustements effect the gain of the video signal
The "Drive" adjustment is after the variable gain stage which
the "Contrast" adjustment controls.  It is also after the point where
the brightness control voltagel is mixed in.

The "Bias" adjustments seem to have a completely different function.  In
the CM2086 these adjustments effect the level of the contrast control signal
in each of the variable gain stages.  Proper adjustment of the "Gain"
and "Bias" adjustments should allow the Contrast to have a flat
white balance over its whole range.

There are no longer are "Cutoff" adjustments.  The "Background" adjustment
is actually what used to be "Cutoff", which is to say it sets the DC level
of the final Cathode drive and does not change the effect of the "Brightness"
control.

In this design the final amplifier is DC coupled to the cathodes.  The
brightness control signal now establishes the clamp voltage at the input
to this stage (this was set by the "Bias" controls in the 2073).

Given all of the controls it should be possible to adjust so that
low and high level whites are balanced and so that they stay balanced
over the full range of the "Brightness" control and the "Contrast" control.
However given all of the interactions I have not yet developed a procedure
for doing this.  I will have to try a few things on a monitor first.  
(I haven't really had a chance to adjust any of these since getting
the schematics.)

[I never did write the procedure down ... It turns out that the individual
monitors will have large differences in how much the "Background" and
"Gain" adjustments will interact.  The result is that the adjustment
process is something of an art learned best thought practice.  On some 
you find you actually have to adjust one something like 1/4 to 1/2 of 
the error, and then go to the other, otherwise you end up in an infinate 
loop trying to get proper adjustment.  The monitors released after the 
CM2086 were much kinder to the technician in this respect.]

Will McCown, Rhythm & Hues, Inc., Hollywood CA. (213) 851-6500
INTERNET: will@rhythm.com     or willmc@alumni.caltech.edu