Atari color generator tidbits

In fixing my Star Wars board, I've picked up some information on how the
color generation works. This might be obvious to some, but I don't recall
ever seeing this posted, so . . .

I'm not sure if I blew the r/g/b output myself this time, or not,since the
resistors had already been replaced, and badly. The resistors and inductors
had black carbon on them, but once cleaned they were all intact and tested
ok.

Fact #1: The r/g/b output circuits from the transistors on don't contain
any loops, so as far as I can tell, you can actually test all those
components in circuit. a nice bonus. Of course, everthing tested fine on my
board and it still didn't work, so I just replaced everthing that had
already been replaced.

I recall seeing it asked a few times how comes the color boards have a
single Z output test point (when using a scope as a monitor, for example).
I don't think I ever saw an answer, other than an assumption it was all the
outputs summed. Nope -- they cheat on the color output.

The assumption is that the three r/g/b levels are controlled independantly.
They aren't. Since most people probably have worked on the monitor and see
three separate Z amps, this is a reasonable assumption. The monitor looks
like it will take three different input intensities for r/g/b.

Fact #2: However, as I read the schematic the AVG only generates a single Z
intensitiy. The three r/g/b outputs look like just switches that either tie
that color's output to Z or not. So, the Z testpoint will give a true black
and white version of the picture

I can see why they did this: It simplifies both the circuitry and the
programming. It would be interesting though to see what effects could be
made on a color vector display with true color shade blending.

This leads to the problem I had with my board, which I still don't
understand:

Fact #3: When one of the 3 color ouput 'latch' circuits fails, you will see
a colored dot in the center of the screen corresponding to the one that
failed. The intensity of the dot will reflect how must of that color should
be on the screen.

I don't understand why this so. Unless someone knows why can can explain it,
I'm going to dig back into the schematics and figure this one out, because
it is realy eating at me. The beam should still be deflecting, just not
turning on that gun. Why would it be turning on when the beam was at the
origin ?

Fact #4: You can screw up your color outputs if you put the AVG connector
on wrong. Be carefull.

==========================================================
Chris Candreva -- chris@westnet.com -- (914) 967-7816
WestNet Internet Services of Westchester
http://www.westnet.com/

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Received on Wed Aug 8 10:56:44 2001