800XL Color Unstable

[+Larry]

For the past few months I've been using an 800XL as my main computer, upgraded with a Newell 256K, Supervideo 2.1, and the Bob Puff "Stable XL" mods. But this XL has always had an issue with color stability. In my case this XL changes the screen color from a "greenish-blue" to a more "redish-blue" (more purple) after being on for several hours. The only cure is to increase the color level then adjust the tint. Changing only the tint makes the color look way off.

 

Is anyone aware of a particular cause, and perhaps cure for this?

 

Thanks,

Larry

[dmlloyd]

The 600/800XL has a bizarrely complex circuit for "biasing" the color adjustment level. I imagine that if any of the components involved were starting to fail, it might affect the color setting a bit. One thing which amused me is that in the 1400XL (the "flagship" Atari system which never shipped), they replaced all that circuity by just connecting the CADJ potentiometer to +5V and GND, removing something like a dozen components in the process.

 

If you're feeling mod-y, you could try doing the same thing: remove R37 and C47 and connect the side of "R38" (the color trim pot) which used to connect to those two components directly to +5V (maybe on the anode side of CR2). That is, if my schematics are right. :-)

 

If the problem still persists after that, then the cause must lie either with the chroma output circuit (maybe a failing capacitor?) or GTIA itself (if you have a spare, swapping it out might be an easy way to rule it out early). It's also possible that the color trim pot itself is failing, but this seems less likely to me.

[a8isa1]

For the past few months I've been using an 800XL as my main computer, upgraded with a Newell 256K, Supervideo 2.1, and the Bob Puff "Stable XL" mods. But this XL has always had an issue with color stability. In my case this XL changes the screen color from a "greenish-blue" to a more "redish-blue" (more purple) after being on for several hours. The only cure is to increase the color level then adjust the tint. Changing only the tint makes the color look way off.

 

Is anyone aware of a particular cause, and perhaps cure for this?

 

Thanks,

Larry

For a long time my 800XL faded from "BASIC blue" to a dull aqua color after some minutes. At that point the 800XL would not display blue color though once in a while the color would snap back, but only briefly.

 

One day I accidentally bumped the power connector and the colors instantly changed. From there it was easy to see that I had a loose connection.

 

I re-soldered all the joints for the power connector and the problem was solved.

 

Can't say this will solve your problem but weak solder joints are at least something to consider.

 

- Steve Sheppard

Edited December 26, 2009 by a8isa1

[+Larry]

Thanks for both the replies, but Steve, I think you have the answer to my issue. I wiggled the power connector and the color changed. Then I remembered some years ago that I had a 130XE that had the same symptoms. I ultimately removed the power connector from the PCB and replaced it with a new one from Best Electronics. I can remember how tight and solid the new connection felt and that completely cured the issue. I'll try cleaning and re-soldering this one first.

 

Thanks again,

Larry

 

For a long time my 800XL faded from "BASIC blue" to a dull aqua color after some minutes. At that point the 800XL would not display blue color though once in a while the color would snap back, but only briefly.

 

One day I accidentally bumped the power connector and the colors instantly changed. From there it was easy to see that I had a loose connection.

 

I re-soldered all the joints for the power connector and the problem was solved.

 

Can't say this will solve your problem but weak solder joints are at least something to consider.

 

- Steve Sheppard

[dmlloyd]

Guess the simplest solution is usually the right one