I just brought back to live my old Atari 800XL (it's has 30years with our family), any way, the video composite output is on black and white, I had another 800xl and works fine with that cable, so it's a hardware problem.

Quick question, what I should check? the potentiometer? resistance? (or anything else like ram, antic, etc etc)

Thanks in advance for the feedback,

screenshot of the colors problem


  • 1
    First, inspect the board thoroughly with a microscope to look for burned components, broken traces, etc. Commented Jun 3, 2021 at 5:10
  • 2
    This smells like the colour subcarrier has drifted so far out of spec, or is completely missing, and that the TV no longer recognises it as a colour signal. If you have a 'scope, you'll be able to check for a signal and thus know if the crystal is off-spec or dead and in need of replacement. Some machines have a trimmer which can be used to adjust the frequency. Ideally you'd have a frequency counter so you can set it correctly for all TVs, but just twiddling it until the colour appears on yours would be sufficient.
    – pndc
    Commented Jun 3, 2021 at 7:42
  • I think the "trimmer" is the "potentiometer" that I was speaking with my facebook friends, thanks for your reply Commented Jun 3, 2021 at 18:57

2 Answers 2


Based on the information provided, I'd recommend the following first steps:

  1. Clean the monitor jack on the 800XL using contact cleaner.
  2. Inspect the monitor jack while inserting and removing the cable several times. It should slide in and out smoothly and the pins remain clean. Also the monitor jack itself should not flex or move appreciably.
  3. If there is any doubt about the jack after cleaning and inspecting externally, disassemble the computer and use a magnifier to check the 5 solder joints for the monitor jack on the bottom of the PCB. Reflow any suspect solder joints.
  4. Most 800XL machines support S-Video output. If you can obtain a compatible Atari S-video cable and monitor with S-Video input, then you can test with this setup to see if the Chroma signal is present at all. If S-Video shows color output, then the problem is just the Composite output pin; if not, then the problem is upstream of the monitor jack.
  5. If none of the above works, then you have to evaluate the signal path for the Chroma signal from GTIA, through the mixer and modulator, to see where it is "lost".
  6. A possible simple check if you see nothing wrong with any of the components is to swap out the GTIA chip from the working machine. This is only easy if they are both in sockets.
  • I vote up your answer, but in this case it's not the solution, from a facebook group (from Chile), a friend discover that is the potentiometer the problem (R38), so, It's has to be replaced, thanks any way for the tips ;) Commented Jun 3, 2021 at 18:49
  • @EnriqueSanMartín Thanks. Glad you found the problem. I think that fault is relevant to my step #5, right?
    – Brian H
    Commented Jun 3, 2021 at 19:11

from: https://www.facebook.com/groups/840633719349481/permalink/4031384456941042

Photo of the potentiometer of my motherboard

The R38 potentiometer it's has to be replaced.

Edit: the potentiometer was replaced (and also a "hack" on the motherboard to enable the S-Video signal), here is the sample:

Alternative Reality Color demo

  • 2
    Looks like the pot is gone. Am I seeing that right?
    – Brian H
    Commented Jun 3, 2021 at 19:20
  • 2
    Looks like the invisible pots. They were overstock from a production run used to build Wonder Woman's airplane. Commented Jun 4, 2021 at 1:53
  • 1
    The emperor's new pots. Commented Jun 4, 2021 at 14:01
  • That's escalate quickly. Commented Jun 4, 2021 at 16:06

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