Task: Repair an Atari 800XL That Does Not Boot Cartridges by Replacing the GTIA Chip
Needed: Working GTIA Chip, Atari 800XL, Screwdriver, Chip Lifter
Time: About 1 Hour or Less
Introduction
I acquired a non-working Atari 800XL in 2025 that would boot to a black screen. I was able to get it to boot to BASIC by replacing the delay line chip at U29 as I have previously described for another computer. However, this computer would not boot cartridges presenting an interesting fault. My suspicion was that there was another bad chip interfering with the data or address bus in a way that did not interfere with BASIC.
Instructions
I started down the path of diagnosis feeling all the chips for any that were unusually hot. None were. I knew the 6502 was good because I had probed that with the oscilloscope and that is how I found the U29 failure. My strong suspicion was that one of the other chips was failing and creating some noise on the bus. So, I decided to do a quick swap of the major chips before starting the more tedious process of probing them all with the scope. The first one I tried was the GTIA and that fixed it. I got lucky this time 🙂 Fortunately, I had two GTIA chips in my supply.
Comments
I decided to write this up as a post because the GTIA would not be a likely suspect for this kind of fault.
With a potential bus issue all chips are suspect.