you probably know that there is a batch of ATARI XE computer out there with Bad GTIA chip, these computers where sold in the early 90ies in east Europe.
here is a image of an ATARI 130XE with the bad GITA chip onboard
the obvious fix is to replace the bad GTIA (CO14889) with a good one, but the GTIA is soldered to the board on these models which makes is hard to remove and replace.
here is a hack by adding a 74LS175 Chip you can fix the Bad GTIA (This fix was originally designed my Simius from Poland)
Step 1 is to make a small Cut the traces that connects Antic Pins 2,3,5 TO GTIA Pins 18,29,20 (see image below)
- Piggy back the 74LS175 on top of the connecting only Pin 8 (GND) and pin 16 (vcc) Legs - Connect Pin1 of 74LS175 to pin16 of 74LS175 - PIN 28 OF GTIA (CO14889) to pin 9 of 74LS175 - Pin 18 of GTIA (CO14889) to pin 10 of 74LS175 - Pin 19 of GTIA (CO14889) to pin 15 of 74LS175 - Pin 20 of GTIA (CO14889) to pin 6 of 74LS175 - Pin 2 of Antic (C021698) to pin 12 of 74LS175 - Pin 3 of Antic (C021698) to pin 13 of 74LS175 - Pin 5 of Antic (C021698) to pin 5 of 74LS175
A very interesting fix. I love this kind of workaround in electronics. If only I had the time to read again some books I studied when I was at the Uni and practice a little... Well done and thanks for sharing the fix.