O2 fixer is a replacement for 74LS08 chip in all Atari XL/XE machines. It provides four additional BO2 signals that provide option for separation Atari chips on the BO2 (Buffered O2) clock bus. It also lowers the parasitic capacitance on the BO2 path on the motherboard. Idea by Piotr D. Kaczorowski, design/production by Lotharek.
Installation manual
Photos below are made with Atari 65XE motherboard. For other models and motherboard versions steps are the same, but the location and appearance might be dfferent.
Locate 74LS08 chip on the motherboard. If it is socketed, skip point 2 below.
Desolder the chip and solder back a 14 pin DIL socket (not supplied).
Insert O2 Fixer into the socket. Make sure that it is oriented properly. 4 soldering pads at the PCB should be next to pin 1.
Atari should start normally at this stage. Test it!
Locate O2 signal from the CPU on O2 Fixer: with the multimeter check for the continuity from pin 39 on the CPU to subsequent pins of the fixer. For most XE machines and 1200XL it is pin #1, for Atari 600XL it is pin #5, for 800XL - pin #12, but for 800XLF, it is pin #2.
Connect the located pin with a wire or a bit of solder to the middle pad called O2_IN.
From now on, the soldering pads marked OUT_1, OUT_2, OUT_3 and OUT_4 will provide 4 buffered and noise free O2 signal outputs. You can use them for:
- BO2 signals
- Ultimate 1MB connection
- cart slot O2 signal replacement - this is most suggested use, resolves problems with fast cartridges like SIDE 3.1
- connecting PIA O2 / POKEY O2 signals
Additional info
- 3 of 4 outputs have been connected to GND with 680pF capacitors - look at the bottom side of the PCB. Ultimate 1MB needs no capacitor - please use soldering pad marked OUT_1 for U1MB.
- if you use "capped" output (OUT_2, OUT_3 or OUT_4) and Atari behaves odd or crashes, please remove corresponding capacitor for the given OUT soldering pad. Installing O2 fixed in the socket allows quick removal of the capacitor when required.