68705 BYPASS MODIFICATION
Written by Matteo Marioni - 8 March 2005
On early Dragon's Lair / Space Ace PCBs, the main function of U7 (the 68705 microprocessor), was to generate a clock signal named RTC and use it to drive the INT signal of the Z80 main CPU. On later revision boards (Rev. C) the 68705 was eliminated by generating the needed clock signal by using two unused counter ICs on the board. The modification described here is designed to eliminate the 68705 IC by soldering three wires on the back of an early Version 1 Cinematronics PCB.
Early Version 1 PCBs (80-12035-01) have no revision marking and copper-colored traces. This is the only PCB this document applies to.
Version 1 Rev. C PCBs (80-12035-01 REV C) are marked REV C, and the traces are green.
Version 2 Rev. A PCBs (80-12035-02 REV A) are marked REV A, and the traces are green.
Make sure your board does not have the 68705 mounted (of course not!). Check if your board has U6 and U13, and if they're missing, install them. They are the two 74LS393 counter ICs. Prepare three small wires, and solder them to the back of the PCB as shown in the picture below: one wire from U6 pin 2 to GND, another wire from U6 pin 6, to U7 socket pin 12, and the last wire from U13 pin 13, to U19 pin 22.
That's it! No traces to cut or other solder joints to do. Your board should now work fine without the 68705. This modification has been successfully done on two PCBs, serial numbers 657 and 4186.
The notes above are believed to be correct and work fine for the author. No one is forcing you to do this. If you break your Dragon's Lair or Space Ace PCB, it's your fault!
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