Identifiable by small daughterboard at the bottom of the board and 5770-09656-00 etched above battery holder.

This modification allows us to modify a Revision - Stargate CPU to allow us to use it with later games such as Robotron, Joust, Sinistar, Splat! and with further modifications Bubbles. Stargate games were shipped with two revisions of CPU board; revision - and revision B. The revision B (also C and D) boards can as standard run all the later games but, unfortunately the revision - boards can only run Stargate in their standard form. Why do later games not run on a revision - board? Well if you examine the schematics, the BS.BA signal is inverted to be /BS.BA on revision B and later boards but not on revision -. Stargate ROM boards do not use the BS.BA signal so this modification will have no affect for Stargate. If you look at a revision B board you will see that the BS.BA inversion was actually a factory modification done by cutting two tracks and inserting two jumper wires to route the signal through a spare NOT gate. Fortunately the same gate is spare on a revision - board so we can do this modification ourselves. View a High Resolution Image of this PCB (123kb).

To implement the modification simply follow these steps:

The location of IC 6F Click image to enlarge (???kb)	The reverse of IC 6F Cut the track shown Click image to enlarge (???kb)	Step 1 - Cut PCB track carrying BS.BA signal The BS.BA signal that we want to invert is routed from pin 3 of IC 6F to pin 32 of connector 1J4 (the header of the ROM board ribbon cable). The first step is to cut the track carrying the signal. The easiest place to do this is at pin 3 of IC 6F. The pictures to the left show IC 6F's location and on the reverse the track which you need to cut.
The location of IC 5A Click image to enlarge (???kb)	The reverse of IC 5A Cut the track shown Click image to enlarge (???kb)	Step 2 - Cut PCB track grounding IC 5A input To invert the BS.BA signal we will be using IC 5A which is a 7404 hex inverter. We will be using pin 3 as the input and pin 4 as the output. The boards solder mask grounds pin 3 so before we can use it we need to cut the track grounding it. The pictures to the left show IC 5A's location and on the reverse the track which you need to cut.
The location of IC 5A Click image to enlarge (???kb)	Wires attached to IC 5A pins 3 & 4 Click image to enlarge (???kb)	Step 3 - Connect the wires to IC 5A Solder 2 wires to pins 3 and 4 of IC 5A respectively, these will route the BS.BA signal to IC 5A and the /BS.BA signal from it. The Williams factory modification used 30AWG wire so there's no need for anything heavy duty here. The pictures to the left show IC 5A's location and IC 5A with the two wires attached.
The location of IC 6F Click image to enlarge (???kb)	Wire attached to IC 6F pin 3 Click image to enlarge (???kb)	Step 4 - Connect the wire to IC 6F Solder the wire coming from IC 5A pin 3 to IC 6F pin 3, this will route the BS.BA signal to IC 5A for inversion. The pictures to the left show IC 6F's location and IC 6F with the wire attached.
	Wire attached to plate through hole Click image to enlarge (???kb)	Step 5 - Connect the wire connector 1J4 Solder the wire coming from IC 5A pin 4 to the plate through hole shown in the picture to the left, this will route the /BS.BA signal to pin 32 of connector 1J4. The picture to the left shows the plate through hole location with the wire attached. That's it, you're done! Now all you have to do is recheck your work and try the modified board with a Robotron, Joust, Sinistar or Splat! ROM board.

What to do if it doesn't work???

I have carried out this modification on numerous boards with no problems but it may be possible that something can go wrong. If your modified board does not work, try it with a Stargate ROM board. It does not use the BS.BA or /BS.BA signal so if it works with Stargate then your problem is with the modification. Recheck all your work, look at the BS, BA, BS.BA and /BS.BA signals with a 'scope/logic probe to see what they are doing. As a last ditch attempt replace IC's 5A or 6F, or undo the mod completely.
REMEMBER THIS MODIFICATION IS CARRIED OUT AT YOUR OWN RISK!