Bally/Stern MPU single 2764 EPROM conversion

21 February, 2011 (10:03) | Pinball | By: admin

A conversation with a fellow collector led me to revisit an old problem that I never really solved at the time. Originally I wanted to develop a method to convert Bally and Stern MPUs to use a single EPROM. Bally had the option to use a single 2764 EPROM on their 6802 MPU so it should have been easy enough to just look at that schematic to figure out how to make the earlier Bally and Stern MPUs work with a single 2764 EPROM. This proved to be true, though a different method would be required for each board generation (-35, MPU-200 and -17/MPU-100). I quickly moved on to using 27256 EPROMs for my single EPROM conversions but I don’t currently see a good set of instructions for a single 2764 conversion for Bally/Stern MPUs and I decided it was about time for me to follow through on an old idea and put together some instructions to benefit everyone.

Click “Read more” for complete instructions.

Below you will find instructions to convert the 3 main board types that comprise the Bally/Stern MPU generations to use a single 2764 EPROM.

Bally -35 MPU – 2764 EPROM Conversion Instructions
1. Remove any PROMs or EPROMs from U1, U2, U3, U4, U5 and U6.
2. Bend out pins 1, 2, 27, and 28 on a 28 pin EPROM socket so we can solder wires to the pins and install it in socket U2 on the MPU. Pin 26 of the EPROM is not connected. I usually remove this pin from the socket but you can just bend it out and not connect anything to it.
3. Ensure that there are no jumpers connected to E4, E13, E13a, E11, E9, E12, and E35.
4. Connect jumpers E7 and E8. This connects EPROM A9 to CPU A9.
5. Connect EPROM socket pins 1, 27 and 28 to +5V. I chose to use pin 24 of U3 as the source for +5V. This wire is RED in my examples.
6. Connect EPROM pin 2 (A12) to jumper location E4 (A14). This wire is pink in my examples.
7. Connect jumper location E12 to ground. I use the large ground trace closest to E12. That will connect EPROM CE to ground. This wire is BLACK in my examples.
8. Connect jumper positions E9 and E13A. That will connect EPROM A11 to CPU A11.
NOTE: There is an error on the Bally schematic which shows jumper position E13 connected in common with E13A. This is an error and E13A will need to be used for this step.
9. Connect jumper position E11 to the side of resistor R14 closest to the EPROM socket. That will connect EPROM OE to the output of the gate at U17A. That is an enable that combines CPU A12, VMA and phase 2 of the clock creating an enable that we can use for our EPROM. We could also use jumper location E35 but R14 is closer and makes for a cleaner installation, in my opinion. This wire is yellow in my examples.






Bally -17 And Stern MPU-100 – 2764 EPROM Conversion Instructions
1. Remove any PROMs or EPROMs from U1, U2, U3, U4, U5 and U6.
2. Bend out pins 1, 2, 20, 23, 27, and 28 on a 28 pin EPROM socket so we can solder wires to the pins and install it in socket U2 on the MPU. Pin 26 of the EPROM is not connected. I usually remove this pin from the socket but you can just bend it out and not connect anything to it.
3. Ensure that there are no jumpers connected to E3, E4, and E8.
4. Connect EPROM socket pins 1, 27 and 28 to +5V. I chose to use pin 24 of U3 as the source for +5V. This wire is RED in my examples.
5. Next we need to connect EPROM A12 (pin 2) to CPU A14 (pin 24 of U9). The problem is that there are no jumper locations on the -17 and MPU-100 that are connected to A14 so we have to connect directly to pin 24 of the Motorola 6800 (U9). One option is to run the wire over the top of the board to the back (solder side) but I wanted to make my conversions look as clean as possible so I use pin 20 of U3 as a through hole to get to the other side of the board. So we connect pin 2 of the EPROM to pin 20 of U3 on the components side of the board then connect pin 20 of U3 to pin 24 of U9 on the solder side of the board. This makes for a clean looking conversion on the components side of the MPU. If you’re going to use this method you need to ensure that there are no jumpers connected to E8. This wire is pink in my examples.
6. Connect EPROM pin 20 (CE) to ground. I like to use the large ground trace closest to the EPROM socket. This wire is black in my examples.
7. Connect jumper E3 to the side of resistor R14 closest to the EPROM socket. That will connect EPROM OE to the output of the gate at U17A. That is an enable that combines CPU A12, VMA and phase 2 of the clock creating an enable that we can use for our EPROM.
8. Connect EPROM pin 23 to jumper location E4. That will connect EPROM A11 to CPU A11. This wire is green in my examples.







Stern MPU-200 – 2764 EPROM Conversion Instructions
1. Remove any PROMs or EPROMs from U1, U2, U3, U4, U5 and U6.
2. Bend out pins 1, 27, and 28 on a 28 pin EPROM socket so we can solder wires to the pins and install it in socket U2 on the MPU. Pin 26 of the EPROM is not connected. I usually remove this pin from the socket but you can just bend it out and not connect anything to it.
3. Ensure that there are no jumpers connected to E2, E4, E5, E6, E10, E17, and E18.
4. Connect EPROM socket pins 1, 27 and 28 to +5V. I chose to use pin 24 of U3 as the source for +5V. This wire is RED in my examples.
5. Connect EPROM pin 2 to jumper location E10. This wire is pink in my examples.
6. Connect jumper location E5 to E4.
7. Connect jumper location E17 to E18.
8. Connect jumper location E2 to E6




Once your board is converted you’re going to need a ROM image to burn to your EPROM. The original game code has to go in the EPROM in the correct order for this conversion to work. I’ve created 2764 ROM images for all of the Bally and Stern titles. Bally can be found here and Stern here. Keep in mind that most of these have not been tested. There is no reason they shouldn’t work because they were created in the correct way from the original game code but if you have problems and you suspect the ROM image please email me or create your own ROM image from original game code.

To create a 2764 ROM image from 2732 based ROMs you will need to split each ROM and then put the pieces back together in this order U2 first half, U6 first half, U2 second half and finally U6 second half. I cover this process in greater detail in my 27256 conversion article.