So reading back to old posts, you may know that Mark has a Williams Flash pinball machine. There is a lot of info on the machine in previous posts, but it is an old version of the machine and was running really old code. (Yellow flipper PROMs). One issue is that it sometimes takes up to 1/2 second to register points. We believe this is because it is using the old version of the code. The other issue is that nobody has images of the old PROMs (yellow). Because of this, you can’t just replace a bad Flipper ROM or game ROM. Pinside has a great post on the differences.
Since we wanted to update the PROMs to the newer version, why not combine all the PROMs into a single chip? The cheapest PROM chips that I can get are the 39SF010 which are $2/qty 1. This EEPROM chip has more than enough space for the Flipper and GameROMs. It even has enough space to put an image of the testROM code. The modification to the board needed to be easily switched back to the original configuration if necessary. Looking at the schematics, I found a way to do the modification so that it would not modify the MPU board at all except for adding a socket! No cuts and no adds to the board, so the MPU board should remain pristine. Read all directions and make sure you understand them before attempting to do this. If you have any questions, send me an email. If you don’t know how to solder, or don’t know what solder wick is, don’t attempt this! Here is how to do it:
1. Remove IC15 and replace it with a socket. (This is a 16 pin PDIP chip which is a 74LS139.) It is used as the address decoder for the PROM chips.
2. Put IC15 into the socket. Test your machine. Your machine should now work exactly as it worked before doing the modification. If not, you may have destroyed the chip when you desoldered it from the board.
3. Program the 39SF010 chip with the PROM images. In my case the GAMEROM is at 0x1e000, GREEN1 is at 0x1f000, and GREEN2 is at 0x1f800, and the testrom is at 0x1b000.
4. Remove all ROMs/FPROMs/etc. This should be IC21, IC22, IC14, IC26, IC17 and IC20. Your pinball machine will have a subset of these installed. Make sure that you mark their locations so you can put them back if necessary.
5. The 39SF010 chip will be installed in position IC20. The old IC20 has 24 pins, while the 39SF010 chip has 32 pins. I built and interface board, but if you are careful, you can do this without the interface board. Attach pin1 to pin 2 to pin3 to pin 28 to pin 30 to pin 31 to pin 32 on the 39SF010 chip. Put a piece of wire across the top of the pins and solder where they come out of the chip so that you can still plug the chip into the socket. This connects power (VCC or 5V) to the unused address lines and the write enable signal to disable writes to the flash chip. I put a jumper so I can swap between the testROM code and the game code. This involves using a 3 pin header with pin 1 attached to 39SF010 pin 28, pin 2 attached to 39SF010 pin 29, and pin 3 attached to 39SF010 pin 16. This allows you using a jumper to switch address line 14 (A14) to either VCC or VSS (power or ground). If you don’t want this ability, simply attach 39SF010 pin 29 to VCC. (VCC is the wire that you added above to all the unused address lines). Bend pin 25 of the 39SF010 so it doesn’t go into the system 6 PROM socket. (You can either bend this straight out, or straight up. Be careful to not break the pin off the part). Put the 39SF010 into the IC20 socket. Pin 5 of the 39SF010 should be in Pin 1 of IC20. This means eight pins (4 pins on each side will overhang the socket).
6. Remove IC15 from the socket. Make a jumper wire and connect IC15 socket pin 1 (push the wire into the socket) to IC15 socket pin 6. Make another jumper wire and attach IC15 socket pin 2 and solder to the 39SF010 pin 25. (This is the pin that was bent so it didn’t go into the original socket). Make the last jumper wire and attach IC15 socket pin 3 to 39SF010 pin 4.
That’s it. It may look like a difficult mod, but once you read and understand the instructions, it should take you less than an hour to do it. The most difficult part is removing IC15 and installing a socket. The mod can also be used on the original Firepower to combine all of the different PROM chips using the FPCOMBO IC14 chip. Information on burning the EEPROM chips and making your own burner can be found in previous posts.
To switch between your old PROMs and the new ones, pull out the jumper wires from the IC15 socket, reinstall IC15, uninstall the 39SF010, and plug in all the old PROMs. Easy as pie.