The late great Leon Borre posted a mod that can be found on PinWiki to use a single 2764 EPROM. It gets lost in translation a bit so i thought I would try and simplify the instructions.

This mod lets you use a single 2764 ROM in sys 3-4 MPUs. This includes FirePower on a single ROM.

To assemble the 8KB binary file for the 2764 EPROM you need the game ROM to be in the first 4KB half of the binary and the two flipper ROMs in the last half(flipper 1 then flipper 2) of the binary. For 2KB games(everything but firepower) double up the GAMEROM. For FirePower, use the FPCOMBO 4KB image found on planetary pinball.

So command prompt for combining the ROM binaries is going to look like this.

FirePower
copy /b fpcombo.732 + green1.716 + green2.716 fpcombo.764

All other games (timewarp example)
copy /b timewarp.716 + timewarp.716 + green1.716 + green2.716 timewarpcombo.764

To prep the board start with desoldering IC15.

Remove all ROMs.

Stack a 24 pin leaf socket on top of IC17(you may skip this step depending on socket styles). Stack another 28 pin leaf socket on top of IC17 again so pins 1,2, 27 and 28 are the extra ones hanging out of the 24 pin socket below. Also bend out pin 22 and pin 23 of the 28 pin socket so can have a lead soldered to them.

On the new IC17 28 pin socket, solder pins 1, 27, and 28 to +5v. I used the nearby bypass cap.
IC17(28pin) Pin 2 goes to IC15 Pin 13
IC17(28pin) Pin 22 goes to IC15 Pin 12
IC17(28pin) Pin 23 goes to IC15 Pin 14

I used a prototyping board and some socket headers to plug into a socket at IC15. That way the mod would be easily reversible. The leads could also be hard soldered in at IC15

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All credit goes to Leon. Here is the link to his info.

http://www.pinwiki.com/wiki/index.php?title=Leon_Borre_Converting_Williams_System_3_%26_4_to_System_6

Han's adapter serves a slightly different purpose. It attaches at the CPU socket and only works on sys 6 MPU boards.

This mod is for 3-4 MPU boards. It replaces IC15 and attaches to IC17. I could make adapter PCB(s), but you don't really need them. I figure this mod isnt needed too often. Most cases it may make more sens to use 2716 at IC14, 17, & 20. This is nice for FirePower tho.

Both save you replacing a few sockets.

74LS chips driving to many standard 74 gates? Some boards have nearly all standard 74 logic, some have 74LS at places that are standard 74 on others (in the upper address line range for CEs).

I do the FP combo mod with two 1n4148 diodes at the 74139 jumpers and have never had an issue.

I will draw the schematic out and see if anything stands out.

74LS chips behave OK with floating inputs as they seem to pull themselves high, not sure about 7400

I had some time today to reverse engineer Leon's guide to do this at the CPU socket, i believe this would be the circuit for system 3-4(maybe 6 too). I see Han's used a quad input nand, may be a good reason to do so and im overlooking something.

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I went off of the system 4 schematic and I see no compatibility issues. I will refrence sys 6 schematic soon. There may be issues with Jumpers on system 6. Some of the jumper pads may re-route A12-14 and cause a conflict somewhere.

Blank PCBs are cheap. I may whip these up for my use as it will save from replacing 3 sockets and using 2 more eproms. Assembly time of the adapter should be less than the time of installing new sockets.

system 6 it looks like it combines the VMA with the cpu's external clock (BUS CLK 2)

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I can use the two extra NAND gates in the 74*10 in my schematic to form an AND gate, but that could be where timing issues start to come up. The clock signal timing appears to be fussy on these boards from messing with NVRAMs. I could not get 120ns NVRAM ICs to work reliably in system 3-4 boards after combining chip selects, but 70ns and 100ns varieties worked.

Combined VMA and E signal using gate B and C of the 74LS10 like an "AND" gate.

Added not connected pins 35 and 38 to the MC6800 component library so it will remind me to route tracks for them between the header and adapter cpu socket (i think the 6802 might use one of those in internal RAM mode) .

I wonder how a system 3 and 4 MPUs will behave with the VMA and E combined together like done in system 6 for the eprom p22 OE signal.

EPROM_at_CPU_socket_3-6_(resized).png

I figured as much.

I recently modded some atari 7800 carts to take EPROMs. On those atari cart PCBs i am using 1Mbit 27c301 EPROMs and I have the /OE pin grounded and it works fine. The EPROM @ CPU socket adapters may work with that pin just held low and not fussed with.... i need to do some reading and real life testing i suppose.

Most of the home brew a7800 games are better than the official releases, but im not the kind of guy to buy the homebrews for $60 a pop from that atari age store =D.

/OE has to be used since /CE-P18 of ic14, 17 & 20 are held low (p20 of 2764)