A caveman can do it. Well, that's what I've been in search of anyhow, so just about anyone can do it, and I think I've found it, but it requires a mod to the 3902 board and some soldering skills. It can't be too simple right? If you're comfortable with soldering surface mount devices read on.
First a little background. I've always been in search of an easier way to change games on my GK (and now I-game too...same thing, just more stuff to support the additional row of buttons) The most difficult way was swapping EPROMS/SIMMS out on a single 3902 board to change games. You have to keychip it to set all the options every time, a real pain in the @ss, and you run the risk of bending pins, zapping something with static electricity (ESD for the tech types), etc.
So the simple solution to avoid those problems was to get a 3902 board tray for each of your games, costs some $$$, but cheaper than buying a whole new machine by far, and once the EPROMs/SIMMs are installed there's no worries about bending pins, ESD, etc.
That worked better than the first, but when you swapped out 3902 board tray in the machine you lost your credits, statistics, etc as the EEPROM on the backplane (or motherboard) contains certain information that is specific to the board/game installed. Still a pain in the @ss, but not as much pain as the chip swapping before having a second 3902 tray.
The solution to that seemed apparent, since the backplane EEPROM is socketed, why not get one for each game and swap it out when you change 3902 board tray? That idea worked out pretty well, now you could swap the 3902 tray and EEPROM, keep all your statistics and settings, and it wasn't too difficult. It still required "above average" skills (read, the kids, spouse, etc probably aren't going to do it without training) to make sure that the EEPROM was installed correctly.
Even if you know what you're doing it's still not easy if you don't pay attention because IGT changed the orientation of the backplane EEPROM at some point. On three of my machines pin 1 belongs on lower left (looking straight at the backplane from the front of the machine), but on another one it's on the upper right. I accidentally plugged one in backwards once, and luckily it didn't damage anything, but still it's not something you want to do. It was still better than the first two methods though.
Ok, so that got me looking at the 3902 schematics, and I discovered a way to move the EEPROM from the backplane back to the 3902 board itself. Somewhere along the line IGT had thought about putting a specialized serial EEPROM on the board at U85 (listed as optional). According to the schematics I have it was for a M34R32 chip that has some one time programmable (OTP) information as well as EEPROM storage capability. It's obsolete now, but if you google it you can find a gaming patent that that describes some of the ideas that someone apparently had for it. Yawn.
The good thing about it though is that it is set up the same as the common 24C16 serial eeprom, except the write control line is tied high (through R179). The 24C16 requires a low on pin 7 to enable memory writes. By removing R179 that pin is no longer tied high and internally tied low (write enabled). So now if you install a 27C16 SOIC EEPROM at U85 the MPU will read and write it to vs the backplane EEPROM. Actually, it will read and write to both if you leave the backplane EEPROM installed, but that defeats the purpose of the mod!
Ok, so what does all this mean? If you install a 24C16 EEPROM at U85, remove R179, and remove the EEPROM on the backplane all the information the MPU requires stays on board. You can swap games by simply swapping 3902 trays out.
That's simple enough that a caveman can do it!
I've done this mod to 12 of my 3902 boards and haven't found a downside to it yet. If you leave the game EEPROM on the backplane after installing the 3902 board tray and play enough games, up to 100 I guess, (I never counted), the MPU will update both the EEPROM at U85 and the backplane. Then you can remove the backplane EEPROM and play on. If you remove the backplane EEPROM prior to installing the modified MPU you'll get a EEPROM CRC Failure error, turn the jackpot reset key and it will boot up. You'll get a "verify options" warning screen, but your credits/statics will be intact.
3802 boards don't have the empty EEPROM slot, and the display resolution isn't the same as a 3902 so they won't work.
Here's some before and after mod photos. For the photo detectives, yes, they're different boards. I'm not a photographer and took several photos as I went through the different boards I have. These two were the best focused.
Standard warning, your mileage may vary. I'll be happy to answer any questions though. Please don't attempt this mod if you don't have the skills and equipment, I don't want to see anyone damage their machines! The mouser electronics part number for the serial EEPROM is 511-M24C16-WMN6P.