back in the 80's, when we designed electronics at work, we used a 2kx8 battery backed up SRAM during development. That way we could dump the memory and see what are the variables were when the system crashed. It was pretty cool debugging trick back in the day of 8" floppies and Intel 80xx series processors!
System 4 MPU just has 2 RAM ICs.
Made some unintentional progress on my FLASH. I powered it up for the first time in a month or so to run the "edit_cmos" test ROM and when I powered the game, my displays were all blank... crap! digging around, I found the "displays" test rom and tried that and still no luck, but noticed that i bent some pins on the EEPROM. The way the board is mounted, it's impossible to see the left side of the IC due to the door and hinge.. great!.
Checking the display board, my 5V was only 0.5V. It turned out my ground wire from the power board to the display board was intermittent. Looks to be an issue at the power board molex connector. I fixed that and put the game ROM back in and powered up and all came up good (whew!) and when I started, I got my background sound!!!! I have no idea what caused that to start working... I'll have to start wiggling harnesses and such while it's working and see if I can locate the culprit.
I started to rebuild the drop targets as they were not being detected. Once that gets done I'll see if I can figure out the unlimited ball issues.
At least it's warmer now so i can work in my garage/gameroom a bit more comfortable!
Oh, make it out to the Texas Pinball Festival yesterday - over 400 machines - what a blast! Fun to play the old and new games for the afternoon