Onto troubleshooting why my MPU isnt booting. Ive repinned A1J5, A1J6, all power connectors. 5v is solid, pic of the small amount of battery corrosion I cleaned attached. Clock signal is high low pulsing, IRQ is stuck low, r/w pins all either stuck low or high. When solenoid driverboard is attached, random lights and solenoids fire (I believe this is called the thunk?). Interconnect harness was repinned on both sides.

With an IRQ stuck low, what should I do? I have a spare parts Gottlieb Mpu which I believe to be a System 80. It has a socketed Motorola 6802, is this a drop-in replacement for the 6502? I want to make sure before I do anything because none of the chips on my complete MPU are socketed

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Thanks, I wasnt sure because I know Motorola had their own part numbers. Not sure what MPU my parts one is then, but it looks like I would have to order a 6502 if I needed to replace it

Did some digging, found this:

FROM GUIDE (ON "RDY AND IRQ"): "If the IRQ is low, then chances are
one of the 6532 RIOT chips is damaged. The most likely RIOT culprit is
U4 (switch matrix), or one of the TTL chips that feeds it (Z11, Z12,
Z13, Z14, Z15). Also note IRQ is connected to +5 volts through
resistor R1 (3k ohms); verify that resistor is not damaged. If the IRQ
is stuck high and never starts to pulse, this can often mean the U2/U3
game rule ROMs are damaged."

I will check if one of the chips is getting hot, probe around and see what I find

No, they look fine. I just wasn't sure what affected the IRQ line on it. What explicitly should I check for on U4 U5 U6 and the Z chips which drive them? All the inputs/outputs are stuck either low or high on U4 and Z11, but I'm not sure what this is indicative of. All the RIOT chips were a little warm but I didn't feel one much hotter than the others, and all of the Z chips were cool to the touch.

Just tested Z11-Z15 with a multimeter and they all tested fine. I did pop out the old 5101 and socket in the NVRAM before these issues popped up. Prom is the factory one but isn't getting hot to the touch. I was gonna just program a new one buuuuuut the ROMs aren't available online

I noticed a couple strange things on the MPU:
Pic 1 is of Z36
Pic 2 is connected from a trace to Z10
Pic 3 is of Prom 2 connected to Prom 1

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Okay, I figured they were factory because they looked pretty cleanly done. Would anything else be causing the hanging IRQ line beside U4 U5 U6, and Z11-Z15? Can I check the RIOT chips another way while theyre still soldered in?

Okay so big update! I had read on Pinwiki about the rolling 0s caused by the slam switch being open, and my game was doing that, but I thought since the rolling 0s eventually went away I had other issues. I did the slam switch fix and my game seems to boot (although I'm not sure if it's booting correctly). The IRQ line is no longer stuck, and the displays have all 0s on them.

I currently have the driver board unplugged because of a locked coil I can't figure out (I've checked the coil diode, coil, transistor under the PF, transistor on the driver board, and resistor associated with it and they're all good), and the interconnect harness unplugged because my MPU is doing the thunk. I have yet to do the thunk cure. When I hit the start button, it seems to start a P1 game, but no sound plays despite the sound board being plugged in and working in test. Scoring doesn't change the displays. After a bit of the game being started, all displays started bugging out so I shut the game off. It goes into test mode but it's hard to tell if it's working properly because I don't have everything plugged in, and System 80 is new to me. Seems like I still have MPU issues... I'm thinking bad ROM? It is the factory one, so I will be replacing it.

Got it, thanks. Is the "Thunk" symptomatic of other MPU issues, or is it a byproduct of how they designed the board? Would you happen to know which of the chips (Z1-Z5) I need to do on Mars for the "thunk cure"?

I did Z1 through Z5 as the flippers.com page suggested. I misread the instructions and cut completely away the clear pins instead of keeping them bent out of circuit to tie together. I was able to rectify this using cap leads I had, then I made the "reset circuit" on the back. I used a 100uf 50v cap because its all I had on hand. Daisy-chained all the pin 1s together, isolated them by cutting any traces parts and solder side, connected it to the other components, still getting the thunk. Pics attached. I likely did something wrong

Edit: The lamps which are getting stuck on are different, but the coils are the same, if that means anything.
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For the sake of testing I did disconnect the 3 coils which are getting locked on (ramp and ball release caused by thunk, right launch lane which is caused by something else). Game turns on and boots, Game Over light in backbox is flashing.
When the game starts, the trough coil does kick the ball into the lane but no scoring works, just says -----0 on P1 display, and draining does not advance the ball. Sound still does not work. Drop targets dont reset.

Something odd is that all 3 coils having issues are the ones which are driven by the bottlecap transistors under the playfield. Is this just a coincidence?

Pulsing Pin1 of Z1-Z5 shows HI
Pulsing Pin1 of Z6, Z8, Z9, Z12 shows LOW
Looks like the solenoids thunking are both fed from Z4, which should be cured. I'm definitely thinking I've done something wrong in my thunk cure. Maybe daisy-chaining them is the problem, and I should connect each one individually? I doubt that but I'm not seeing anything else I've done wrong. I've double-checked my work 3 times now