Hi all,
I have a Gottlieb System 3 Super Mario Bros which doesn't boot. The Display controller board LED is stuck on and the MPU LED is flashing fast. I can get the game to boot sometimes, if I cycle the power 30-50 times.
When this happens, I can play a game as if nothing happened. Display is working as expected.
This leads me to thinking that GAL U8 is fine? any thoughts?
I suspect a reset problem, but I can't seem to locate the root cause.
all voltages check out fine.

Hope you guys can help out here.
Any help is appreciated.

Hi Marco, I think you are right about the watchdog circuit. I already replaced the C20 and C21. I also exchanged the resistors to 680k.
Sadly this did not solve my problem
I have been doing some measurements on the reset circuit, and everything looks fine.
The DS1 (CPU board U9, pin5) is pulsing 300ns low every 250ms. If I understand this correctly, this signal resets the Display controller board? at least the CPU on the controller board is being reset every 250ms?
Any ideas anyone?

Thanks to Marco (MarAlb), I now have the MPU and display controller up and running. Marco was very kind to share his test roms for the MPU board, which lead me to the root cause of this problem; the battery
I already tested the battery, which measure ~3V, and I did not give it more thought.
Using the Test ROM, the tests stalled at the RAM test. i pulled the battery and the DS1210, jumpered pins 1->8 and 5->6 and the board booted on the bench
I tested all I/O's and the watchdog circuit which all tested ok.
I mounted the board back in the game and it now boots as expected.
All settings are of course lost, so I wen't into the test menu.
To my great disappointment, I'm unable to navigate the menu!!! The flipper buttons are not registering.
This is strange, because before, when I got the game booted by cycling the power numerous times, the menu was working fine!!!

So back to fault finding

Does anyone have any idea what could be wrong to make this happen?

So now I found out why the buttons didn't work; several fuses were blown. I guess it must have happened during testing??
Replaced the fuses and now the games runs perfectly.
Thank you Marco, you have been a great help.