Quoted from T-Tommy:Just wanted to pass this along. This has been talked about before but this fix wasn't mentioned. Working on a Bally -35 mpu that had some lamp flickering and random rebooting during attract mode and game play. The clock signal to the 6800 looked good with a logic probe. The pulse points along the VMA were pulsing as they should. I decided to check the voltages along the VMA and compare them to the schematic. I got to U15 output pin 6 which was 2.4v ; this made it's inputs at pins 1,2 also 2.4v ; (the schematic says these legs should be 3.0v). On a hunch, I pulled out U15. My little chip tester said it was bad. A new 74S37N at U15 fixed the rebooting and flickering.
I think that exact fail mode has popped up a few times over the years. If a non socketed chip goes bad on the Bally MPU that locks it up, its almost always U15. I think that might be the gate with the super stiff 150 ohm pullup resistor. That chip is stressed out and I think Bally Voltage regulator running at sometimes up to 5.5vdc does not help. When 12v is accidently dumped down the 5v rail, if you are lucky U15 will short out and pop the fuse before the rest of the board is wrecked. If I dig through my piles of junked out bally MPUs there are a couple where U15 has a hole burned into it or literally got blown apart.
