Quoted from Aflacjack:
Lol yeah I completely missed the single battery. Displays don’t come on, just a couple orange dots. And no solenoids fire nor will a game start. There’s issues somewhere, just gotta find them. Might be easier once I get it unloaded and can check fuses, etc
Start at the rectifier board. Slide the clear shield up and remove it. Get a high quality VOM if you don't have one already. Get familiar with pinwiki.com Bally Stern section. It's a great resource for volatge specs for each board. Also mod fixes and other info.
A locked on or steady illumination of the mpu red led indicates a few different problems. First could be no 5 or 12 bolts reaching the mpu to activate the reset section for booting up. Can also be a bad transistor on mpu reset section. IIRC Q1 thru Q5? Corrosion in the reset section also. Bad U9 IC or deeper problem. Can also possibility be that the sound board ribbon cables are dirty or misaligned or installed incorrectly.
Start measuring voltage test points at rectifier board. Be careful not to touch with bare hands as there is 200+ volts for the score displays there and on solenoid driver board.
Record all voltage test points for the rectifier, solenoid driver and mpu.
Report results here.
Compare results to pinwiki and look for discrepancies.
This will atleast get you started and possibility be able to get game to boot up.