Quoted from slochar:
Sounds like it's time to go through the entire game one board at a time and get each up to snuff. (see pinwiki.com in the bally/stern section).
Basically, rebuild the rectifier board first, get that solid, then do the solenoid driver board, then the mpu, finally the lamp board.
The only things I can think of in the head that could produce such a bang like that are the 2 large capacitors on the solenoid driver board C23 and C26.... they're over 40 years old if they're original, eventually all the electrolyte evaporates.
Simple things first as well, did you smell anything when it went bang? Make sure nothing metal/conductive fell into the transformer.... transformers are really reliable, but don't take kindly to having outputs/inputs shorted.
Personally I replace the rectifier board now in every single bally/stern I'm working on as they are cheap (I buy the kit from Weebly's and build it up myself) and rebuilding original vs. just replacing with new is so much easier (you don't have to deal with hammered 40+ year old traces/connections). The 323 regular is pretty reliable, and I'll evaluate the caps on the solenoid driver board with a scope to see the AC ripple, anything over 1/3 of a volt or so I replace. These parts are relatively cheap as well.... you start to get into expensive costs when you have to replace the back boards (SDB, MPU) with new. Used originals can be had in various states of repair as well, and it is helpful to have a spare set to swap around when you are fixing/testing things.
Whatever you do, keep in mind that as you are testing things, they might go boom again, this time with your head/hands around.... not good.
Yes, Yes , Yes, Just sliding the connectors on and off one of the NVRAM rectifier boards with ease makes replacing those feel like a must. That and getting rid of all that grubby, crusty, hacked on, (border line) under powered crap in one fell swope, goodbye to those future headackes for only 35 bucks. NVRAM are Built to a healthier standard and arranged better IMO than the originals. Also, in home use it won't be used anywhere near as much as the original was.