Your MPU is not booting, so something is wrong there, but first, I'd check & fix the test points on the SDB.

http://www.pinwiki.com/wiki/index.php?title=Bally/Stern#Solenoid_Driver.2FRegulator_Test_Point_Values

Find the readings (TP1 thru TP5) that are not in spec and work from there to get the SDB working 100%, then move to the MPU. If the big cap is original, it needs to be replaced--that will probably fix the speaker hum.

I almost always rebuild/replace the high voltage section and 5V sections on my Stern and Bally SDBs as part of the bulletproofing process. Big Daddy Enterprises is a good vendor and has a variety of helpful kits with the needed parts already gathered and itemized, and some linked pictures, for the 5V and the high voltage sections. Great Plains Electronics is my favorite source (usually faster shipping to me and the best parts labeling in the business) for electrical components and should have the parts you need as well. As mentioned above, test the power setup one leg at a time. If the power to the rectifier board checks out, then move on to the SDB, and test it without the further-down-the-line items attached. After that, you can rule out power issues as you deal with potential MPU problems (bad chip, battery acid leakage, and so on). Also, expect to replace and repin connectors on this era of pin, both header and socket pins, in the .100 and .156 flavors.