Quoted from frunch:
Chris Hibler here at Pinside has a good reputation here as a board repair tech.
The Coin-Op Cauldron does good work, as well.
I definitely recommend having your original boards repaired instead of buying new 3rd party replacements. Some are good, but original boards are usually quite reliable once they've been gone through by a good tech.
Even tho they have repair prices posted on their website, I was sad to learn this weekend that The Coin-Op Cauldron no longer repairs early Bally MPUs even if they're free of corrosion and previous non-professional repairs. Here's a portion of the email I got after I inquired about sending one in:
"We no longer repair -17 or -35 CPU boards due to their age and always recommend Altek boards as a replacement."
They still do Williams boards, so you may still be in luck!