Howdy Dave.
Sounds like Galaxy has more than one issue. No big surprise - it's a 30 year old game.
For a quick test, you could try unplugging all the connectors from the sound board.
Has the battery been removed from the MPU board? Is there evidence of battery leakage along the bottom of the MPU board?
Did you buy the game recently or has it been in the playroom for 5 10 20 years?
Here's our favorite shop manual:
http://www.pinrepair.com/bally/index.htm If the game hasn't been worked on much or recently, the items under headings "before turning the game on" are worth doing.
This was a $75 D.O.A. pin a few years back......

Rebuilt power supply. Replaced missing driver board. No battery rot but bad RAM kept game from booting. New caps on sound board. Display bugs mostly fixed. Made new clear p/f plastics to replace warped yellowed broken set. Replaced a bunch of worn broken cheapo Stern slingshot parts with nice sturdy Williams bits.
Think I'll take a break from Skill Pool and go play it.
Cheers, Mark
"Knowledge is Good." - Emil Faber