While poking around Sopranos, I found a code to get into the service menu without the keys on most(?) Stern WhiteStar games. The memory protect switch in the coin door prevents you from changing anything, but maybe a fun parlor trick if you want to check on the settings or something. 
8L 1R 9L 1R 4L 1R 5L 1R (spells “HIDE”)
The game should play a sound (depends on the game)
then:
1L 1R 2L 1R 3L 1R. Then press R again and hold it for a few seconds.
It will pop into the service menu