What are the actual AC voltages did you get?
How I normally test...black meter lead wedged in ground braid. Left or right side should be about 6.8-7.2VAC and the return ground on each should be about 1.2-1.8VAC.

N9.it definitely should be 1.2 - 1.8 volts AC if the lamps were working. Actually before it gets to the first bulb, it goes through a small relay board which you would find mounted on the underside of the playfield. Normally this relay has a big yellow covering. Look at the male and female pins going to that board and see if you see any that are burnt. You would need to deal with those. If you don't see any pins burnt, then you can swap to relay board with the one in the back box mounted on the backside of the light board to see if the problem moves to the back box

As nerdygrrl mentioned, resolder the pins first on the connector and on the relay. If that does not fix the issue, then swap the whole relay board with the one in the backbox mounted on the back of the light board door.

Giddy up!! Enjoy your game.

I suggest changing to lithium batteries since they will last 5 years and will not leak as do alkaline batteries do. Easiest upgrade you can do to protect your MPU from future damage.