Remove all the balls, go to switch test and see if there are any closed switches. Then slowly put in the balls and verify machine sees each switch is closed by the ball

Did you verify it’s on free play? Drop a few quarters or simulate coin drops and then try.

Does the switch test show the start button being pushed?

Ok. I’m stumped. Someone else needs to chime in.

Verify the switches register with balls, not you manually doing it with your hand. Verify all three are engaged in switch test with All 3 balls.

The coils work normally?
Checa que todas las bobinas funcionen, igual con los transistores

when i first started in the business we had about 1/2 dozen Firepower pins....one of the common problems were the original leaf switches that were used in the ball trough. they sucked then so much that Williams came out with a mod to convert the leaf switches to microswitches with wires. if you have the old leaf switches, a big giveaway is to push on those two leaf switches while pushing the start button...(make sure all 3 balls are in the trough.....). I ran into that very problem a couple weeks ago on a home repair......i wonder if there are still mods available for those that did not get updated back in the eighties....

Have you already tried to jumper the card? You replace the 0 ohm resistors for a cable or wire by bridging, or even reheating each pin of the two cards, the one for the batteries and those of the transistors