I would remove the big cap in the center top of the board.
If you are lucky, you will find that it has leaked and severed the trace beneath it.
Those boards mostly either work or don't work.
Other possibilities are:
...failed ROM
...failed 555 timer
...failed 7404 (IIRC)
...failed LM380
...rocker switch failed

The 6503 is tough to find.
Most often the 6530 has failed. It contained Gottlieb specific code and is NLA. In which case, the board is of no use.

Pascal Janin makes a nice replacement board, I think called the PI-FX.
--
Chris Hibler - CARGPB #31
http://www.ChrisHiblerPinball.com/Contact
http://www.PinWiki.com - The Place to go for Pinball Repair Info

The best way to answer that question is to use a logic probe to probe around and see if the 6503 is running, accessing the 6530, the 555 is running, etc.
If you don't have a logic probe, there is a good "how to use a logic probe guide" here on PinSide. The Elenco LP-560 is about $18 or so on eBay/Amazon. And, you have the schematics. You can't go very far wrong.

Or you can send it to me and I can give you a diagnosis. Happy to help.
--
Chris Hibler - CARGPB #31
http://www.ChrisHiblerPinball.com/Contact
http://www.PinWiki.com - The Place to go for Pinball Repair Info

Sound signals do pass through the driver board where they are inverted.
—
Chris Hibler - CARGPB #31
Http://chrishiblerpinball.com/contact
http://www.PinWiki.com/ - The new place for pinball repair info

This particular board was a bit of a mess.
Both 7404s on the board were crocked.
But worse, the 6530 on the board had failed.
I replaced all of those. Board returned to normal operation.
--
Chris Hibler - CARGPB #31
http://www.ChrisHiblerPinball.com/Contact ... for board repairs
http://www.PinWiki.com - The Place to go for Pinball Repair Info