Back in the day when we had these games on the route, whenever they'd come into the shop for shopping out, I wired in connectors to be able to unplug the whole drop target assembly. Sure made easy to work on that way: just unplug & unbolt it from the playfield and take it to the bench to work on it. Plus if the assembly was real dirty, I could spray it with cleaner, hose it down, rinse it, then blow dry it with a hair dryer. Came out looking like new again.

The circuit board is actually 5 separate circuit boards (one for each target), all hardwired together. Each board has two screws. If you take off all the screws, you can remove the boards, but you'll still have to deal with the coils. While you've got it apart it's very important to clean or replace those boards, biggest source of issues with that style of drop target