This, https://pinside.com/pinball/forum/topic/gtb-as-match-stepper-rebuild-step-by-step, is a great in-depth look at the unit. Yours probably doesn't need the full rebuild, just cleaning and adjusting. It is located in the back box, and can be removed to your work bench by unplugging the two jones plugs below.

image (resized).jpeg

Yes, the Alt relay A would be the first place to look, if it checks out ok then the AX since it activates A.