Looks like it's one of the big TIP36C transistors ultimately controlling the ball launcher coil (if I'm reading this correctly). It's the one at Q5 on the PPB board on the schematic here, just follow the line from the bottom of the ball launch coil to the transistor at Q5:
To confirm we're looking at the correct circuit, can you verify you have an violet/orange wire on the non-banded lug on the ball launch coil? The banded side of the coil should have a yellow/violet wire.
Note that whether or not the TIP36C is bad, there are other transistors that work in conjunction with it--a TIP122 and a smaller 2N4401 pre-driver transistor. They're located in this region of the cpu board:
I've read that if there's problem with any of the 3 transistors in one of these coils circuits, all 3 should be replaced at the same time (and the diode on the coil for good measure).
So I'd find the 2 transistors for coil 3 (I'm guessing it's coil 3 from the marking "3L" on the schematic with the coil on it, but i might be wrong ) on the CPU board and check them in addition to Q5 on the PPB board.
I'll admit these schematics are very difficult to follow, so you'll want to study then carefully to make sure we're checking the correct components.