Flippers are enabled by the Q relais. Try to check this by pressing it manualy.

If the Q relais is'nt activating check your scematics to see witch transistor drives the relay. But dont forget that the CPU - Driver cable is real troublesome. I (and this is not the nice looking way to do) always solder the wires to the boards.

One of the important thing if you buy a system 1 flipper is to see if the displays are working... (if thay work the game probably boots up).

Gottlieb is just my cup of thea

Well i repaired a lot of Gottliebs and once you got them bulletproof (goundmod) and did all mandatory fixes thay are more relible than Bally or Williams pinballs.