Is the knocker coil the right value? The current backwash of these can be quite large and its normally diode protected to prevent the backwash travelling up the signal path and through the associated transistor... 7XXX buffer chip and then the interface chip or 4XXX signal chip.
Sounds like you had a large current backwash. It MIGHT have stopped at the transistor but if it went further the buffer could be short etc which would possibly make the coil fire every time you turn the game on even if you replace the TIPXXX