Actually handy for me to find this link because I recently acquired a Playboy which I think uses the same board .

The Stern soundboard is totally different to the Bally board Dave  

As Dave suggests though the fault could be one of the 4013 ICs causing the problems with this particular board.
I just had a look at the schematic for this board.. I'd say U9, judging from the voltages quoted for TP2 and TP5 in Sinbad79s post above, would be a good place to start.