I had a very simular problem. Mini playfield would not lock 2 balls during a game and test modes were showing errors with opto 71-75 but they all functioned properly. I replaced optos, resoldered headers, confirmed blacked out target box, etc. There were many other strange problems as well. Really frustrating.. I sent the board to Clive at CoinOp Caldron and bought a GLM 10 opto board while I waited for the repair. After installing the GLM board everything worked perfect..all issues resolved. And the returned repaired board also worked perfect from CoinOp! Clive identified many problems with the old components. : )