My problem started with "Gnd. short Row 1 is stuck closed" error, which I think I still have. After doing some trouble shooting and a lot of reading, I'm now at the point where I have rows 1,2,3 all showing all closed switches in the T1 test mode. If I remove J6 from the opto board on the bottom of the playfield all of the closed switches on each of the rows then show open (as I believe they should).

The only changes I have made was to remove U3 LM339 IC from the opto board, socket it and install a new LM339N. The chip rings out fine with respect to continuity. The resistors and diodes associated with this chip all read normal. Seems like the issue should be associated with the board, but am I on the wrong track? Should I be looking somewhere else?

I tested the MPU be pulling the row and column connectors and shorting the pins in test mode, the matrix checks out fine from that stand point.

Looks like I may have found my error. Leg 10 of the LM339N socket was not making contact.