Blowing the under playfield fuse usually indicates a short somewhere. Q3 is the knocker, Q5 top thumper, Q10 lower left thumper, 11 one of the drop target coils. First check all of these areas for shorts (particularly the drop target coil), or incorrectly wired coils (the common wire should go to the same lug as the banded side of the coil diode). I have never had a board that blew transistors on its own, but it would be simple to check, just dont, put any of the large plugs on the SDB, and disconnect the knocker.
Hope this helps...
