JDW is quite correct -- poor focus is not caused by a logic board problem (except, perhaps, indirectly by loading the power supply; that possibility is quickly checked by simply measuring voltages).

Check the board that holds the CRT socket. Perhaps some connections there came loose during the repair work. Reseat the socket. Inspect the pins and socket for corrosion, etc. and clean if necessary.

I don't recall if there is a focus control on the SE/30, but if there is, give it a tweak. The control may simply have some dirty contacts.

Check the connections to the flyback transformer. Freshen the solder joints if any look marginal (I generally freshen them as a matter of course). Don't just reheat them; use fresh solder.