Was 12V shorted before you worked on the boards? What did you do on the MPU? Could the other J headers on the MPU have been affected by your repairs--like a shorted pin or bad component?

Did you make any changes to the connectors (could a pin be swapped in the wrong hole)?

Someone mentioned this in another thread, but its better to make one repair at a time and test, so when there's a problem, you know what repair caused the problem (for future reference).

You could try powering up with the sound card disconnected (and everything else connected) -- but it may blow your last fuse.

You could consider these, if you do even a few repairs, these are great:

https://pinside.com/pinball/forum/topic/making-and-using-a-fuse-breaker#post-1309179

or you're probably going to need more fuses.

^^^^great idea.

For testing, you might be able to power it with your bench supply before you put it in the machine--save that last fuse