Yep, the playfield fuse, which is to protect the SD transistors in case of a bad coil, was blowing and the fix was to but a larger fuse in. Why didn't they just short out the fuse holder??
What you need to do is to review all the playfield coils for nice smooth operation. I'm guessing you are going to find one that doesn't operate smoothly and that was causing the fuse to blow.

Of course, it's possible the fuse blew and the 20amper was all the tech had at the time. :}

Good luck, let us know what you find.