Picked up a dead early 80s Bally that I started to work on. Just got thru the rectifier board. I then moved onto the solenoid driver board. Removed power to MPU and started taking voltage measurements. I believe I had my meter on tp5 and my hand slipped and I believe I discharged 12v to the 5v regulator heat sink. Fuse F4 blew hard. Nothing left of it. Replaced the fuse and it blew almost immediately. Any ideas on why or how to correct?