Quoted from monkfe:
I'm wondering if this meltdown occured initially, and the smoke didn't become apparent until your daughter played the game later.. sometimes that smoke can take some time to linger out of the box...But you said you disconnected the coil after you saw that it was locked on...meaning to me that the transistor was shorted and locked on...when I've had this happed, the transistor smokes too...what was the condition of the coil when you disconnected the wire?
The coil was definitely working when I disconnected the +50 wire. The game was even playable before this. I turned the game back on to test other things, went across the room to get my multimeter, daughter was playing F14 next to it and smelled smoke.
After I turned the game off, the coil was blazing hot, wrapper burned, and the sleeve melted (the smoke was the plastic burning).
Quoted from Pin_Guy:
Don't read too much into this, I didn't say your MOSFET isn.t blown, it probably is; just that a ground short wouldn't blow it. The subway mounting bracket shouldn't be grounded, but you can easily tell if is by performing a continuity test between the cabinet ground braid and the subway bracket.
I was just going to replace the mosfet and diode since it looks like it got hot (see below). Maybe there is more going on here than just the coil being stuck on for extended period of time? Anything to test? Only thing I can think of is its still a tieback issue.
Good idea on testing the bracket for ground... it is NOT shorted to cabinet ground. So what that tells me is that for this coil to have melted without its power wire attached, both coil lugs had to be contacting their brackets and voltage went from the second coil through the subway bracket and over to the first coil bracket, and back through the black/white return (since it was the only path to ground). Just like bobukcat said.
Here's a picture I took before I unsoldered the power to the top coil. Coils were upside down here. Notice how the bottom coil is turned, the lug HAD to be touching the bracket at this angle. Also notice the wrapper isn't burned here.
I just put the burned coil back in upsidedown and took this picture to show how close the lugs are to the bracket...