Because it locked on, and got hot. They're not supposed to be activated continuously at full power.. now you get to replace that coil again.

You said you replaced the PRE-driver transistor? How about the primary? (oops, I saw you said you did that in the original post) Did you replace them at the same time? Otherwise you may have just screwed the one you replaced as well. I've *never* had to replace a pre-driver, but I've replaced a bunch of the main ones right before the coil. But when you're blowing full collapse voltage back into the system, no telling what or how much you're going to blow up..

*Edit* For that matter, have you taken apart the diverters and cleaned them? All of the diverters on STTNG are NOTORIOUS for getting gummed up, so even if you "fix" the problem, they may not move correctly!

At this point, I would....but you melted a sleeve into the coil already, so it's a moot point. I wouldn't replace any diodes on the PCB unless you're super comfortable with soldering and/or you've already tested them and they're broken in some way (Exploded, caught on fire, allowing current in both directions or NOT allowing current in the proper direction. You'd have to test that when the driver board is out of the game anyways.). You have to replace that coil and assuming you're purchasing the correct part it should have diodes on it, but I'd be real sure to check the orientation is correct and use your meter to make sure it only allows current in the correct direction BEFORE you solder it in. I've bought coils before that were missing diodes or they were mounted backwards.

Okay, gotcha. It's been a while since I had a STTNG and I never had to do anything with the aux boards, but I'm pretty familiar with WPC in general.

That's because the diverter isn't working for whatever reason. Checking the optos on the underside of the subway is a good idea. Suspect one is the one at the "top" of the subway for the scoop behind the drop target. This the same diverter that had the melted sleeve? Heh.

Awesome, nice work. The good thing about STTNG is..at least in my experience owning two of them, generally once you get past the drop target and diverter issues, you've bascially fixed the worst of it! Everything else is really accessible and makes sense, but the logic for the diverters and optos is a bit wonky at times. Any one of those not working 100% (and the cannon optos too for that matter) can cause really odd things to happen.