I have a monopoly pinball where the trough up-kicker got locked on and fried the coil. I replaced the coil and diode and replaced the MOSFET with a IRL540N. powered on machine to find the coil stuck on still. figured i had a bum FET or a dead U1 so I just shotgunned it and replaced the chip and the FET. checked the diode was in the right direction... still locked on.

i have a pack of STP40NF10L's and seen some folks mentioning that it could be used (it was more expensive or whatever) in any case, I put one of those in there. powered on, still stuck on. replaced C270 just to be sure. still locks on.

pulled the wire out of the IDC, the coil stays off... so the playfield wiring is good. (not grounded out)

used a logic probe to check the output of U1 when the solenoid test is performed. i can see the low to high pulse... so the logic is working.

can the STP40NF10L maybe NOT replace the STP20N10L? I can't for the life of me figure out why this continues to stay locked on.

Oddly enough, I found the very very edge of the through-hole for the gate signal had pulled up off the board, breaking the connection from U1. It looked like it was connected but had a tiny break where the sliver of a trace connected.

A jumper wire solved this issue.

So yes, in fact the STP40NF10L does replace perfectly well... It just needs the gate hooked up!