I've been fixing a great deal of WPC boards as of late.
In my experience, resets are most often caused by cold solder joints or connectors.
You would be amazed at how much the +5 volt supply will come up by doing some simple things. Removing the solder on the 2 legs of Q1 and resoldering. Resoldering the legs of BR2. Resoldering C4 and C5. Resoldering J101 and F113. Note that I didn't say *reflow*, I said *replace*.
I've removed all the caps on a WPC driver board, and thrown them on my cap tester. On many 20 year old WPC driver boards, the caps are all fine.
There are some where the caps and bridges do fail. But, those are a whole lot less common.
The whole 'replace BR2 and C5' thing has worked for so many people because when they replace BR2 and C5... they remove the solder, and replace it.
I've fixed my fair share of boards, too, where someone took the 'replace all the bridges and caps' method, and managed to run a whole bunch of plated through traces. They ended up spending twice as much as they would've, if they'd just sent the boards out to begin with.
Don't get me wrong, we all learn somewhere. But I try to convince people to solder on some old junk boards before diving into pinball boards. Mess up an old alarm clock or something, learn, and them apply what you've learned to your pinballs :)
