Likely you blew the LN2803, which may have blown the 12v fuse as well..

I'm on my phone right now, so I can't pull the schematics and point to chip references.

However, the really short and silly is that *likely* by sending +50v or +20v back through the switch matrix, the chip on the MPU was fried. This means sending the board off to get fixed if you can't do it yourself.

Can someone chime in here on how to test the chip? That appears to have been blown in the past as that solder job is not factory.

Is that burn a symptom of sending 50v through the switch matix?

I have a chip tester that clay reccomended that I bought on eBay for cheap and it works really well. And no, I think those burns are just prior rework.

Got a link? I can't find a tester that is denoted for ln series

ebay.com link: itm

Can’t find the exact link I used, but those one lists the ULN as supported.

It's already got a socket, just replace the uln2803 chip and check.

https://www.greatplainselectronics.com/proddetail.asp?prod=ULN2803AP

Jon

Theres really no way to test ic u20 on cpu board unless you have a very good logic probe,did you replace the fuse?if you replaced fuse and still no go then you have to replace u20.have that u20 ic socketted

Good to hear!

For future reference, http://www.pinwiki.com/wiki/index.php?title=Williams_WPC#Switch_Matrix_Problems

The Wiki has an awesome switch matrix diagnosis section.
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Chris Hibler - CARGPB #31
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http://www.PinWiki.com - The new place for pinball repair info

It's common in ALL WPC games. Every time a switch wire hits one of those high-voltage terminals, it'll fry. I went through about 13 of them trying to knock down an intermittent issue in my game. (Turned out the Mini PF switch was shorted against the ball entry metal guide; and the right wireform ramp was shorted to 50v. So, if the ball rolled into the powerfield just right, and made contact between the ramp and metal guide, it'd fry the 2803.)