Start with using a Slow blow fuse. If the manual says S.B it should be that, otherwise it will blow too fast.

A broken U14 (U23 on WPC-S and WPC-95) can take out U20 after a while. U20 normally breaks within a couple of minutes if U14 is broken.

One way to check that there isn’t any short on the playfield is to disassemble the U20 chip, then start the game, and measure in the socket that it isn't higher than +12 VDC on any of the pins 11 to 18. If it’s higher voltage than that on any of those 8 positions, then there’s probably some type of short on the playfield.
If the short is intermittent, you probably won't see a higher voltage in this test, but it could be worth a try.

Yes I meant removing U20 from it's socket and measuring between the socket and ground.
On the second last page in the manual, you'll find the switch matrix, and there you can see which column goes to which pin on U20.
Column 1: U20-18
Column 2: U20-17
Column 3: U20-16
Column 4: U20-15
Column 5: U20-14
Column 6: U20-13
Column 7: U20-12
Column 8: U20-11
If you get higher than 12 VDC on any of the pins, you'll have to look for a short around the switches and their cabling in that column.