Check Fuses F114/F115 (or F106/F101 on WPC-95) Error Message.
This error message on the score display is a very common problem on WPC games. Not so much because these fuses are blown, but because often these fuses are *not* blown, yet the error message persists.

If fuse F114 (or F106 on WPC-95) is indeed blown, this usually indicates an open or shorted BR1 bridge (D11-D14 on WPC-95), or cap C6 or C7, or the LM7812 voltage regulator Q2. If this fuse is OK, check fuse F115 (or F101 on WPC-95). If this is blown, typically it is caused by a shorted U20 chip (and possibly U14, or U23 on WPC-95) on the CPU board.

The way the game determines if fuse F114/F115 is blown is through the game's switch matrix. Looking at the switch matrix chart, it can be seen that switch 24 on every WPC game is named "always closed". This switch 24 is monitored by the CPU's software. If the software see this switch is not closed, it assumes the power to the switch matrix is gone, and hence the F114/F115 (or F106/F101 on WPC-95) fuse error is displayed. The problem with this assumption is that the switch matrix fuse can be fine, but the switch matrix can be otherwise blown, thus giving a false fuse error.

So the first thing to check are the fuses themselves. If this pair of fuses (F114/F115 or F106/101 on WPC-95) are bad, replace them and power on the game. If they blow immediately, chances are good bridge rectifier BR1 (or diodes D11-D14 on WPC-95) have shorted. This is a fairly common problem. Less common is capacitor C6 or C7 being shorted or a bad LM7812 voltage regulator at Q2.

Another thing to try is this:

Power off.
Remove the fuse F115 from the powerdriver board.
Insert a new fuse at F114.
Switch the game on and see if a fuse blows.
If the fuse F114 does blow, chances are good either bridge rectifier BR1, or caps C6 or C7 are shorted. Using a DMM and measure the voltage at TP8. This should read about 18 volts - if lower (but more than 2 volts), the caps at C6 and/or C7 are suspect. If fuse F115 blows either the 7812 voltage regulator is bad or there is a short on the playfield.

If neither F114 (the fuse before BR1) or F115 (the fuse after the 7812 voltage regulator) blows, there are a number of other potential issues. The following checks can be performed with only connector J101 installed (J129 on WPC95) on the power driver board. Make sure J114 and J116/J117/J118 and the ribbon cable are removed (on WPC95 remove J101 and J138/J139/J140/J141). This will remove the playfield and anything that uses regulated 12 volts from the equation.

First check for voltage at TP8 for 20 volts DC (TP102 on WPC95). If there's voltage at TP8, then the BR1 bridge and C6/C7 caps are fine (D11-D14 and C11/C12 on WPC95). No voltage there and either fuse F114 is bad (fuse F106 on WPC95) or the BR1 bridge is open (diodes D11-D14 on WPC95). Now check for voltage at both sides of D1 and D2 (1n4004). Input is the non-banded side of D1, which should show 20 volts DC. The banded side of D2 should show about 16 volts DC. If nothing here, then the D1/D2 have gone open. Now the unregulated DC voltage goes to Q2 pin 1 (7812), the voltage regulator. Q2 pin 1 should show 16 volts DC. The output of Q2 is pin 2, and this should show 12 volts. If there's input voltage at pin 1 but nothing at output pin 2, replace Q2. Next the (now regulated) 12 volts DC goes to cap C2 which is 100 mfd 25 volts (C40 on WPC95). I have seen this cap leak and fail, and the circuit board trace break at C2, so check that. Finally the 12 volts goes through fuse F115 and to TP3 (F101 and TP100 on WPC95).

If the F114/F115 (or F106/101 on WPC-95) fuses are good yet the error message still persists, there is a switch matrix problem. As mentioned above, the CPU monitors switch 24 for its "always closed" status. Switch 24 is actually physically closed by having a loop of wiring going from the CPU board connector J212 down to the coin door interface board connector J3. Here the switch matrix column 2 and row 4 are joined together through a 1N4004 diode, band connected to switch column 2 on the interface board. If one of these connectors are disconnected, or the wiring broken to the coin door interface board, the blown fuse error will be displayed (note this is rarely the problem).

Assuming all is good so far (no blown fuses and CPU board connector J212 and coin door interface connector J3 are attached), the next thing to check is CPU board chip U20 (ULN2803). Often this chip is blown, usually because the solenoid voltage somehow touched a playfield switch. On WPC-S and later games, Williams socketed CPU board U20 because this problem was so common. If U20 has failed, the game will display the F114/F115 (or F106/F101) fuse error (even though these fuses are good). Replace U20 with a new ULN2803 chip (use a socket if one is not there). If the error still persists, replace CPU board chip U14 (74LS374, or on WPC-95 U23, a 74HC237).

Regarding U14 (74LS374 on WPC/WPC-S) and U23 (74HC237 on WPC-95). For some reason, this chip can die *without* the U20 (ULN2803) failing. This is very strange, as the chip in question is between the ULN2803 and the cpu logic. But it does happen! Just keep this in mind. It doesn't happen a lot, but it does happen.

Here is a further indepth, step-by-step approach to see exactly what is causing the F114/F115 (or F106/F101) error message, assuming the fuses themselves are *not* blown. With the game on and the coin door closed:

Test for AC voltage at J101 pins 4 and 7 (or J129 pins 4 and 7 on WPC-95). A reading of 13 to 18 volts AC should be seen. This is the AC voltage coming from the transformer. If no voltage here, check the Molex connectors around the transformer and at the power driver board.
Test for DC voltage at TP8 (or TP102 on WPC-95) and ground. A reading of 16 to 18 volts DC should be seen. If no voltage here, replace BR1 (or D11 to D14 on WPC-95). Also no voltage here can occur because the solder pads are cracked around bridge BR1 (or D11 to D14 on WPC-95). Using jumper wires for BR1 (as described in the Game Resets section) helps prevent this.
Test for DC voltage at TP3 (or TP100 on WPC-95) and ground. A reading of 12 volts DC should be seen. If no voltage here, check or replace diodes D1 and D2 (1N4004, all WPC version).
If still no voltage at TP3 (or TP100 on WPC-95) and diodes D1/D2 are OK, replace Q2 (all WPC versions) which is a LM7812 voltage regulator for the 12 volts. Also check cap C2 (C40 on WPC95) and the circuit board traces for cracks.
If the above still does not fix the problem, replace U20 (all WPC versions) on the CPU board (ULN2803). If U20 died "hard", it could also blow the 74LS374 at U14 (on WPC-95 it's U23, a 74HC237) on the CPU board.
If voltage is still not right, or BR1 (or diodes D11-D14 on WPC-95) are REALLY hot, check all the TIP107 transistors on the power driver board. If these test good, check/replace the ULN2803 at U19 (or U11 on WPC-95) on the power driver board, or maybe the 74LS374 at U18 (or U10 on WPC-95) on the power driver board.