Check this thread and the post in particular as it sounds to be exactly your issue.

"Here is follow up to the solution to this problem that I was able to implement.

I found the following in a print copy of Clay's WPC guide I have as a known issue that is relative here.

Exploding +20 volt C11 Capacitor (or C10 on WPC95).

There are cases when the +20 volt capacitor (Driver board C11 on WPC-S and prior, C10 on WPC-95) can just explode. This happens when a shorted flipper coil diode or shorted transistor on the Fliptronics board causes the 70 volt coil power to feedback into the 20 volt flashlamp circuitry. Because of reverse voltage, this blows the 20 volt capacitor. Also installing one of the ribbon cable connectors in the backbox on the header pins (top row of header pins to bottom row of housing) can do the same thing. And lastly, if connector J124 is mistakenly plugged into the driver board connector J128 (they are keyed alike!), this can cause capacitor C11 to explode.
First check the ribbon cable header pins to make sure they are attached correctly. Then check the flippers. If when the flippers are activated, one of the flashlamps dimly lights, there may be a bad flipper transistor on the Fliptronics board.
There is a preventive measure which can be taken for this. Install a blocking diode on the driver board ceramic 10 watt resistor R224 (or R9 on WPC-95). To do this on a WPC-S or earlier driver board, first remove the lower leg of resistor resistor R224 (the leg just above TP7). Connect the anode (non-banded end) of a 1N4004 (or 1N4007) diode to the resistor's leg. Then solder the cathode (banded side) of the diode back into the driver board (where one leg of R224 was removed). This will prevent the problem.

First off, I replace both bridge rectifiers at BR1 and BR2 and both of their associated capacitors at C11 and ???. C11 was obvious and the other was because I was already in there.

I then installed the blocking diode as Clay noted above. I checked for shorted diodes on the fliptronics board and on the flippers and found no issues.

So after all of that I plugged it all back together and the game came up fine and has been ever since.

I suspect I inadvertently plugged J124 into J128 as Clay noted can happen. I had just been rebuilding a driver board for a customer's game and was using my BSD as the test bed. So I probably induced the problem in this instance.

I hope this helps others.

Mike O.
Team-EM"

https://pinside.com/pinball/forum/topic/tech-bsd-pdb-issue-blown-c11

Double check your ribbon cable is on the correct pins as that can cause the failure as well. What was the issue in the first place that caused the board to be sent for repairs?