Do you have 12 volts at the driver board - TP5 ?

Check TP 5 on SD board. It should be 11.9v unregulated. This voltage is them regulated down to 5v. If 11.9 is not present then you must figure out why. If it is present there is something wrong with the 5v regulation circuit. The Mata Hari schematics is on the IPBD website.

I agree with oldschoolbob and @mikat11. Before you conclude that your voltage regulator is bad, make sure you have 12 volts at TP5. If you don't check the voltage coming into J3 pin 12. This pin carries the 12 volts from the power supply (rectifier board) to the solenoid driver board.

The unregulated 12 v comes from the rectifier board J3 pin 8. Check for voltage there also.

Those voltages are correct. It seems you are losing the 12 v from rectifier board J3 pin 8 to solenoid driver board J3 pin 12. Reseat those 2 connectors. there may be a bad solder joint or dirty/ bad connection.

What is the voltage at TP 5 now?

Ok so now we know there is 12 v present at solenoid driver board J3 pin 12 but not TP 5. Check continuity from pin J3 pin 12 to the positive side of capacitor C23.I think its the big one next to the black heatsink. Then check continuity from the negative side of C23 to any ground or more specifically to J3 pin10. That cap is a common failure point and could be causing a problem.

There is a good chance that it is bad. It is most likely the original. if you can test it or just go ahead and replace it. It will probably fail sometime in the future. There are 2 other caps in that circuit c24 and c29 that may be causing problems as well. Those caps are in the circuit before the 5v regulator. The IPDB(internet pinball database) website has the Mata Hari schematics that will help you understand the 12v to 5v circuit you are troubleshooting. I've been using it to help you tonight.

Leakage is bad news for sure. I would bet a $ bill you've found the problem.