I'd take a good look at page 19 of your manual (page 19 in pdf, but numbered page 16 in a real manual). Verify that the connectors coming from the p900 power supply and wire colors match what in listed on that page. If that got flipped, or someone rewired it wrong, you might be sending an extra 5V to a ground line on the Board. You may actually be supplying 10V to the system.
This might be one of those fun ones that has more than one problem - enjoy!
For starters, disconnect all connectors from your power supply (other than AC in....) and verify that each +5V on the PS is good. I.E. 9P connector - put black lead of multimeter in pin one, and touch each of pins 5, 6, 7, and 8 with the red lead of the multimeter. Verify 5V. Lastly, check 24V by putting black lead in one of the GND pins, and red lead on +24V.
Now, Turn your game OFF!! Test continuity between all ground pins on that connector -i.e. put black lead on pin one, red lead on pin 2, verify continuity. Repeat for pins 3 and 4. Test continuity between +5 pins and GND pins on that same connector - you shouldn't have any.
Now, do the same procedures for the 12P connector. Power on to test voltages, power off to test continuity.
Once you verify your power supply, it is out of the equation.
Then, if it is good, you probably have a short. Turn off your game!!!!!! Then, plug in your board but Don't power it up!!!! Test one of the chips on the board. Find a 74lsxxx chip and test continuity between Vcc and GND for that chip. If you have continuity, then you have a short on your board .... somewhere.....most likely (small value resistors can make a board appear shorted when they really aren't, but generally, you shouldn't have continuity between +5 and GND on a single chip).
With so many 5V supplies to this board, I can easily see a shorted component leaking an extra 5V into the system. Shorts are hard to find... not only that, but finding the short is only the start - you may have many shorts. You need to figure out what component caused all the others to short. And, you need to find all the shorted components and replace them, or you might end up right back where you started.