I realize you can Google just as good as the best of them, but I found this here:

http://www.allteksystems.com/pdfs/Complete_Manual_Ver_J.pdf

The +5vdc or +5vac voltages are not correct or
The over voltage LED (D17) is on.
• There are only two important components that make up the +5vdc. The voltage regulator (Q20)
and the big capacitor (C23) on the solenoid board. If you have too much AC voltage, then replace
the big capacitor. If you have too much DC voltage, then replace the voltage regulator. Recheck
the voltages before plugging in the MPU again. Sometimes you might have to replace both
components and with the game being over 20 years old, it’s not a bad idea to replace both
components at this time.

What did you do to fix it?

This is one of those threads Google will index and, eight years from now, some poor soul with this exact same problem will find this thread. On that night, you'll hear the scream, "MRSODAPOP!!!!"