Here is the description for error code #7 :
System failure. Occasionally, the following components can contribute to this problem: U21 (4MhZ crystal), components in the IRQ circuit, broken leads or failed C9 (22uF) in the reset section, or loosely seated ICs on the CPU board. Note that boot code "7" is the hardware default and characteristic of the CPU "reset" signal not complying with the proper low-to-high spec at pin 40.
I would investigate these suggestions first. The easy ones being the Ic's (if socketed reseat / if soldered look for bad joints), visually inspect the cap (pull and test or better yet replace).
Where are you testing for 12 volts? What are the test point readings?
Check for loose connectors from the power supply to the CPU. Also confirm proper output voltages from the PS. A logic probe on pin 40 of U17 would tell a lot.
Check this out - http://gamearchive.askey.org/Pinball/Manufacturers/Williams/pdfs/pinball_troubleshooting_sys9.pdf Pages 48-51 have some valuable information on troubleshooting this problem.