As retroActive said, the power connector for the coin door supplies the ground as well as the 6.3VAC to run the lights and the coin lockout solenoids.
There are other problems with running a switcher in a Williams cabinet thet you should be aware of. Due to the design of the 6809 CPU, when it is powering down, the CPU will randomly write to the memory address space. If that is a ROM or dynamic RAM address, no big deal. The ROM will ignore it and the RAM will be reinitialized the next time anyway. But if that memory address is in the CMOS, you can lose your high scores or the settings. The Williams original power supply was designed to minimize this risk by having that large capacitor on the power supply board. This keeps the power to the CPU on for a few extra milliseconds, which is long enough for the CMOS protection circuit to detect that the 12VDC has gone down and to turn off the CMOS chip enable. This protects the CMOS from shutdown writes by the CPU. You lose that protection when you replace the original board with a switcher.
Is there some reason you went with a switcher? There are a couple of us on the board that rebuild Williams power supplies or if it is missing, we can sell you a rebuilt one for very reasonable prices. PM me for details.
ken
I just hate to see people drop switchers in and then wonder why they are losing their scores or setting all the time.
ken