The most common reason the credit knocker on Williams electromechanicals
doesn't work is because the normally closed portion of the make-break coin
switch #1 (the left-most when looking at the door when opened) is dirty,
bent, misadjusted, or broken. If you're not going to be using coins on the
machine, and the switch or wires are broken you can get around the problem
by just soldering together the two wires (Green-Orange and Yellow-Red on
most coin doors).