One of the most common and important part of the Williams games that is often over looked is the Power Supply (PS). If you have a Stargate, Joust, Robotron, Sinistar or Bubbles game, your original PS will look like the one below. After 25+ years the PS needs to be rebuild or replaced. You could replace it with a switcher but if you don't want to lose the originality or mess around with rewiring the game it's a lot easier to rebuild the PS. It takes about an hour and parts are around $15-$20.
Below is a picture of a Defender's PS. Also pictured is the transformer that is interchangeable with all Williams games of the same time era.