Some basic questions:
1) Are you using an original Williams power supply or a switching power supply?
--- Williams PS boards typically attach to the big transformer in the bottom of the cabinet. Older PS boards have two big heat sinks on the board (Defender style). Newer PS boards have a separate heat sink (Joust style)
--- Switching power supplies typically attach directly to the 120VAC wall power and look like vented steel bricks.
If you have a Williams power supply, there are 3 LEDs on it that indicate +5V, +12V and -5V are approximately correct. If those are not lit, then the power supply probably needs to be rebuilt.
Always start with the power. If the power is not right, then nothing is going to run right.
2) When testing, make sure the fuses are good and the back door interlock switch is pulled out. There is a 3A slow blow fuse on the power brick next to the big transformer. This is the main power fuse. If it is out, you are not powered up.
The back door interlock switch on Williams machines is the white switch in the upper right corner as you face the back. This is an interlock switch just like your refrigerator uses. When the door is closed, it is pressed in and power will go to the components. The difference from a refrigerator switch is that it can pull out and that turns the switch on in a "cheater" mode so you can work on the internals of the machine. You wouldn't believe the number of "I took the back off and it wouldn't play anymore" stories there are floating around.
3) Once you have verified that you have the correct power to the boards, normally there are some diagnostic messages that flash on the 7 segment LED on the ROM card. These messages are typically in the form of three numbers that flash. The first number is whether it is a RAM or ROM error. The second two numbers tell the positions of the bad chip. For RAM chips (first number 1) it is the bank and chip numbers. For ROM chips (first number 2) it is the ROM chip number.
If you don't see any numbers flashing is when the fun begins.
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