The most likely suspect were the 4116 DRAMs. These were a pretty standard part "back in the day". What's annoying about these parts is they require 3 voltages to operation, a +5, -5 and +12v supply. I also recall (from back when I had a TRS-80 that used these same memory chips) that these chips were finicky regarding how the power is supplied. My guess was when the +5 went out it took the RAM chips down too.
Luckily you can still get these chips, so I ordered a complete set (24, for 48K of RAM) from Bob Roberts. They came in today. I pulled all the old chips and put in the new. I turned the system on and Defender came up.
I'm not playing the game yet I still have a couple of issues to address. I need a new speaker, the one in the game has a rather large tear, though it still sounds good. The bigger problem is I need to fix the battery backup on the game. This is a common problem, when batteries leak their acid tends to corrode the board and eventually the battery holder and traces are eaten away. That's the case for my board so I have some work ahead of me to clean this up.
Battery backup for a Defender game is important, it's not just about the high score. Defender and other Williams games use software configuration. In other words the configuration of the machine is stored in memory (RAM), and if that memory isn't backed up by battery it will "lose" the configuration and the game will be unplayable until it's re-configured. This is a major pain in the ass to perform every time you want to play a game.
So the solution is to either fix the battery holder (AA type batteries), replace with a Bob Roberts replacement (Lithium battery) or try to find the SimTek replacement that swaps out the 5101 with an NVRAM - the no battery solution.
For now I need to move on to the cap kit for the GO7 and make decisions about how far I want to take restoration of the cabinet.
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