As I’m getting back into my system 11 board repairs again I’ll start documenting them. Maybe you’ll find this information useful when troubleshooting your own boards.
This is a board someone tried to repair before. It wouldn’t boot up and was giving a single flash indicating a U25 RAM failure. The U25 RAM is soldered direct to the board from the factory.

When I got the board the 6116 was removed and a socket in its place. I guess after replacing the RAM chip didn’t fix it the tech shelved it. I put my test ROM in and it ran but failed the RAM test.

If you run the board with no ROMs the processor will run through all the addresses looking for instructions and will do this indefinitely if none are found. This is handy for tracking down stuck lines.
I checked the address and data bus on the RAM chip itself and all showed activity. The R/W (read/write) pin also had activity. Pin 18, the chip enable pin, also known as CS (chip select) was stuck low.

This pin is tied to the collector of Q40, a 2N3904 transistor. I measured the transistor and it read fine. The next thing is to check continuity. The line was open. I hate running wires across the back of the board but I didn’t want to risk further damage to traces by removing the socket and repairing the trace from the top.
With continuity restored, the board is running again. This is actually not the first time I’ve seen this exact problem so do yourself a favor, when troubleshooting a U25 RAM failure, check continuity between Q40 and pin 18 first.

One last note, if you’re repairing a board that uses a 28 pin RAM chip you’ll need to check the schematics since CE is a different PIN number. I think it’s 23 but don’t quote me on that since I don’t have a clear copy on hand at the moment.
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