Thanks for the info. I wasn't sure where to begin.
I think you're right about the extra resistors and wire. While searching I saw that someone else has a motherboard with the same connections. I'm still worried about the mystery gunk on the pins and PCB around the RAM chip though.
1) The power supply is good. 9v and 1.45 amps. It works on my genesis.
2) The switch appears to be working, but I'm not sure how to test it. I get 0.5v, 3v, 5v, 6v, and 9v when testing random combinations of the 6 pins.
3) Nothing seems visually wrong with the IC or C134, but here are the results of some tests you said to try in other threads:
From this thread:
I get 0v between pin 2 (CART_IN) and 4 (GND) on U38, and a near-zero reading for continuity between pin 2 of U38 and the anode of SD1. I guess this confirms that the switch and cart are OK.
Later in the thread you say to check the voltage across C150. I'm getting 0.2v across this and you say it should be around 5v. Does this alone mean the U38 is bad?
In another thread you suggest some other tests:
1) Where is R168? If it's the one directly under U38 with "103" written on it then I'm getting 9v across that.
2) 9v
3) 9v
4) 5v (REG1)
according to a later post, those voltages are expected.
So, it looks like the only test that failed was the C150 one. Time to replace the U38 chip? What about C150 and C134?