So I finally got around to installing the composite video mod from Vintage Gaming and More. When I first hooked it back up, it worked fine, but, after putting the top back on, it didn't even power up.
After inspection, the black ground input wire had come loose from the board. The trace had de-laminated from the board and broken loose, so I resolldered the black wire to another place nearby on the same trace.
Now it works against, but I've got no sound.
Any suggestions?