necro bumping this. i have a sys 16b that has almost the same exact - similar issue. dip 1 and dip 2 both switch 1's are stuck on, as well as my jump button appears grounded in some games and not in others. this is for both player one and two, but either the inverse i cannot use it in game. all other buttons work. i can see the inputs pass through the optocouplers and the first two resistor networks.
there is then a LS253 at C16 where the input passes i can see the inputs ground as i press the button, but the output seems a little suspect, which is similar to the Sys 16a board in this thread, but obviously on the sys16b. the ls139 appears ok on my probe, i did try piggybacking the 253, but since its a tri-state i know it can cause issues.