that board with the red/green leds is the interface between the top and bottom cpu's, when operating correctly they will flicker to indicate data being transferred back and forth. the two cables that go into that box run directly from the top connector to the sound module that plugs into the mp4 cpu. it has to be in the right connector, there are two connector in that opening, one is correct and the other does nothing. the bottom connector from the led board goes down to the s plus mother board ,progressive output, three yellow wires, however, from the led board to that connector there is another inline board with one connection on each end.
locate this board and its connections, also check the connection on the s plus motherboard, the three connector with the yellow wires and the single connector with the red wire on it, now, remove the hopper, leave off the coin tray , close the door and reach in and wiggle these connections, see if you can get the leds to start to flicker, I have seen these connections become intermittent.
Jim