I have the same issue right now on a Star Trek next generation. Mine says soundboard U6 CKSUM error. I bought a new U6 ROM for the mpu board and tried that, No difference. I tried a new ribbon cable from the Mpu to the audio board, no difference. And I tried a known good mpu with no difference. I don’t have another soundboard to try. Anyone have any other ideas? If you figure it out, let me know! Thanks