The 4231A handles the AUX IN function for the system. If you believe that the rest is working, then it does in fact look like an isolated chip failure. Getting the chip is not a problem. However, getting it off the board without damaging the tracking, and then sucessfully soldering a new one on, is. I have done it sucessfully twice, in two attempts, so I must be lucky - however, PLEASE don't just make an attempt because you know how to solder - if you pull the tracking off the board, the player is DEAD. Think carefully before you do this.