What you describe sounds like a broken contact point. Sometimes on knobs, RCA and other inputs the point where the component connects to the board gets loose or actually breaks away from the circuit board. Sometimes the solder cracks and makes the connection imtermitent. Anyway... That may be an easy fix,but, if you are not comfortable with a soldering iron I wouldn't mess with it. The circuit boards for the AD500/1000 are super packed with components and are not a good place to start learning how to solder. Hopefully the seller will take it back. If not, try to get someone here to look at it for you. If I knew it was going to sell that cheap, I would have bid on it. It's a great sounding unit.